WORKSHOP LOCATION:  

Agile Lab L70 room in the Central Kings Building of NJIT. 

Workshop Chair:

Andrei Sirenko, NJIT

SCIENTIFIC COMMITTEE: 

Sang-Wook Chong (Chair), Valery Kiryukhin, and David Vanderbilt, all from Rutgers U.

LOCAL ORGANIZERS

Junjie Yang and Trevor Tyson, and Andrei Sirenko, all from NJIT

GROUP  PHOTO  Jan 9th, 2025

Program of the Workshop: 

Total: 37 Talks (45 min, 30 min, and 20 min); 18 Posters; 57 Abstracts; Total: 80 registered participants

SEVERAL PRESENTATIONS are in the FOLDER  BELOW

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LIST of PRESENTATIONS POSTER SESSION

Thursday, January 9th, 2025 17:00 – 18:30

Posters will be displayed on individual big-screen monitors connected to the Presenter's laptops. Several slides can be presented during multiple discussions with the other Workshop Participants. Preparation of a Backup copy on a memory stick is strongly advised. 

P1. Tuning the magnetic ordering of SrFeO3 thin films via epitaxial strain

Lucas Barreto,  Singh Center for Nanotechnology, University of Pennsylvania, Philadelphia, PA 19104, USA.  Center for Natural Sciences and Humanities, Federal University of ABC - UFABC, Santo André 09210-580, SP, Brazil.   Department of Physics, University of Johannesburg, PO Box 524, Auckland Park, 2006, South Africa

P2. Study of altermagnetism in Ni-based compounds with non-collinear spin structure

Deepak K. Singh

P3. Alter-fast Dynamics: Optical Studies of Engineered Altermagnetism

Eunice Paik,  Army Research Laboratory, USA

P4. Raman Scattering Spectroscopy in the intercalated transition metal dichalcogenide VNb3S6

Shreenanda Ghosh,  Johns Hopkins University, USA

P5. Electrical 180o switching of Néel vector and anomalous Nernst effect in altermagnet

Lei Han, Xizhi Fu, Junwei Liu, Cheng Song

School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

P6. Materials with Broken Mirror Symmetries: Physical properties and altermagnetism

Junjie Yang,  Department of Physics, New Jersey Institute of Technology, USA

P7. Imaging Atermagnetic domains and their dynamics

Valery Kiryukhin, Rutgers University, USA

P8. Ferroaxial phonons in chiral and polar NiCo2TeO6

V. A. Martinez, Department of Physics, New Jersey Institute of Technology, USA

P9. Unconventional insulator-metal transition in Mn3S2Te6 

Jan Musfeldt, University of Tennessee, USA

P10. Symmetry Tuning of Altermagnets: Exploring Spin and Structural Interplay Using In-Situ Uniaxial Strain with X-ray Scattering

Philip J. Ryan Magnetic Materials Group, Argonne National Laboratory, USA

P11. Neutron Investigation of RuO2 Thin Films

Shelby Fields, U.S. Naval Research Laboratory Washington D.C., USA

P12. Magnetic imaging of domains and domain walls in antiferromagnets

Weida Wu, Rutgers University, USA

P13. High Pressure Synthesis of Novel Altermagnetic Candidates

Weiwei Xie, Department of Chemistry, Michigan State University, USA

P14. Origin of large effective phonon magnetic moments in monolayer MoS2

Wencan Jin , Auburn University, USA

P15. Emerging magnetism in candidate altermagnet Fe0.75Cr0.25Sb2

Xiao Hu, Brookhaven National Lab, USA

P16. Symmetries in Quantum Materials – Antiferromagnetism and Beyond

Xianghan Xu, University of Minnesota, USA

P17. Antiferromagnetic driven odd-parity magnetism

Yue Yu,   U. of Wisconsin-Milwaukee, USA

P18. Double-Q chiral stripe order in the anomalous Hall antiferromagnet CoNb3S6

Ben Zager, Brown University, USA

P19. Electrical Switching of an unconventional odd-parity magnet

Qian Song, MIT, USA

P20. Optical Studies of RuO2 Thin Films

Benjamin Mead, UPenn, USA

Wednesday, January 8th, 2025,  8:00-10:00

W1   Altermagnetism Foundations I; Chair: Andrei  Sirenko and Libor Šmejkal

W1-1  From superfluid 3He to altermagnets

Tomas Jungwirth

Institute of Physics, Czech Academy of Sciences, Cukrovarnick´a 10, 162 00 Praha 6, Czech Republic

School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom

The Pauli exclusion principle combined with interactions between fermions is a uni- fying basic mechanism that can give rise to quantum phases with spin order in diverse physical systems. Transition-metal ferromagnets, with isotropic ordering respecting crys- tallographic rotation symmetries and with a net magnetization, are a relatively common manifestation of this mechanism, leading to numerous practical applications, e.g., in spin- tronic information technologies. In contrast, superfluid 3He has been a unique and fragile manifestation, in which the spin-ordered phase is anisotropic, breaking the real-space rotation symmetries, and has vanishing net magnetization. The recently discovered alter- magnets share the spin-ordered anisotropic vanishing-magnetization nature of superfluid 3He. Yet, altermagnets appear to be even more abundant than ferromagnets, can be robust, and are projected to offer superior scalability for spintronics compared to ferro- magnets. The talk revisits the decades of research of the spin-ordered anisotropic phases with vanishing net magnetization including, besides superfluid 3He, also theoretically conceived Pomeranchuk instabilities of Fermi liquids [1,2]. While all sharing the same ex- traordinary character of symmetry breaking, we highlight the distinctions in microscopic physics which set altermagnets apart and enable their robust and abundant material re- alizations. We show coordinate-space and momentum-space microscopies, experimentally demonstrating and exploring the altermagnetic ordering in MnTe [3,4].

References

[1]    L. Smejkal, J. Sinova, T. Jungwirth, Physical Review X (Perspective) 12, 040501 (2022).

[2]   T. Jungwirth, R. M. Fernandes, E. Fradkin, A. H. MacDonald, J. Sinova, L. Smejkal, (Perspective) arXiv:2411.00717

[3]   O. J. Amin et al., Nature in press, arXiv:2405.02409.

[4]   J. Krempasky et al., Nature 626, 517 (2024).

W1-2   Altermagnetism and Kinetomagnetism

Sang-Wook Cheong

Keck Center for Quantum Magnetism, Rutgers University, NJ, USA

 Altermagnetism is introduced as a category of magnetic states with ‘collinear’ antiferromagnetic spins and alternating variations of local structures around spins in such a way that the symmetry allows typical ferromagnetic behaviors such as spin split bands. Altermagnets exhibiting ferromagnetic behaviors without any external perturbations (type-I) turn out to belong to the ferromagnetic point group. Other altermagnets (type-II and type-III) can have ferromagnetic behaviors only with external perturbations such as electric current or stress, which conserve parity-time-reversal (PT) symmetry. All types of altermagnets themselves have broken PT symmetry. The concept of altermagnetism can be extended to accommodate non-collinear spins and multiple local-structure variations. All these altermagnets are classified in terms of magnetic point groups. Strong altermagnets have spin split bands in the non-relativistic limit, and spin splitting in weak altermagnets requires spin-orbit coupling.

Kinetomagnetism refers to magnetization induced by electric current, encompassing longitudinal or transverse effects and even- or odd-order effects. The precise relationship between Altermagnetism and Kinetomagnetism warrants exploration. The intersection of Altermagnetism and Kinetomagnetism presents both opportunities and challenges in science and technology.

References

1, Anomalous Hall effect arising from noncollinear antiferromagnetism, H. Chen, Q. Niu, & A. H. Macdonald, Phys. Rev. Lett. 112, 017205 (2014).

2, Emerging research landscape of altermagnetism, L. Šmejkal, J. Sinova, & T. Jungwirth, Phys. Rev. X 12, 040501 (2022).

3, Altermagnetism with Non-collinear Spins, Sang-Wook Cheong and Fei-Ting Huang, npj Quantum Materials 9, 13 (2024).

4, Emergent Phenomena with Broken Parity-Time Symmetry: Odd-order vs. Even-order Effects, Sang-Wook Cheong and Fei-Ting Huang, Phys. Rev. B 109, 104413 (2024).

W1-3   Manipulation of the altermagnetic order via crystal symmetry

Cheng Song

  Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.

Crystal symmetry guides the development of condensed matter. The unique crystal symmetry connecting magnetic sublattices not only distinguishes altermagnetism from ferromagnetism and conventional antiferromagnetism, but also enables it to combine the advantages of ferromagnetism and antiferromagnetism. Altermagnetic order is essentially magnetic crystal order, determined by the magnetic-order (Néel) vector and crystal symmetry. Previous experimental works were concentrated on manipulating the altermagnetic symmetry by tuning the Néel vector orientations, but the manipulation of the crystal symmetry remains challenging, which holds great promise in opening a new paradigm of manipulating altermagnetic order. Here, it is realized in altermagnetic CrSb films. The locking between Dzyaloshinskii-Moriya (DM) vector and magnetic space symmetry helps to reconstruct the altermagnetic order, from collinear Néel vector to canted one. It unprecedentedly generates room-temperature spontaneous anomalous Hall effect in a metallic altermagnet. The relative direction between the current-induced spin polarization and DM vector determines the switching modes of altermagnetic order, i.e. parallel for field-assisted mode in CrSb(11�00)/Pt and non-parallel for field-free mode in W/CrSb(112�0). The DM vector induces asymmetric energy barrier in field- assisted mode and generates asymmetric driving force in field-free mode. Particularly, the latter is guaranteed by the emerging DM torque in altermagnets. Reconstructing crystal symmetry adds a new twist to the manipulation of altermagnetic order. It not only underpins the magnetic-memory and nano-oscillator technology, but also inspires crossover works between altermagnetism and other research topics.

______________________________________________________________

Wednesday, Jan 8th, 2025,  10:30-12:30

W2    MnTe-I;   Chair:  John Tranquada, Brookhaven National Lab

W2-1   Emergent ferromagnetism, magnons and phonons in altermagnetic candidate MnTe films grown on InP (111)

Matthew Brahlek1, Liang Wu2

1.      Oak Ridge National Lab

2.      University of Pennsylvania

To push into new generations of spintronic devices requires understanding new magnetic phenomena and also how to control both known and emerging material platforms as high-quality epitaxial thin films. Specifically, altermagnets are a new phase that is predicted to exhibit a strong spin splitting of the band structure, which can form the basis for new spintronic applications. MnTe, a candidate altermagnet, is a room-temperature antiferromagnet (TN ≈ 310 K) semiconductor (energy gap of order 1 eV) with a NiAs structure. Here, we present results on the molecular beam epitaxy growth and properties of MnTe/InP(111). Using polarized neutron reflectivity and magnetotransport, we find that there is emergent ferromagnetic behavior likely driven by a combination of charge transfer and strain. The ferromagnetic component is likely a slight canting of the bulk-like A-type antiferromagnetic state, as seen by neutron diffraction. We have also performed time-resolved magneto-optical effect on these samples to study the magnons and phonons. This high level of tunabililty of MnTe opens the door to tailoring interlayer magnetic interactions in this layered material system. Together these results provide a potential mechanism of tuning antiferromagnetic ordering for applications in high-speed, next-generation spintronics.

1.  I. Gray, Q. Deng, Q. Tian, M. Chilcote, J. S. Dodge, M. Brahlek, and Liang Wu* Time- resolved magneto-optical Kerr effect in the altermagnet candidate MnTe, Appl. Phys. Lett. 125, 212404 (2024)

2.  M. Chilcote, A. R. Mazza, Q. Lu, I. Gray, Q. Tian, D. Moseley, A. Chen, J. Lapano, H. Ambaye, J. S. Gardner, G. Eres, T. Z. Ward, H. Cao, M. McGuire, R. Hermann, D. Parker, M.-G. Han, Liang Wu* , T. R. Charlton, R. G. Moore, M. Brahlek Stoichiometry-induced ferromagnetism in altermagnetic candidate MnTe Advanced Functional Materials, 2405829 (2024)

W2-2   Interplay of anisotropic paramagnons and phonons in altermagnetic MnTe

George Yumnam1*, Eleanor Clements2, Jiaqiang Yan1, Huibo Cao2, Songxue Chi2, Bing Li2, Benjamin Fransen3, Michael E. Manley1, Raphaël P. Hermann1

1 Materials Science and Technology Division, Oak Ridge National Laboratory

2 Neutron Scattering Division, Oak Ridge National Laboratory

3 Department of Physics and Astronomy, Brigham Young University

* email: yumnamg@ornl.gov

Altermagnets can exhibit an unusual d– or g–wave magnetization order parameter due to their combined breaking of rotational and time-reversal symmetry, which is explained by multipolar magnetic moments. MnTe is an altermagnet candidate with large spin, S=5/2, and correspondingly strong magnetic neutron scattering signal.

 Recent inelastic neutron scattering work observed chirally split magnons in MnTe. Furthermore, theory work suggested coupling between the altermagnetic order parameter and the lattice dynamics, even in the absence of a magnetic field. This coupling would lead to a hybridized paramagnon-polaron mode that provides direct access to altermagnetic excitations based on phonon data. We will report on a study of the chiral magnon modes and their possible interplay with phonons. This work was carried out with triple-axis neutron spectroscopy and reveals different dynamical structure factors for the transverse and longitudinal paramagnon-phonon interactions.

W2-3   Spin structures of pure and Ge-doped Mn3Si2Te6

Feng Ye, ORNL

Recently discovered colossal magnetoresistance (CMR) Mn3Si2Te6 has shown stark contrast with conventional CMR materials. The resistivity in Mn3Si2Te6 drops by orders of magnitude leading to an insulator-metal transition with an applied magnetic field just above 4 T. The CMR occurs only when the field is applied along the hard c-axis, while such effect is absent with the field applied in the basal plane, where magnetization is fully saturated. We have conducted systematic single crystal neutron diffraction study of the ground state spin structures of pure and Ge-doped Mn3Si2Te6 and the evolution of the spin configurations as function of temperature, electric current, and external magnetic field up to 14 Tesla along the c-axis. Prominent short- range magnetic diffuse scattering near Tc, the close connection between the square of spin-spin correlation lengths and electric resistivity, strongly emphasizes the relevance of fluctuating moments contributing to the transport properties. Application of magnetic field along the c axis renders a swift occurrence of CMR but only a slow tilting of the magnetic moments toward the c axis. The results provide direct evidence that the modification in spin order is not sufficient to explain the giant magnetoresistance response when the field is applied along the spin hard axis and reveals new mechanism to describe the novel magnetotransport behavior.

W2-4   Beyond the ground state of MnTe: Short-range magnetism at high temperature and magnetic evolution under pressure

Benjamin Frandsen

Brigham Young University

Most studies of altermagnetism in MnTe and other candidate materials have focused primarily on properties of the ground state, providing a vital starting point for understanding these fascinating materials. However, much can be learned about the underlying physics of these materials by investigating their behavior as they transition out of the ground state and into neighboring phases, such as the paramagnetic phase at elevated temperature. Moving beyond the ground state provides a more complete context in which to understand various altermagnetic materials, while also potentially providing insights relevant for practical applications. Here, we present detailed neutron scattering studies of the magnetism and structure of MnTe as a function of temperature and applied hydrostatic pressure. We demonstrate the persistence of robust short-range magnetic correlations in the high-temperature paramagnetic state that preserve the local magnetic environment of the low-temperature ground state, while also driving a pronounced spontaneous magnetostriction response in MnTe. We also show that pressure increases the Neel temperature significantly, while simultaneously reducing the magnitude of the ordered moment at low temperature. These results reveal strong magnetostructural coupling in MnTe, potentially providing another tuning knob for modifying the properties of this altermagnet. More generally, this work raises interesting questions about the influence of short-range magnetism in altermagnets.

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Wednesday, Jan 8th, 2025,  14:00-16:00

W3   Anomalous Hall and multipole Hall Effects, Chair: Badih A. Assaf

W3-1   Altermagnetic Anomalous Hall Effect Emerging from Electronic Correlations

Jeroen van den Brink

Institute for Theoretical Solid State Physics, IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany

Altermagnetic materials are characterized by collinear magnetic order with a vanishing net magnetic moment, but nevertheless have a spin-splitting in their non-relativistic electronic band structure. From ab initio calculations we have identified around 60 altermagnetic materials. From a theoretical point of view several physical properties that render altermagnets different from canonical antiferro-, ferro- and ferri-magnets will be discussed. These include certain spin and heat transport features and piezomagnetic responses. By symmetry in principle also an anomalous Hall effect (AHE) is allowed in certain altermagnets. In particular we introduce an altermagnetic model in which the emergence of an AHE is driven by interactions. Quantum Monte Carlo simulations show that the system undergoes a finite temperature phase transition governed by a primary antiferromagnetic order parameter accompanied by a secondary altermagnetic one. The emergence of both orders turns the metallic state of the system, away from half-filling, into an altermagnet with zero net moment but a finite AHE.

Y.  Guo,  H.  Liu,  O.  Janson,  I.C.  Fulga,  J.  van  den  Brink,  and  J.I.  Facio, Spin-split collinear antiferromagnets: A large-scale ab-initio study, Materials Today Physics, 32, 100991 (2023)

T.  Sato,  S.  Haddad,  I.C.  Fulga,  F.F.  Assaad,  and  J.  van  den  Brink, Altermagnetic anomalous Hall effect emerging from electronic correlations, Phys. Rev. Lett. 133, 086503 (2024)

O. Gomonay, V. P. Kravchuk, R. Jaeschke-Ubiergo, K. V. Yershov, T. Jungwirth, L. Šmejkal, J. van den Brink, J. Sinova, Structure, control, and dynamics of altermagnetic textures, npj Spintronics 2, 35 (2024)

C. Li, M. Hu, Z. Li, Y. Wang, W. Chen, B. Thiagarajan, M. Leandersson, C. Polley, T. Kim, H. Liu, C. Fulga, M.G. Vergniory, O. Janson, O. Tjernberg, and J. van den Brink, Topological Weyl Altermagnetism in CrSb, arXiv:2405.14777 (2024)

W3-2   Spin Hall and Edelstein Effects in Novel Chiral Noncollinear Altermagnets

Mengli Hu

Leibniz Institute for Solid State and Materials Research, IFW Dresden, Helmholtzstraße 20, 01069 Dresden, Germany

Altermagnets are a newly discovered class of magnetic phases that combine the spin polarization behavior of ferromagnetic band structures with the vanishing net magnetization characteristic of antiferromagnets. Initially proposed for collinear magnets, the concept has since been extended to include certain non-collinear structures. A recent development in Landau theory for collinear altermagnets incorporates spin-space symmetries, providing a robust framework for identifying this class of materials. Here we expand on that theory to identify altermagnetic multipolar order parameters in non-collinear chiral materials. We demonstrate that the interplay between non-collinear altermagnetism and chirality allows for spatially odd multipole components, leading to non-trivial spin textures on Fermi surfaces and unexpected transport phenomena, even in the absence of SOC. This makes such chiral altermagnets fundamentally different from the well-known SOC-driven Rashba-Edelstein and spin Hall effects used for 2D spintronics. Choosing the chiral topological magnetic material Mn3IrSi as a case study, we apply toy models and first-principles calculations to predict experimental signatures, such as large spin-Hall and Edelstein effects, that have not been previously observed in altermagnets. These findings pave the way for a new realm of spintronics applications based on spin-transport properties of chiral altermagnets.

W3-3  Harnessing magnetic octupole Hall effect to induce torque in altermagnets

Seungyun Han1, Daegeun Jo2,3, Insu Baek1, Peter M. Oppeneer2,3, and Hyun-Woo Lee1

1Department of Physics, Pohang University of Science and Technology, Pohang 37673, Korea

2Department of Physics and Astronomy, Uppsala University, P.O. Box 516, SE-75120 Uppsala, Sweden

3Wallenberg Initiative Materials Science for Sustainability, Uppsala University, SE-75120 Uppsala, Sweden

d-wave altermagnets have magnetic octupoles as their order parameters [Phys. Rev. X 14, 011019 (2024)]. We theoretically show that magnetic octupoles injected from outside generate torque on the d-wave altermagnets. The injection can be achieved by the magnetic octupole Hall effect in an adjacent layer. We calculate the magnetic octupole Hall conductivity of the heavy metal Pt and find a sizable value comparable to its spin Hall conductivity. Our work generalizes the spin Hall phenomenology (generation by heavy metals and detection by torque in ferromagnets) to the magnetic octupole Hall phenomenology (generation by heavy metals and detection by torque in altermagnets), which may be utilized to electrically control magnetic configurations of altermagnets.

W3-4  Magnetic octupole Hall effect in altermagnets

Hye-Won Ko and Kyung-Jin Lee

Dept. of Physics, Korea Advanced Institute of Science and Technology (KAIST), Korea

Altermagnets have recently attracted significant attention due to their unique combination of momentum-dependent spin splitting and antiferromagnetic ordering [1]. The order parameters in altermagnets are the antiferroic Néel order and the ferroic magnetic octupole order [2, 3], with the latter driving the momentum-dependent spin splitting. Since the order parameter dictates all responses, magnetic octupole currents are inherently present in altermagnets. In this talk, we will explore the magnetic octupole Hall effect in altermagnets, with a focus on the crucial role of orbital transport.

References

[1]  Libor Šmejkal, Jairo Sinova, and Tomas Jungwirth, Phys. Rev. X 12, 040501 (2022).

[2]  Sayantika Bhowal and Nicola A. Spaldin, Phys. Rev. X 14, 011019 (2024).

[3]  Paul A. McClarty and Jeffrey G. Rau, Phys. Rev. Lett. 132, 176702 (2024).

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Wednesday, Jan 8th, 2025,  16:30-18:30

W4   RuO2    Chair: Junjie Yang

W4-1   Hunting antiferromagnetic ordering in altermagnetic candidate RuO2 thin films

Steven Bennett 

US Naval Research Laboratory

 Altermagnetism is an emerging phenomena of significantly high interest to the spintronics community. While spintronics has persisted for decades across a wide array of memory and computing paradigms, it has reached a threshold and Ferromagnetic spintronics have clear downsides that are hindering the progression of novel spin related paradigms. Their low switching speeds, stray fields requiring large device sizes, and low packing densities are preventing progress toward a more efficient and higher speed future of non-volatile memory, especially for brain inspired neuromorphic computing/machine learning.

Altermagnestism promises to yield materials which are inherently antiferromagnetic (having no net magnetic moment) but nonetheless posses a non-relativistic (non-topological) spin polarization. However, before we as a community can adopt rigorous research efforts on the Altermagnets there are critical fundamental questions which need to be answered about the amplitude of the spin polarization, spin filtering efficiency and controllability at high switching frequencies of these materials.

If the Altermagnets prove to be what we are hoping, a subset of antiferromagnetism containing the ultra-high switching speeds of an antiferromagnet, combined with the spin polarization typically characteristic of a ferromagnet and the spin momentum locking reminiscent of a topological insulator then there is once again great hope for a new paradigm in ultra-efficient spin-centric electronics.

At the Naval research laboratory in Washington D.C we are undergoing a multi-pronged effort to experimentally verify the presence of these spin polarized states in the theoretically predicted Altermagnets. In this talk I will summarize recent developments in our efforts to measure the recently debated presence of antiferromagnetic ordering in epitaxial films of RuO2 using neutron scattering and SQUID magnetometry. [1]

[1]  Shelby Fields et. Al. “Orientation Control and Mosaicity in Heteroepitaxial RuO2 Thin Films Grown Through Direct Current Sputtering”, Crystal Growth and Design, Vol 24/issue 11 (2024)

W4-2   Tunable Localized Currents at Crystallographic Domain Boundaries in Altermagnet RuO2

Gina Pantano, 1,2 Eklavya Thareja, 1 Libor Smejkal, 2,3 Jairo Sinova, 2 Jacob Gayles 1

1Department of Physics, University of South Florida, Tampa, FL USA

2Institut für Physik, Johannes Gutenberg Universität Mainz, Mainz, Germany

3Institute of Physics of the Czech Academy of Sciences, Cukrovarnická, Czech Republic

Research on interfacial phenomena in condensed matter physics has garnered significant interest due to the discovery of new properties and phases distinct from the bulk, enabling the manipulation of materials for technological applications. In this work, we investigate the novel effects that arise from the presence of a locally chiral crystallographic domain boundary in the altermagnet ruthenium dioxide (RuO2). Conventionally, crystallographic domain boundaries are seen as detrimental to transport due to increased electron scattering and the cancellation of opposite anomalous responses [1]. However, we uncover more intriguing phenomena at the interface. Altermagnets are characterized by having a substantial non-relativistic spin splitting comparable to ferromagnetic materials but with compensated magnetic ordering. The spin splitting originates from the Heisenberg exchange interaction combined with the anisotropic octahedral crystal field reducing the symmetry between the two opposite spin sublattices to an antiunitary rotation or mirror symmetry. This introduces a new mechanism for controlling spin-dependent transport phenomena by altering the crystal configuration, specifically through its local chirality, such as the crystal Hall effect. RuO2 was chosen for this study due to its high Néel temperature, metallic nature, and exhibiting one of the largest spin splittings in this new material class. We use first- principle calculations to characterize the interfacial states and their contribution to electronic transport. We observe an induced magnetization at the domain boundary and enhanced anomalous transport along the interface when spin-orbit coupling is considered, due to the change of symmetry. We theorize the localized currents are tunable by the direction of the magnetization at the interface. Our findings will contribute to the understanding of how altermagnetic properties evolve toward interfaces with the reduction in dimensionality and symmetry and contribute to advancements toward the design of sustainable, energy-efficient devices.

References

[1]  I. I. Mazin, Altermagnetism in MnTe: Origin, predicted manifestations, and routes to detwinning, Phys. Rev. B 107, L100418 (2023).

[2]  L. Smejkal, J. Sinova, and T. Jungwirth, Emerging research landscape of altermagnetism, Phys. Rev. X 12, 040501 (2022).

[3]  E. W. Hodt, P. Sukhachov, and J. Linder, Interface-induced magnetization in altermagnets and antiferromagnets, Phys. Rev. B 110, 054446 (2024).

W4-3   Magnetism in bulk RuO2: insights from inelastic and nuclear resonance scattering

Raphael Hermann, George Yumnam

Materials Science and Technology Division Oak Ridge National Laboratory hermannrp@ornl.gov

Ruthenium dioxide has attracted significant interest due to predicted altermagnetic behavior of this rutile-structure material. A key point of contention is whether ruthenium exhibit a magnetic moment. Whereas neutron diffraction provides some indication for an antiferromagnetic ground state, muon spin rotation indicates a much lower limit and possibly zero magnetic moment. Here, we will report on new analysis of ruthenium-99 Mössbauer spectroscopy and nuclear forward scattering of synchrotron radiation. We will compare fit models for hyperfine splitting from the electric field gradient and magnetic hyperfine field, and discuss implications for magnetism. This data will also be complemented with inelastic neutron scattering and inelastic x-ray scattering measurements compared to lattice dynamics calculations.

We acknowledge collaboration with Dimitrios Bessas, Ilya Sergueev, Lars Bocklage, Parul Raghuvanshi, Lucas Lindsay, David Parker, Valentino Cooper, Michael Manley, and Shaofei Wang, and thank especially John Budai and the late Fritz Wagner, for initial data acquisition of some data we reanalyzed here.

W4-4 Questioning Altermagnetism in RuO2?

John Q. Xiao, David T. Plouff, Laura Scheuer, Shreya Shrestha, Weipeng Wu, Nawsher J. Parvez, Subhash Bhatt, Xinhao Wang, Lars Gundlach, M. Benjamin Jungfleisch

   Department of Physics and Astronomy University of Delaware Newark, DE 19716

Altermagnets (AMs) represent a new class of magnetic materials that exhibit properties common to both antiferromagnets (AFMs) and ferromagnets (FMs). Like AFMs, AMs have zero net magnetization, while also displaying spin band splitting, a characteristic of FMs. However, unlike the global spin band splitting in FMs, AMs exhibit momentum- dependent splitting along specific crystalline directions. This unique combination of traits enables AMs to offer superior performance compared to FMs in various applications. A prominent example of an AM candidate is metallic RuO2, where the spin band splitting arises from the 90-rotated crystal fields between neighboring Ru atoms in the rutile structure. Experimental evidence supporting this includes spin-torque ferromagnetic resonance, laser-induced THz emission, and earlier neutron scattering data. However, more recent neutron and muon experiments suggest that Ru atoms do not carry a magnetic moment. In this presentation, we confirm that RuO₂ is not an altermagnet based on our latest experimental findings from laser-induced THz emission, magneto-Kerr rotation, and neutron scattering experiments.

==================================

Thursday, January 9th, 8:00 – 10:00

T1   Altermagnetism Foundations II;   Chair: Sang-Wook Cheong, Rutgers University

T1-1  Spin group theory of unconventional magnetism: altermagnets and beyond

Libor Šmejkal1,2,3

1 Max Planck Institute for the Physics of Complex Systems, Nöthinzer Str.38, 01187 Dresden, Germany

2 Institut für Physik, Johannes Gutenberg Universität Mainz, D-55099 Mainz, Germany

3 Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00, Praha 6, Czech Republic

                  lsmejkal@pks.mpg.de

Spontaneous symmetry breaking is systematically studied using group theory across various areas of physics, such as high-energy physics, superfluidity, and superconductivity. In this talk, we will demonstrate that spin groups enable the analogous systematic exploration of exchange symmetry breaking in magnetic crystals [1]. This approach has recently led to the discovery of collinear altermagnets[1] and noncollinear p-wave magnets featuring compensated even and odd-partial-wave spin orders[2].

In the first part of the talk, we will discuss how the prediction and observation of the unconventional anomalous Hall effect[3-5] inspired the development of a systematic spin symmetry classification for magnetic phases. In the second part, we will overview experimental confirmations of altermagnetic symmetries and effects in both reciprocal and direct space. Specifically, we will explain the band structure features described by spin group theory, which have been experimentally observed using photoemission studies in MnTe and CrSb[6-7]. Additionally, we will elaborate on how the unconventional time-reversal symmetry breaking in altermagnets was confirmed through combined XMLD-XMCD mapping of altermagnetic domains in the direct space[8]. Finally, in the last part of the talk, we will provide an overview of proposed applications of altermagnetism and spin group theory in spintronics, topological matter and other research areas [9].

References

[1]  L. Šmejkal, Jairo Sinova, T. Jungwirth, PRX 12, 031042 (2022).

[2]  A. Birk Hellenes et al., arXiv:2309.01607 (2024).

[3]  L. Šmejkal et al., Science Adv. 6, 23 (2020).

[4]  I. I. Mazin et al., PNAS 118 42 (2021).

[5]  H. Reichlová et al., Nat. Commmun. 15, 4961 (2024).

[6]  J. Krempaský, L. Šmejkal et al., Nature 626, 517(2024).

[7]  S. Reimers, et al. Nat Commun 15, 2116 (2024)

[8]  O.J. Amin et al., arXiv:2405.02409 (2024), Nature, in press.

[9]  I. Mazin et al., arXiv:2309.02355 (2023)

T1-2  Spin crystallographic symmetry for classification of altermagnets and noncollinear magnets

Hikaru Watanabe

Department of Physics, the University of Tokyo

In recent years, antiferromagnetic spintronics has undergone rapid advancements and has garnered significant interest. Notably, a class of collinear antiferromagnets known as altermagnets plays a critical role in spintronic phenomena due to their unique spin- momentum coupling [1]. These materials exhibit substantial spin-charge interconversion and various physical responses stemming from their nonrelativistic spin-orbit coupling, which has been systematically elucidated through symmetry analysis using the spin group framework [2]. It is anticipated that the intriguing physics observed in collinear antiferromagnets will be similarly reflected in noncollinear antiferromagnets. For example, previous studies have suggested the presence of large spin-charge-coupled electrical responses and the topological Hall effect in noncollinear magnets [3].

In this work, we extend symmetry analysis to encompass both collinear and noncollinear antiferromagnets [4]. Utilizing the spin space group, our analysis reveals magnetic symmetries that do not rely on relativistic spin-orbit coupling. The spin-space-group symmetry captures the intricacies of specific spin structures and provides deeper insights into the physical implications of various spin orders.  

[1]  L. Šmejkal et al., Nat. Rev. Mater. 7, 482 (2022), and references thein.

[2]  L. Šmejkal et al., Phys. Rev. X. 12, 031042 (2022); P. Liu, et al., Phys. Rev. X 12, 021016 (2022).

[3]  J. Železný et al., Phys. Rev. Lett. 119, 187204 (2017).; Y. Zhang et al., New J. Phys. 20, 073028 (2018); J. Ye et al., Phys. Rev. Lett. 83, 3737 (1999).

[4]  H. Watanabe, K. Shinohara, T. Nomoto,A. Togo, and R.Arita, Phys. Rev. B. 109, 094438 (2024).

T1-3   Switchable p-wave magnetism

Riccardo Comin

MIT  rcomin@mit.edu

Altermagnets are a new class of magnetic materials that combine aspects of ferromagnets and antiferromagnets, possessing zero net magnetization like antiferromagnets but exhibiting (potentially large) spin splitting of the electronic bands and anomalous Hall responses like ferromagnets. Recent research has focused on magnetic systems with odd-parity spin splitting of nonrelativistic origin (p-wave magnets), which are promising for spintronic applications. Symmetry considerations suggest the possibility of coupling altermagnetism with ferroelectricity in polar chiral magnets, potentially allowing for novel mechanisms for electric-field control of magnetism.

Nickel iodide (NiI2) is a van der Waals magnetic insulator and multiferroic when in its chiral magnetic phase (T < 59 K). The spin helices (pitch ~ 7 unit cells) that characterize this phase breaks inversion symmetry, leading to an induced electrical polarization of purely electronic origin. I will start by discussing prior work on the evolution of the multiferroic down to the two-dimensional (single-layer) limit. I will then focus on more recent work characterizing the nonrelativistic spin splitting of electronic bands in NiI2, and its connection to symmetry and chirality. I will present a demonstration of the electrical (voltage-based) switching of chirality and consequent reversal of the momentum-space spin polarization. I will conclude with an outlook for potential applications of spin-chiral multiferroics for information storage.

T1-4  Two-dimensional altermagnets from high throughput computational screening: symmetry requirements, chiral magnons and spin-orbit e;ects

Joachim Sødequist and Thomas Olsen

We present a high throughput computational search for altermagnetism in two- dimensional (2D) materials based on the Computational 2D Materials Database (C2DB). We start by showing that the symmetry requirements for altermagnetism in 2D are somewhat more strict compared to bulk materials and applying these yields a total of 7 altermagnets in the C2DB. The collinear ground state in these monolayers are verified by spin spiral calculations using the generalized Bloch theorem. We focus on four d-wave altermagnetic materials belonging to the P21'/c' magnetic space group – RuF4, VF4, AgF2 and OsF4. The first three of these are known experimentally as van der Waals bonded bulk materials and are likely to be exfoliable from their bulk parent compounds. We perform a detailed analysis of the electronic structure and non-relativistic spin splitting in k-space exemplified by RuF4. The magnon spectrum of RuF4 is calculated from the magnetic force theorem and it is shown that the symmetries that enforce degenerate magnon bands in anti-ferromagnets are absent in altermagnets and give rise to the non-degenerate magnon spectrum. We then include spin-orbit eQects and show that these will dominate the splitting of magnons in RuF4. Finally, we provide an example of i-wave altermagnetism in the 2H-phase of FeBr3.

[1] J. Sødequist and T. Olsen, Appl. Phys. Lett. 124 182409 (2024)

_________________________________________

Thursday, January 9th, 10:30 – 12:30

T2   MnTe-II , Chair Vivek Bhartiya

T2-1   Electronic structure and symmetry analysis of altermagnetic system MnTe

 Chang-Jong Kang1*

1Department of Physics, Chungnam National University, Daejeon 34134, Korea

Altermagnetism is a recently identified fundamental form of magnetism characterized by a vanishing net magnetization and a broken electronic structure with time-reversal symmetry. In this talk, we employ a combination of symmetry analysis and first-principle calculations to reveal that the crystallographic symmetry groups of numerous magnetic materials, featuring negligibly small relativistic spin-orbit coupling (SOC), are significantly larger than conventional magnetic groups. Consequently, a symmetry description incorporating partially decoupled spin and spatial rotations, termed the spin group, becomes essential. We establish the classifications of spin point groups that describe collinear magnetic structures, encompassing altermagnetic phases. Using MnTe as an example, we provide direct evidence for altermagnetism in MnTe.

T2-2   Spin splitting effect in strained g-wave MnTe and in anisotropic ferromagnets

 Kirill Belashchenko

Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Nebraska 68588, USA

The spin splitting effect is the generation of a pure transverse spin current without the need for spin- orbit coupling, which is a promising application of altermagnets in spintronic devices [1]. First, I will discuss strain-induced spin splitting effect in g-wave altermagnets, focusing on the case of the hexagonal MnTe semiconductor. The strain-induced spin splitting effect in hole-doped MnTe is predicted to have a giant gauge factor of more than 30 [2]. To calculate this effect, we use the spin-orbit-coupled 𝑘𝑘 ⋅ 𝑝𝑝 Hamiltonian for the quadruply-(almost)-degenerate valence band maximum at the A point derived from symmetry and fitted to eigenvalues calculated from first principles. It is noteworthy that MnTe has several Rashba-like spin-orbit terms which have a strong effect on the spin polarization and the transport properties. The large spin splitting gauge factor in MnTe is associated with the lifting of the A-point degeneracy by the strain-induced crystal field, whereas this gauge factor is usually of order 1 in metallic g-wave altermagnets. Next, I will show that the spin splitting effect is generically allowed in non-cubic ferromagnets thanks to the crystallographic anisotropy of the conductivity tensor. Under open boundary conditions, the charge-to-spin conversion is proportional to the difference in the transport spin polarizations along the inequivalent principal axes. First-principles screening of 41 noncubic ferromagnets combined with the Boltzmann calculations in the relaxation-time approximation revealed that many of them, when grown as a single crystal with tilted crystallographic axes, can exhibit large spin splitting angles comparable with the best available spin-orbit-driven spin Hall sources [3].

[1]  R. González-Hernández et al., Phys. Rev. Lett. 126, 127701 (2021).

[2]  K. D. Belashchenko, arXiv:2407.20440.

[3]  K. D. Belashchenko, Phys. Rev. B 109, 054409 (2024).

T2-3   X-ray magnetic circular dichroism in MnTe and rutile altermagnets

Jan Kuneˇs

Masaryk University, Brno

 X-ray magnetic circular dichroism (XMCD) is a well-established technique for investigating magnetism. Here, we present theoretical studies of XMCD at the manganese L2,3 edge in two altermagnets, MnTe and MnF2, with experimental data available for MnTe. Our calculations reveal that spin-orbit coupling in the valence states has a negligible effect on the XMCD spectra. In other words, the spectra computed in an idealized non-relativistic system—where altermagnetism is well-defined by symmetry—closely resemble those obtained in a more realistic model that includes valence spin-orbit coupling. This feature distinguishes XMCD from optical or transport measurements, where spin-orbit coupling is essential for observing magneto-optical or anomalous Hall effects.

Using MnTe and MnF2 as examples, we demonstrate that crystal symmetry dictates the origin of the finite XMCD signal, with different terms in the Hamiltonian contributing depending on symmetry. In the case of rutile MnF2, the core-valence exchange interaction causes a minor modification of the XMCD signal, which exists even without this interaction. Conversely, in hexagonal MnTe, the core-valence interaction is necessary to observe any finite XMCD signal at all. These distinct origins of XMCD account for the different magnitudes of the effect in these isolectronic materials (Mn d5).

Finally, we will discuss how XMCD can be utilized to determine the orientation of the N´eel vector.

T2-4   Unexpected Tuning of the Anomalous Hall Effect in Altermagnetic MnTe Thin Films

Sara Bey1, Shelby S. Fields2, Nicholas G. Combs2, Bence G. Márkus1,3, Dávid Beke1,3, Jiashu Wang1, Anton V. Ievlev4, Maksym Zhukovskyi5, Tatyana Orlova5, László Forró1,3, Valeria Lauter6, Steven P. Bennett2, Xinyu Liu1, Badih A. Assaf1

1 Department of Physics and Astronomy, University of Notre Dame, Notre Dame IN, 46556, USA. 2 Materials Science and Technology Division, U.S. Naval Research Laboratory, Washington DC, 20375, USA 3 Stavropoulos Center for Quantum Matter, University of Notre Dame, Notre Dame IN, 46556, USA. 4 Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA 5 Notre Dame Integrated Imaging Facility, University of Notre Dame, Notre Dame, IN, 46556, USA. 6 Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831 USA

The discovery of an anomalous Hall effect (AHE) sensitive to the magnetic state of antiferromagnets can trigger a new era of spintronics, if materials that host a tunable and strong AHE are identified. Altermagnets are a new class of materials that can under certain conditions manifest a strong AHE, without having a net magnetization. But the ability to control their AHE is still lacking. In this study, we demonstrate that the AHE in altermagnetic α-MnTe grown on GaAs(111) substrates can be "written on-demand" by cooling the material under an in-plane magnetic field. The magnetic field controls the strength and the coercivity of the AHE. Remarkably, this control is unique to α-MnTe grown on GaAs and is absent in α-MnTe grown on SrF2. Our findings are a result of systematic magnetotransport measurements done on well-characterized films grown by molecular beam epitaxy. In conjunction with transport measurements, I will discuss results from temperature dependent X-ray diffraction, atomic force microscopy, transmission electron microscopy, SQUID magnetometry and polarized neutron reflectance that reveal the structural and magnetic properties of our α-MnTe films. The tunability that we reveal challenges our current understanding of the symmetry-allowed AHE in this material and opens new possibilities for the design of altermagnetic spintronic devices.

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Thursday, January 9th, 14:00 – 16:00

T3   Symmetry analysis  Chair: David Vanderbilt

T3-1  Topological properties of altermagnets

Rafael M. Fernandes

Department of Physics, University of Illinois Urbana-Champaign, IL 61801, USA

The properties of a magnetic state depend on which symmetries of the lattice leave the state unchanged when combined with time-reversal, i.e., with flipping all the magnetic moments. In a ferromagnet, no such symmetry exists, resulting in a nonzero magnetization and a uniform Zeeman splitting of the spin-up and spin-down bands. In contrast, this type of symmetry is present in a collinear antiferromagnet, since a lattice translation or inversion “undoes” the flipping of the spins, leading to degenerate spin-up and spin-down bands with no Zeeman splitting. Between these two types of magnetic states, however, lies a broad range of systems for which the symmetry that relates configurations of flipped spins is a rotation (proper or improper). Called altermagnets, these states have no magnetization, like an antiferromagnet, yet their bands display a nodal Zeeman splitting, resembling a "d-wave" (or higher-order) ferromagnet. In this talk, I will discuss the various connections between altermagnets and phenomena of interest in correlated electronic systems, such as multipolar order and Pomeranchuk instabilities. I will then show how spin-orbit coupling endows altermagnets with interesting and non- trivial topological properties, including mirror-protected nodal lines, Chern bands, and Weyl nodal lines in the electronic spectrum

T3-2   Altermagnetism viewed from multipole representation

Satoru Hayami

Graduate School of Science, Hokkaido University

The concept of electronic multipoles in condensed matter physics has been used to systematically describe charge, spin, and orbital degrees of freedom in electrons. There are four types of multipoles according to the spatial inversion and time-reversal parities: electric multipole (time-reversal-even polar tensor), magnetic multipole (time-reversal-odd axial tensor), magnetic toroidal multipole (time-reversal-odd polar tensor), and electric toroidal multipole (time-reversal-even axial tensor). Since these multipoles are independent of each other and their bases cover any internal electronic degrees of freedom in solids owing to their completeness in the Hilbert space, they provide not only a deep understanding of physical phenomena but also a further exploration of functional materials [1].

In this presentation, we introduce a close relationship between multipoles and altermagnetism. We show that the emergence of magnetic toroidal quadrupole leads to the symmetric spin-split band structure [2], while that of anisotropic magnetic dipole leads to the anomalous Hall effect in both collinear and noncollinear anti- ferromagnets [3]. Furthermore, the multipole description enables us to search for further intriguing functions in antiferromagnets without the spin-orbit coupling. We show the case of the antisymmetric spin-split band structure in noncollinear antiferromagnets [4] and that of the nonlinear nonreciprocal transport in noncoplanar antiferromagnets free from the spin-orbit coupling [5].

[1]   S. Hayami, H. Kusunose, JPSJ 93, 072001 (2024).

[2]   S. Hayami, Y. Yanagi, H. Kusunose, JPSJ 88, 123702 (2019).

[3]   S. Hayami, H. Kusunose, PRB 103, L180407 (2021).

[4]   S. Hayami, Y. Yanagi, H. Kusunose, PRB 101, 220403(R) (2020).

[5]   S. Hayami, M. Yatsushiro, PRB 106, 014420 (2022).

T3-3   Unconventional magnetism

Qihang Liu

Southern University of Science and Technology, Shenzhen 518055, China  Email: liuqh@sustech.edu.cn

With the advancement of antiferromagnetic (AFM) spintronics, magnetic materials with diverse magnetic structures have garnered widespread attention. Of particular interest are “unconventional magnets”, which simultaneously exhibit AFM configurations while displaying properties reminiscent of ferromagnets (FMs). Thus, unconventional magnets promise to combine the advantages of both FM and AFM materials, offering, e.g., high storage capacity, low power consumption, electrical manipulation and read-out, and ultrafast dynamics. The rapid evolution of the emerging field of unconventional magnetism has inevitably engendered some degree of conceptual ambiguity and uncertainty, particularly regarding the intricate entanglement between spin-split-AFM (including altermagnets) and anomalous-Hall-AFM. In this talk, we start with symmetry theory describing magnetic geometry—spin group theory—to discuss the symmetry design strategies and material pools for these two types of unconventional magnets, aiming to inspire further exploration in the field of unconventional magnetism. We also introduce a homemade online program, FINDSPINGROUP (https://findspingroup.com/), which is applied to diagnose the symmetry classification of magnetic materials.

  [1]  Liu et al. Phys. Rev. X 12, 021016 (2022).

[2]  Chen et al. Phys. Rev. X 14, 031038 (2024).

[3]  Chen et al. arXiv:2307.12366 (2023).

[4]  Zhu et al. Nature 626, 523 (2024).

[5]  Liu et al. to appear (2024).

T3-4   Understanding Altermagnetism Through Multipole Representation: Bulk and Surface Implications

Sayantika Bhowal

Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India

In this talk, I will discuss the multipolar framework to understand the non-relativistic spin splitting in the centrosymmetric antiferromagnets with broken time-reversal symmetry, commonly referred to as "altermagnets." Using the well-known rutile-structure transition metal difluorides as case studies, I will discuss how magnetic octupoles provide a unified framework for understanding broken time-reversal symmetry and non-relativistic spin splitting. This framework offers insights into manipulating spin splitting and explains phenomena such as (anti-)piezomagnetism, "chiral" magnons, and unconventional magnetic Compton scattering unique to this class of materials. I will also discuss the emergence of surface magnetoelectric multiferroicity driven by magnetic octupoles, revealing a fascinating bulk-boundary correspondence in these unconventional antiferromagnets. Our work highlights the importance of the multipolar approach in uncovering key insights into the complex interplay between bulk and surface phenomena in altermagnets, opening avenues for innovative applications and advancing theoretical understanding.

Reference:

Sayantika Bhowal and N. A. Spaldin, Ferroically Ordered Magnetic Octupoles in d-Wave Altermagnets, Phys. Rev. X 14, 011019 (2024).

X. H. Verbeek, D. Voderholzer, S. Schären, Y. Gachnang, N. A. Spaldin, and S. Bhowal, Nonrelativistic ferromagnetotriakontadipolar order and spin splitting in hematite, Phys. Rev. Research 6, 043157 (2024).

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Friday January 10th, 2025  8:00 - 10:00

F1   Ferroelectric Altermagnets   Chair: Riccardo Comin

F1-1   BiFeO3 is an Altermagnet

Andrea Urru, Daniel Seleznev, Yujia Teng, Se Young Park*, Sebastian Reyes-Lillo^ and Karin Rabe

Department of Physics and Astronomy, Rutgers

*Department of Physics, Soongsil University, Seoul, Korea

^Departamento de Fisica y Astronomia, Universidad Andres Bello, Santiago, Chile

For over two decades, BiFeO3 has been intensively studied for its promise as a multiferroic material. Because of its relatively high R3c symmetry, the electronic bands are spin-degenerate along the high-symmetry lines in reciprocal space, and its altermagnetic character has not been previously recognized. To make the altermagnetic character of BiFeO3 evident, we compute the bandstructure using first principles methods. To plot the bandstructure, we introduce a new type of generalized path in reciprocal space that highlights the behavior of the electronic bands both along high-symmetry lines and at general k points and captures the sign alternation of spin-splitting typical of altermagnets. We also generate maps of the spin-splitting in planar cross-sections of the entire Brillouin zone, showing complex patterns consistent with its g-wave character that can be used to construct effective Hamiltonian models.

F1-2   Interfacial Hall Effect in an Orthoferrite Based Heterostructure

Takahiro C. Fujita

Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, 113-8656, Tokyo, Japan

Perovskite orthoferrites, LnFeO3 (Ln: lanthanides), are classified as anti-ferromagnetic insulators with Néel temperature of ~700 K depending on Ln. They have been studied especially in the context of multiferroics resulting from the orthorhombic distortion and interplay of magnetic interactions. Recent theoretical studies shed light on altermagnetic aspects in this classic class of compounds such as spin current generation [1] and anomalous Hall effect [2]. Due to their insulating nature, direct electrical transport measurements on these materials are challenging. An ingenious route to avoid this issue is utilizing heterointerface structure with non-magnetic conducting compounds as it has been revealed that mobile electrons in the conducting layer capture microscopic spin textures in the adjacent magnetic insulating layer via proximity effect, leading to emergent transport phenomena [3]. In this study, we report on magnetotransport properties of a heterointerface structure composed of a paramagnetic CaRuO3 and an orthoferrite DyFeO3 [4]. As lowering temperature, Fe3+ moments of DyFeO3 exhibit so-called GxAyFz spin configuration at ~650 K that is followed by a spin-reorientation transition to AxGyCz configuration at ~50 K. Furthermore, a transition from AxGyCz to GxAyFz configurations can be induced by magnetic field below ~50 K, where anomalous Hall conductivity is active in GxAyFz but not in AxGyCz [2]. Our transport measurements reveal an abrupt increase of Hall resistivity below ~50 K plausibly triggered by the field-induced magnetic transition. Our findings will pave the way for electrical detection of spin textures in altermagnetic insulators.

[1]  M. Naka, Y. Motome, and H. Seo, Phys. Rev. B 103, 125114 (2021). [2] M. Naka, Y. Motome, and H. Seo, Phys. Rev. B 106, 195149 (2022). [3] M. Ohno, T. C. Fujita, and M. Kawasaki, Sci. Adv. 10, eadk6308 (2024). [4] T. C. Fujita, K. Omura, and M. Kawasaki, Appl. Phys. Lett. 125, 011602 (2024).

F1-3   Ferroelectric Switchable Altermagnetism

Mingqiang Gu,1 Yuntian Liu,1 Haiyuan Zhu,1 Kunihiro Yananose,2 Xiaobing Chen,1 Yongkang Hu,1 Alessandro Stroppa,3,† Qihang Liu1,4,*

1Department of Physics and Guangdong Basic Research Center of Excellence for Quantum Science, Southern University of Science and Technology, Shenzhen 518055, China

2Korea Institute for Advanced Study, Seoul 02455, Republic of Korea

    3CNR-SPIN, c/o Dip.to di Scienze Fisiche e Chimiche - Università degli Studi dell'Aquila - Via Vetoio - 67100 - Coppito (AQ), Italy.

4Guangdong Provincial Key Laboratory of Computational Science and Material Design, Southern University of Science and Technology, Shenzhen 518055, China

 We propose a novel ferroelectric switchable altermagnetism effect, by synergistically correlating the switching of ferroelectric polarization and the altermagnetic spin splitting. We demonstrate the design principles for the ferroelectric altermagnets and the further symmetry constraints for switching the altermagnetic spin splitting through flipping the electric polarization based on the state-of-the-art spin-group symmetry techniques. 22 ferroelectric altermagnets are found by screening through the 2001 experimental reported magnetic structures in the MAGNDATA database and 2 of them are identified as ferroelectric switchable altermagnets. Using the hybrid improper ferroelectric material [C(NH2)3]Cr(HCOO)3 as an example, we show how the altermagnetic spin splitting is tightly coupled to the ferroelectric polarization, providing an ideal platform for designing electric-field-controllable multiferroic devices. Finally, we find that such manipulation of altermagnetism can be detected by monitoring the physical quantities that are related to the non-vanishing Berry curvature dipole, such as the linearly polarized photogalvanic spin current.

Acknowledgement This work has been funded by the European Union - NextGenerationEU, Mission 4, Component 1, under the Italian Ministry of University and Research (MUR) National Innovation Ecosystem grant ECS00000041 - VITALITY - CUP B43C22000470005.

 [1]  K. Yananose et al, Inorg. Chem. 2023, 62, 17299-17309;

[2]  A. Stroppa et al., Adv. Mater. 2013, 25, 2284-2290;

[3]  A. Stroppa et al., Angew. Chem. Int. Ed. 2011, 50, 5847-5850;

[4]  M. Gu et al., https://arxiv.org/abs/2411.14216.

F1-4   Chiral spin liquids in AMnBi2 (A = Ca, Yb) 

Pengcheng Dai

Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA

Time reversal (T) and inversion (P) are the most basic symmetries in condensed matter systems. In magnets, T is spontaneously broken by magnetic long-range-order (LRO), and P is broken simultaneously in multiferroics. On the other hand, the possibility of broken T- and/or P-symmetries in the collective state of spins without magnetic LRO, i.e., a chiral spin liquid (CSL) phase by quantum and/or thermal fluctuations, has been intensively studied theoretically. The order parameter characterizing the P-symmetry breaking is the vector spin chirality (VSC) while that for T-symmetry is the scalar spin chirality (SSC) , where   are spins at neighboring sites i, j, k, respectively. Despite its tremendous theoretical implications, a CSL phase has never been found experimentally. Here we use polarized neutron scattering to show that tetragonal lattice AMnBi2 (A=Ca,Yb), a C-type collinear antiferromagnet with TN =260 K and 290 K, also has an coexisting in-plane VSC that vanishes above TVSN =420 K (TVSN > TN.) and does not follow the Mn2+/ Mn4+ magnetic form factors. On cooling from TVSN to TN, low-energy paramagnetic spin excitations change from isotropic to anisotropic in spin space, forming a spin nematic state around  that breaks the in-plane fourfold rotational symmetry of the tetragonal lattice29-32 before gapping out below TN.  While both YbMnBi2 and CaMnBi2 have VSC, only in YbMnBi2, the Yb3+ moments interact with VSC and nematic spin excitations to induce static/dynamic SSC, giving rise to anomalous Hall (AHE)33 and anomalous Nernest effect (ANE) by breaking time-reversal symmetry. Our results, therefore, provide compelling evidence for a CSL and its impact on AHE and ANE35,36, and open new avenues of research to unveil the properties of CSLs.

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Friday January 10th, 2025  10:30 - 12:30

F2  Analytical  Methods  Chair: Jan Musfeldt

F2-1  Nanoscale lattice engineering towards emergent magnetic states

Yue Cao

Materials Science Division, Argonne National Laboratory

 In quantum materials, conventional structure-property relationship often correlates the atomic structure and space group symmetry with the electronic and magnetic properties. However, realistic materials host a plethora of lattice imperfections, leading to a deviation of the nanoscale lattice structure away from the ensemble average. In this talk, we will quantify such deviations using nanoscale strain and shear and discuss their consequences in terms of nanoscale breaking of lattice symmetries. In the context of altermagnetism, we will outline a nanoscale lattice engineering approach towards new emergent magnetic states. Our quantification of the nanoscale strain tensor is enabled by coherent diffraction imaging or more broadly Bragg microscopy that takes advantage of the brilliant coherent X-ray photons from upgraded synchrotron facilities. We will propose a multimodal framework where Bragg microscopy will work in tandem with magnetic imaging e.g., magnetic force microscopy and Lorentz transmission electron microscopy to reveal the nature and dynamics of these magnetic states.

The work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division, through the Early Career Research Program.

F2-2  Structural Studies of Altermagnet Systems: Spin-Lattice Coupling in Fe2Mo3O8

T. A. Tyson1,3, S. Liu1, S. Amarasinghe1, S. K. Wang2,3,

S. Chariton4, V. Prakapenka4 , T. Chang4, Y.-S. Chen4, S.-W. Cheong2,3, and M. Abeykoon5

1Department of Physics, New Jersey Institute of Technology, Newark, NJ 07102 

2Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 

3Rutgers Center for Emergent Materials, Rutgers University, Piscataway, NJ 08854

4Center for Advanced Radiation Sources, University of Chicago, Argonne, IL 60439, USA

5National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973

A systematic structural study of the Fe2Mo3O8 altermagnet system as a function of pressure, temperature, and magnetic field was conducted. The results reveal that the P63mc space group of this material remains stable for a broad range of these parameters. The long-range magnetostructural response (Dc/c ) for a magnetic field transverse to the displacement and the local structural magnetic field response were determined. Hydrostatic pressure structural measurements reveal a large compression anisotropy. The results indicate the sensitivity of the lattice parameters in tuning the magnetic phases in the general A2Mo3O8 system.

F2-3  Linear magneto-birefringence: an optical probe of complex (alter)magnetism

Veronika Sunko

UC Berkeley

The combination of ferromagnetic and antiferromagnetic properties in altermagnets is both fundamentally intriguing and functionally promising. A central challenge in this emerging field is the detection and control of altermagnets: distinguishing them from time-reversal invariant antiferromagnets and achieving single-domain samples are critical for uncovering new phenomena and leveraging their functionalities. Optical probes, with their sensitivity to symmetry and spatial resolution, offer a promising solution to these challenges. Notably, Kerr rotation has been proposed as a tool to identify altermagnets and map their domains. However, symmetry constraints mean that not all altermagnets exhibit Kerr rotation, as it is forbidden by vertical mirror planes.

In this context, I will discuss linear magneto-birefringence (LMB), an optical probe of time- reversal symmetry breaking that complements Kerr rotation. I will introduce LMB as both a phenomenon and an experimental technique, using EuIn2As2 as an example to illustrate its ability to reveal complex magnetic structures. Additionally, I will present a symmetry-based argument demonstrating that LMB can identify altermagnets even in cases where Kerr rotation is prohibited and explore a mechanism by which this effect can arise in altermagnets. Remarkably, the same experimental setup can be easily adapted to measure both Kerr rotation and LMB, making LMB a powerful addition to symmetry-sensitive optical tools, as well as a valuable technique for research in altermagnets.

F2-4   Nonlinear optical investigations of unconventional magnetism

Liuyan Zhao

University of Michigan

Altermagnetism is featured by compensated magnetization but e=ective broken time reversal symmetry that lifts the Kramers degeneracy and distinguishes from conventional antiferromagnetism. In this sense, the order parameter of altermagnetism is magnetic multipole moment (e.g., magnetic octupole moment) that requires a tensor field to directly couple with. My group proposes to and practices using nonlinear optics as a probe to study novel magnetism whose order parameter is magnetic multipole moment. In this talk, I will mainly show how we use nonlinear optics to track two magnetic phase transitions in a kagome magnet Co3Sn2S2, one at 175K that corresponds to the ferromagnetic ordering temperature for the out-of-plane spin component and the other at 120K that is explained to be the all-in- all-out noncollinear antiferromagnetic ordering temperature for the in-plane spin component [1]. I will briefly show our results on a helimagnet Cr1/3NbS2 [2] and a vdW magnet CrSBr [3]. And finally, I will discuss our ongoing e=ort in using nonlinear optics to study altermagnetism [4].

References

[1]                     Nature Photonics 18, 26 (2024)

[2]                     In preparation (2024)

[3]                     Nature Communications 15, 6472 (2024)

[4]                     arXiv 2410.14542 (2024)

F2-5   Manipulation of altermagnetic domains

James Beare, Yiqing Hao, Jie Xing, Avishek Maity, Masaaki Matsuda, Chenyang Jiang, Huibo Cao

Oak Ridge National Laboratory

Altermagnets are a special class of antiferromagnets in which magnetic order lifts spin- band degeneracy through time-reversal symmetry breaking, without the presence of macroscopic magnetization. The absence of ferromagnetic magnetization makes them ideal for high-speed spintronic devices but also poses challenges in controlling magnetic domains. Polarized neutrons, which utilize the intrinsic spin polarization of neutrons, are a powerful technique for studying time-reversal symmetry breaking and are particularly well-suited for investigating altermagnets.

We have investigated altermagnetic domain control in several well-known altermagnet candidates using the DEMAND diffractometer and PTAX spectrometer at the High Flux Isotope Reactor (HFIR) at ORNL. Our findings reveal that even an extremely weak magnetic field can effectively align altermagnetic domains. In this presentation, I will introduce these results and explain the underlying mechanism responsible for their sensitivity to magnetic fields.

The research was supported by the U.S. Department of Energy (DOE), Early Career Research Program Award KC0402020 and used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by ORNL.

F2-6    Inelastic neutron scattering from a candidate altermagnet

Sears Jennifer

BNL, USA

Neutron scattering is a well-established and powerful technique for probing magnetism, which can provide important characterization and insights into candidate altermagnetic materials. We will provide an overview of the technique, and its contributions so far to the development of our understanding of altermagnetism. We also present our work on Fe1- xCrxSb2 which shows critical behavior, with magnetism emerging around x=0.25 as one dopes Cr into the nonmagnetic parent compound FeSb2 [1]. This doping level is of particular interest due to theoretical work suggesting its magnetic ground state is a collinear altermagnetic order [2]. We present our data showing diKuse, inelastic magnetic scattering indicating dynamical magnetism emerging in Fe0.75Cr0.25Sb2, and compare with the trivial antiferromagnetism in CrSb2 [3] as well as the potential q=0 altermagnetic state.

[1]   R. Hu, V. F. Mitrovic, C. Petrovic. Phys. Rev. B 76, 115105 (2007).

[2]  I. I. Mazin et al.. PNAS 118(42), e2108924118 (2021).

[3]   M. B. Stone et al., Phys. Rev. Lett. 108, 167202 (2012).

Work at Brookhaven is supported by the O5ice of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy under Contract No. DE-SC0012704.

==========================

POSTERS

Thursday January 9th, 2025  17:00 - 18:30

P1. Tuning the magnetic ordering of SrFeO3 thin films via epitaxial strain

Lucas Barreto1,2,3

1 Singh Center for Nanotechnology, University of Pennsylvania, Philadelphia, PA 19104, USA

2 Center for Natural Sciences and Humanities, Federal University of ABC - UFABC, Santo André 09210-580, SP, Brazil

3 Department of Physics, University of Johannesburg, PO Box 524, Auckland Park, 2006, South Africa

Materials with non-trivial magnetic ordering give rise to exotic topological phenomena that can enhance spin-based devices' performance. The bulk perovskite SrFeO3 displays a rich magnetic phase diagram with several magnetic orderings varying from helical to multi-q arrangements. Exploring and tuning such properties on thin films is critical for many applications. In this work, we evaluate how in-plane lattice stress influences the structural, magnetic, and electronic of SrFeO3 films. We grow epitaxial SrFeO3 films on different substrates to induce compressive and tensile strains. We obtain structural properties via X-ray diffraction and probe the electronic transport as a function of temperature. Density functional theory calculations support the structural and transport data. The magnetic ordering is probed via resonant x-ray magnetic diffraction, and variation in the projection of the magnetic wavevector as a function of strain is observed along the

[111] direction. We observe that the lattice strain can tune the magnetic propagation vector on the films and maintain the SrFeO3 metallic behavior.

P2. Title: Study of altermagnetism in Ni-based compounds with non-collinear spin structure

Deepak K. Singh

Finding altermagnetic state in AFM compound has emerged as one of the mainstream problems in condensed matter physics. From experimental perspective, the exploration of altermagnetism is mostly focused on antiferromagnetic systems with collinear magnetic structures of SU(2) moments, such as RuO2, MnTe. Ignoring AFM compounds with non-collinear spin configurations in the exploration of altermagnetism is puzzling, especially when the symmetry- centered phenomenon is more likely to be realized in the latter case. We argue that AFM compounds with non-collinear spins can also exhibit altermagnetic state. More specifically, AFM compound with chiral crystal structure, or enantiomorphic systems, provides a novel untapped platform to realize altermagnetic ground state. Besides parity violation, chiral AFM can host chiral magnons that are more likely to exhibit spin band splitting. A chiral AFM compound can have chirality entrenched in the two-dimensional configuration, as in the case of Mn3Sn, or it can manifest a helical order, for example NiI2. Hexagonal NiS is another possible candidate to host chiral state. All three compounds, NiSi, NiS and NiI2 violate PT-symmetry. NiSi is also found to exhibit anomalous Hall effect, which cannot be accounted by the spin chirality of non-collinear Ni spin configuration. We argue that these compounds are potential candidate for altermagnetism. Detailed research works can reveal the altermagnetic spin bands splitting in these compounds.

P3. Alter-fast Dynamics: Optical Studies of Engineered Altermagnetism

Eunice Paik

Army Research Laboratory

 We propose a study to investigate magneto-optical interactions in novel altermagnetic materials, develop tools to interrogate the newfound material class at ultrafast timescales, and realize dynamically controllable spin-polarized states. Altermagnets exhibit ferromagnetic properties such as anomalous Hall effect, spin-polarized current, and the magneto-optical Kerr effect, while also possessing fast, terahertz-range, spin excitation resonance frequencies, typically characteristic of antiferromagnets. This presents unique opportunities for ultrafast magneto- optoelectronic control of spin and angular momentum at timescales and strengths that are inaccessible to traditional magnets, pushing spintronics through a technology-limiting frequency barrier and allowing for devices with novel detection modes and readouts. Motivated by this newfound class of magnetic materials and breakthroughs in growth techniques, we aim to identify optical signatures of altermagnetic materials using ultrafast pump-probe techniques, study the spin dynamics and spin excitations using time-domain optical spectroscopy, and engineer altermagnetism using stacked structures and study their optical properties. These techniques serve as critical tools for engineering and discovering new altermagnets that can overcome barriers associated with switching speeds and exchange interactions of traditional magnets, paving the way ultrafast magnetic memory, multiferroics, photomagnetism and neuromorphics.

P4. Raman Scattering Spectroscopy in the intercalated transition metal dichalcogenide VNb3S6

Shreenanda Ghosh

Johns Hopkins University

The recently highlighted class of materials XNb3S6 (X= Cr, M, Fe, Co, Ni, V) is fascinating from a topological perspective since it breaks both time reversal and inversion symmetry and can show entirely different behavior compared to the pristine TMDCs [1, 2]. In this work, we study the vanadium intercalated layered TMDC VNb3S6, a topological semimetal with intriguing features and a candidate material for altermagnets.

We report polarization resolved magnetic and electronic Raman scattering in VNb3S6. Direct observation of a magnetic excitation near 120 cm−1 is reported in the magnetically ordered phase, originating from two-magnon scattering of S = 1 localized moment at the zone center [3]. It is successfully modeled in terms of light scattering from two-magnon excitations within the framework of the Fleury-Loudon theory using spin wave exchange parameters derived from neutron scattering data. These results provide an independent spectroscopic confirmation of the antiferromagnetic state in this material, in agreement with the recent neutron scattering and diffraction measurements [4,5]. Moreover, the electronic Raman scattering spectrum reveals signature of interband electron-hole excitations across the band crossing reported in this Weyl semi metallic system. This work demonstrates Raman scattering as a promising tool to investigate electronic properties of semimetals and other 2D TMDs.

[1]  L. Šmejkal et al. Phys. Rev. X 12, 040501 (2022).

[2]  Z. Hawkhead et al. Phys. Rev. Mat. 7, 114002 (2023).

[3]  S. Ghosh, C. Lygouras, M. Fu, Z. Feng, S. Nakatsuji, N. Drichko ‘Raman Scattering Spectroscopy in the intercalated transition metal dichalcogenide VNb3S6’, in preparation.

[4]  K. Lu et al, Phys. Rev. Mat 4, 054416 (2020).

[5]  A.E. Hall et al. Phys. Rev. B 103, 174431 (2021).

P5.  Electrical 180o switching of Néel vector and anomalous Nernst effect in altermagnet

Lei Han1, Xizhi Fu2, Junwei Liu2, Cheng Song1

1 School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

2 Department of Physics, The Hong Kong University of Science and Technology,

Hong Kong 999077, China

The emerging concept of "Altermagnetism" has attracted considerable attention. The electrical 180o switching of Néel vector is the prerequisite for developing electrical-controllable altermagnetic memory with opposite Néel vectors as binary "0" and "1". However, the state-of-art switching mechanisms have long been limited for 90o or 120o switching of Néel vector [1-3], which unavoidably require multiple writing channels that contradicts ultra-dense integration. Here, we propose a deterministic switching mechanism based on spin-orbit torque with asymmetric energy barrier, and experimentally achieve electrical 180o switching of an altermagnet candidate, Mn5Si3 thin film [4]. The asymmetric energy barrier is built up by applying an assistant field that parallels to the writing current, which adds additional Zeeman energy due to the presence of tiny net moment. Epitaxial strain in the thin film breaks symmetries, and allows an anomalous Hall effect for the readout of the 180o switching. Notably, we have carried out extensive control experiments to show that, the anomalous Hall responses is determined by the Néel vector-dependent Berry curvatures from the intersections of alternating spin-splitting bands, instead of net moment. The anomalous Nernst effect has also been observed in Mn5Si3 thin films, which undergoes a sixfold enhancement via doping, most likely due to the raised Fermi level [5]. Besides fundamental advance, our work promotes practical functional devices based on altermagnets.

[1] P. Wadley et al. Science 351, 587-590 (2016).

[2] X. Z. Chen et al. Phys. Rev. Lett. 120, 207204 (2018).

[3] Y. Cheng et al. Phys. Rev. Lett. 124, 027202 (2020).

[4] L. Han et al. Sci. Adv. 10, eadn0479 (2024).

[5] L. Han et al. arXiv:2403.13427 (2024).

P6. Materials with Broken Mirror Symmetries: Physical properties and altermagnetism

Junjie Yang

Department of Physics, New Jersey Institute of Technology

Material properties are governed by fundamental symmetries, such as space inversion, mirror reflection, and time-reversal symmetries. For example, breaking space inversion symmetry can induce ferroelectricity, while breaking time-reversal symmetry can result in ferromagnetism. Both ferroelectric and ferromagnetic materials have played crucial roles in modern technologies and industries. Breaking mirror symmetry can also give rise to various exotic phenomena in materials, especially when materials are subjected to external electric or magnetic fields or interact with spin ordering. These phenomena include lattice and magnetic chirality, various electric field tunable properties, non-reciprocal spin waves, ferro-rotational order, and magnetic order-induced ferro- rotational order. In this talk, we will discuss emergent material functionalities in several typical materials with broken mirror symmetries, such as RbFe(SO₄)₂, Ni₃TeO₆, and M₁/₃TaS₂. Specifically, we will discuss their electric transport properties (e.g., magnetoresistance and Hall effect), lattice structures, and magnetic structures (e.g., magnetic chirality), and we will discuss the possible altermagnetism in these materials.

P7. Imaging Atermagnetic domains and their dynamics.

Valery Kiryukhin

Rutgers University, USA

Antiferromagnetic domains are notoriously difficult to measure. Many basic properties of altermagnets are domain dependent. Imaging domain texture and the dynamics of the domain walls is therefore essential for understanding the intrinsic properties of altermagnets, as well as for engineering any devices based on them. In this talk, we review the existing AFM domain imaging techniques, and present a novel technique based on Bragg scattering of coherent x-rays developed in our group. It is capable of real-time in-situ measurements, which is especially useful for studies of prototype devices, as well as for fundamental work on nonlocal effects in quantum materials. We present topological domain textures in altermagnets and show real-space videos of thermally fluctuating domain walls.

P8. Ferroaxial phonons in chiral and polar NiCo2TeO6

V. A. Martinez

Department of Physics, New Jersey Institute of Technology, Newark, New Jersey 07102, USA

Perfect circular dichroism has been observed in the Raman scattering by the optical phonons in single chiral domain NiCo2TeO6 crystals. The selection rules for the optical phonons are determined by the combination of the chiral structure C and the electric polarization P along the c-axis. These two symmetry operations are equivalent to the ferroaxial order (C*P)=A, so the observed optical phonons are referred to as “ferroaxial”. For a given Raman scattering geometry the observed effect may also be described as a complete non-reciprocal propagation of the optical phonons, whose preferable q -vector direction is determined by the sign of C*P. The combination

of Raman scattering and polarization plane rotation of the transmitted white light allows for identification of the direction of electric polarization P in mono domain chiral crystals.

Acknowledgements: Done in collaboration with Y. Gao,J. Yang, F. Lyzwa, Z. Liu, C. J. Won, K. Du, V. Kiryukhin, S-W. Cheong, and A. A. Sirenko. The NSF MPS-ASCEND Award #2316535 supported the Raman scattering experiments and data analysis by V.A.M.

P9. Unconventional insulator-metal transition in Mn3S2Te6 

Jan Musfeldt

University of Tennessee

Altermagnetism manifests in many channels of materials. In this talk, I will discuss properties of Mn3S2Te6 . The nodal-line semiconductor Mn3Si2Te6 is generating enormous excitement due to the recent discovery of a field-driven insulator-to-metal transition and associated colossal magnetoresistance as well as evidence for a new type of quantum state involving chiral orbital currents. Strikingly, these qualities persist even in the absence of traditional Jahn-Teller distortions and double-exchange mechanisms, raising questions about exactly how and why magnetoresistance occurs along with conjecture as to the likely signatures of loop currents. Here, we measured the infrared response of Mn3Si2Te6 across the magnetic ordering and field-induced insulator-to-metal transitions in order to explore colossal magnetoresistance in the absence of Jahn-Teller and double-exchange interactions. Rather than a traditional metal with screened phonons, the field-driven insulator-to-metal transition leads to a weakly metallic state with localized carriers. Our spectral data are fit by a percolation model, providing evidence for electronic inhomogeneity and phase separation. Modeling also reveals a frequency-dependent threshold field for carriers contributing to colossal magnetoresistance which we discuss in terms of polaron formation, chiral orbital currents, and short-range spin fluctuations.

P10.  Symmetry Tuning of Altermagnets: Exploring Spin and Structural Interplay Using In-Situ Uniaxial Strain with X-ray Scattering

Philip J. Ryan Magnetic Materials Group

Argonne National Laboratory.

Symmetry-driven behavior in altermagnets arises from the interplay between spin, lattice and orbital degrees of freedom. Such symmetry enforces alternating spin polarization in momentum space, even in the absence of a net magnetic moment. Such properties are observed in materials where specific geometric or magnetic arrangements, such as collinear antiferromagnetic order, create a natural symmetry-breaking effect. This leads to unique electronic and spin transport properties that are directly tied to crystal symmetry.

Recent advances in experimental techniques provide an unprecedented opportunity to investigate altermagnetism (1-4). This presentation will describe the role of X-ray-based techniques combined with in-situ uniaxial strain to probe and control altermagnetic materials. High-resolution X-ray diffraction (HRXRD) will be employed to map structural distortions and strain-induced symmetry breaking. X-ray magnetic circular dichroism (XMCD) and X-ray magnetic linear dichroism (XMLD) will be utilized to elucidate element-specific magnetic order and anisotropies, respectively, providing critical insights into the interplay between electronic structure and magnetic properties. Additionally, X-ray resonant magnetic scattering (XRMS) will enable direct imaging of spin textures and spatially resolved magnetic domain structures.

In-situ uniaxial strain will serve as a tunable parameter, allowing precise manipulation of the crystal lattice to study its influence on magnetic ordering and the emergence of altermagnetic behavior. By combining these approaches, the proposed experiments aim to reveal the fundamental mechanisms driving altermagnetism and its potential for technological applications. The integration of strain engineering with advanced X-ray techniques offers a transformative pathway to explore the dynamic coupling between lattice, electronic, and magnetic degrees of freedom, ultimately paving the way for the control and utilization of altermagnetic characteristics in quantum and spintronic devices.

1.      Strain-Switchable Field-Induced Superconductivity, Joshua J. Sanchez, Gilberto Fabbris, Yongseong Choi, Jonathan M. DeStefano, Elliott Rosenberg, Yue Shi, Paul Malinowski, Yina Huang, Igor I. Mazin, Jong-Woo Kim, Jiun- Haw Chu, Philip J. Ryan*, Science Advances 9, eadj5200(2023). DOI:10.1126/sciadv.adj5200

2.      Suppression of superconductivity by anisotropic strain near a nematic quantum critical point, P. Malinowski, Q Jiang, JJ Sanchez, J Mutch, Z Liu, P Went, J Liu, P.J. Ryan, J-W. Kim, Jiun-Haw Chu, Nature Physics, 1-5, (2020)

3.      Joshua J. Sanchez, Paul Malinowski, Joshua Mutch, Jian Liu, J.-W. Kim, Philip J. Ryan, Jiun-Haw Chu, The transport–structural correspondence across the nematic phase transition probed by elasto X-ray diffraction. Nat. Mater. (2021).

4.      "Spontaneous orbital polarization in the nematic phase of FeSe." Connor A. Occhialini, Joshua J. Sanchez, Qian Song, Gilberto Fabbris, Yongseong Choi, Jong-Woo Kim, Philip J. Ryan and Riccardo Comin, Nature Materials 22, 985 (2023). doi:10.1038/s41563-023-01585-2

P11.  Neutron Investigation of RuO2 Thin Films

Shelby Fields

ASEE Postdoctoral Fellow at the U.S. Naval Research Laboratory Washington D.C., USA

Since initial prediction of rutile RuO2 as a strong altermagnet, more recent experimental measurements and theoretical calculations have generated controversy by reaffirming that in ground-state, this structure is a Pauli paramagnet, accordingly not an antiferromagnet, and therefore not an altermagnet. However, several experimental observations, including the presence of spin splitter torque, THz emission, and nonlinear hall effect resistance support that antiferromagnetic ordering may still occur within thin RuO2 thin films, alluding to extrinsic contributions to this state. Neutron diffraction measurements, which are sensitive to magnetic ordering, are in general challenged to collect data on thin films due to low interaction volumes. Therefore, extremely thick films are required to characterize a potential magnetic ordering that may occur in RuO2 thin films due to extrinsic factors such as strain or point defects. Within this talk, a high-temperature reactive sputtering process is described, which is capable of growing high-quality heteroepitaxial RuO2 with several orientations on lattice-matched TiO2 substrates. This process is employed to prepare thick RuO2 films on double-sided TiO2 crystals with volume sufficient to collect neutron diffraction patterns. Such samples are subsequently interrogated at BL9-CORELLI (at the Spallation Neutron Source) and HB1- PTAX (at the High Flux Isotope Reactor) at Oak Ridge National Laboratory to examine if magnetic ordering occurs in RuO2 thin films.

[1] Shelby Fields et. Al. “Orientation Control and Mosaicity in Heteroepitaxial RuO2 Thin Films Grown Through Direct Current Sputtering”, Crystal Growth and Design, Vol 24/issue 11 (2024)

P12. Magnetic imaging of domains and domain walls in antiferromagnets

Weida Wu

Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA

Layered antiferromagnets with topological band structure are promising platforms to host many interesting topological phenomena such as quantum anomalous Hall effect and axion insulator state [1,2]. Thus, it is imperative to visualize and to control domains or domain walls in these topological antiferromagnets. Despite many decades’ efforts, it remains a grand challenge to visualize antiferromagnetic domains or domain walls in magnetic field [3]. In this talk I will present our recent efforts on magnetic imaging of domains and domain walls in antiferromagnetic topological insulator MnBi2Te4 family using cryogenic magnetic force microscopy (MFM) [4–6]. Our MFM studies reveal enhanced magnetic susceptibility inside the domain walls due to the winding of the antiferromagnetic order parameter. Furthremore, our MFM data confirms that the A-type antiferromagnetic order persists to the surface of MnBi2Te4 and MnBi4Te7 [4,5]. The robust A-type order is further corroborated by the surprising discoveries of the long-sought surface metamagnetic transitions [10–12]. Our results pave the way for exploration of emergent phenomena such as domain wall magnetism or surface transitions in antiferromagnets including altermagnets.

[1]         Y. Deng, Y. Yu, M. Z. Shi, J. Wang, X. H. Chen, and Y. Zhang, Science (80-. ). 367, 895 (2020).

  [2]          C. Liu, Y. Y. Wang, H. Li, Y. Wu, Y. Li, J. Li, K. He, Y. Xu, J. Zhang, and Y. Y. Wang, Nat. Mater. 19, 522 (2020).

[3]          S. W. Cheong, M. Fiebig, W. Wu, L. Chapon, and V. Kiryukhin, Npj Quantum Mater. 5, 1 (2020).

[4]          W. Ge, J. Kim, Y. Chan, D. Vanderbilt, J. Yan, and W. Wu, Phys. Rev. Lett.

129, 107204 (2022).

[5]          P. M. Sass, J. Kim, D. Vanderbilt, J. Yan, and W. Wu, Phys. Rev. Lett. 125, 037201 (2020).

[6]          P. M. Sass, W. Ge, J. Yan, D. Obeysekera, J. J. Yang, and W. Wu, Nano Lett.

20, 2609 (2020).

[7]   H. Li, S.-Y. Gao, S.-F. Duan, Y.-F. Xu, K.-J. Zhu, S.-J. Tian, W.-H. Fan, Z.-C. Rao, J.-R. Huang, J.-J. Li, Z.-T. Liu, W.-L. Liu, Y.-B. Huang, Y.-L. Li, Y. Liu,

G.-B. Zhang, H.-C. Lei, Y.-G. Shi, W.-T. Zhang, H.-M. Weng, T. Qian, and H. Ding, Phys. Rev. X 9, 041039 (2019).

[8]     Y. J. Chen, L. X. Xu, J. H. Li, Y. W. Li, C. F. Zhang, H. Li, Y. Wu, A. J. Liang, C. Chen, S. W. Jung, C. Cacho, H. Y. Wang, Y. H. Mao, S. Liu, M. X. Wang,

Y. F. Guo, Y. Xu, Z. K. Liu, L. X. Yang, and Y. L. Chen, Phys. Rev. X 9, 041040 (2019).

[9]     Y.-J. Hao, P. Liu, Y. Feng, X.-M. Ma, E. F. Schwier, M. Arita, S. Kumar, C. Hu, R. Lu, M. Zeng, Y. Wang, Z. Hao, H. Sun, K. Zhang, J. Mei, N. Ni, L. Wu, K. Shimada, C. Chen, Q. Liu, and C. Liu, Phys. Rev. X 9, 041038 (2019).

[10]   D. L. Mills, Phys. Rev. Lett. 20, 18 (1968).

[11]    R. W. Wang, D. L. Mills, E. E. Fullerton, J. E. Mattson, and S. D. Bader, Phys. Rev. Lett. 72, 920 (1994).

[12]    U. K. Rößler and A. N. Bogdanov, Phys. Stat. Sol. 1, 3297 (2004).

P13. High Pressure Synthesis of Novel Altermagnetic Candidates

Weiwei Xie

Department of Chemistry, Michigan State University

Our research focuses on translating the physical principles of altermagnetism into chemical frameworks to guide the synthesis of new altermagnetic materials. For some theoretically predicted altermagnetic candidates, traditional solid-state synthesis methods have proven insufficient to stabilize the desired phases. To overcome these limitations, we employed high-pressure, high-temperature techniques to successfully grow these materials.

For instance, CoSb and Cr-doped FeSb2 are predicted to exhibit altermagnetic behavior. However, the optimal doping ratios for Cr in FeSb2 were found to depend heavily on the synthesis conditions. Additionally, we synthesized single crystals of a novel phase, MnSb2, using high-pressure and high-temperature methods. MnSb2 crystallizes in the same structure as FeSb2 and has an identical valence electron count to Cr0.5Fe0.5Sb2, while exhibiting zero net magnetic moment. These properties suggest MnSb2 as a promising new candidate for altermagnetism.

This work was conducted in close collaboration with Haidong Zhou, Jian Liu, and Yang Zhang at the University of Tennessee, Knoxville, and Jiun-Haw Chu at UW.

P14. Origin of large effective phonon magnetic moments in monolayer MoS2

Wencan Jin

Auburn University

Recent helicity-resolved magneto-Raman spectroscopy measurement demonstrates giant effective phonon magnetic moments of ∼2.5 μB in monolayer MoS2, highlighting resonant excitation of bright excitons as a new route to activate Γ-point chiral phonons in transition metal dichalcogenides. However, a microscopic picture of this intriguing phenomenon remains lacking. In this work, we show that an orbital transition between the split conduction bands (∆0 = 4 meV) of MoS2 couples to the doubly degenerate E′′ phonon mode (Ω0 = 33 meV), forming two hybridized states. Our phononic and electronic Raman scattering measurements capture these two states: (i) one with predominantly phonon contribution in the helicity-switched channels, and (ii) one with primarily orbital contribution in the helicity-conserved channels. An orbital-phonon coupling model successfully reproduces the large effective magnetic moments of the chiral phonons and explains their thermodynamic properties. Moreover, the Raman mode from orbital transition is superimposed on a strong quasi-elastic scattering background, indicating the presence of spin fluctuations. As a result, the electrons excited to the conduction bands through the exciton exhibit paramagnetic behavior although MoS2 is generally considered as a nonmagnetic material. By depositing nanometer-thickness of nickel thin films on monolayer MoS2, we tune the electronic structure so that the A exciton perfectly overlaps with the 633 nm laser. The optimization of resonance excitation leads to pronounced tunability of the orbital-phonon hybridized states. Our results generalize the orbital- phonon coupling model of effective magnetic moments to a new material system beyond the paramagnets and magnets.

P15.  Emerging magnetism in candidate altermagnet Fe0.75Cr0.25Sb2

Xiao Hu

The relationship between itinerant electrons and antiferromagnetism remains controversial. The system Fe1−xCrxSb2 presents an interesting opportunity for understanding the relevant fundamental features. The x = 0 parent material is diamagnetic, with temperature-induced paramagnetism [1,2], while antiferromag- netic order at temperatures below 275 K is found at x = 1 [2]. Bulk measurements show that magnetism emerges at x ≳ 0.2−0.25 [3]. It has recently been theoreti- cally predicted that for doping near x = 0.25, the Fe1−xCrxSb2 marcasite structure, among other non-Bravais lattices with low crystal unit cell symmetry hosts alter- magnetic state [4]. Motivated by these predictions, we have carried out inelastic neutron scattering measurements of magnetic excitations in Fe0.75Cr0.25Sb2. In our experiments, we observed a weak, diffuse inelastic signal characteristic of a nearly critical magnetic state. The observed excitation reveals dispersion which at low energy looks centered around zero wave vector and is therefore consistent with the expectations for altermagnetism.

[1]   R. Hu, V. F. Mitrovic, C. Petrovic. Phys. Rev. B 76, 115105 (2007).

[2]   I. A. Zaliznyak et al., Phys. Rev. B 83, 184414 (2011).

[3]   M. B. Stone et al., Phys. Rev. Lett. 108, 167202 (2012).

[4]   I. I. Mazin et al. PNAS 118(42), e2108924118 (2021).

P16.  Symmetries in Quantum Materials – Antiferromagnetism and Beyond

Xianghan Xu

University of Minnesota

In quantum materials, symmetries play a fundamental role in governing the interplay between charge, spin, orbital, and lattice degrees of freedom, giving rise to complex ordering phenomena. In this talk, I will explore how antiferromagnetism, combined with specific lattice symmetries, leads to intriguing phenomena such as multiferroicity and altermagnetism. I will highlight two exemplary materials: CoTe6O13, which preserves PT symmetry, and Ca2Fe2O5, where PT symmetry is broken. Their crystal growth approaches and unconventional magnetic properties will be discussed in detail. These findings offer valuable insights into the design of materials with tailored functionalities and emergent quantum phases.

P17.   Antiferromagnetic driven odd-parity magnetism

Yue Yu

U. of Wisconsin-Milwaukee

Realizing odd-parity, time-reversal-preserving, non-relativistic spin splitting is a goal for spintronics applications. We propose a group-theory-based microscopic framework to induce odd-parity spin splitting from coplanar antiferromagnetic (AFM) states. We develop phenomenological models for 470 AFM systems in non-symmorphic space groups and construct minimal microscopic models for 119 of these, focusing on nonmagnetic Wyckoff positions with multiplicity of two. Our analytical analysis of transport properties reveals nonlinear transport coefficients that are linear in spin-orbit coupling (SOC) and Edelstein coefficients that are independent of SOC. Furthermore, we identify a competing class of collinear odd-parity AFM states that, while lacking spin splitting, exhibit substantial SOC-independent nonlinear transport coefficients. We further discuss results on p-wave magnetism in CeNiAsO and possible $h$-wave magnetism in FeSe-based materials.

P18.  Double-Q chiral stripe order in the anomalous Hall antiferromagnet CoNb3S6

Ben Zager

Brookhaven National Laboratory 

CoNb3S6 is an intercalated transition metal dichalcogenide exhibiting a giant anomalous Hall eEect that cannot be explained by its putative collinear antiferromagnetic order. Possible explanations include a real-space Berry curvature provided by a triple-Q noncoplanar order, the interplay of band topology with a weak out of plane ferromagnetic moment, or crystal chirality eEects. Using resonant elastic x-ray scattering, we have discovered a long-wavelength modulation of the commensurate magnetic structure. This noncoplanar double-Q structure consists of a noncollinear commensurate component and an incommensurate helical component which gives rise to a novel striped pattern in the scalar spin chirality. The incommensurate modulation is naturally explained by the presence of four-spin exchange interactions and breaks the symmetries required for the anomalous Hall eEect in the presence of spin-orbit coupling. These results not only uncover a novel chiral spin structure, but also provide insight into the elusive mechanism of the giant anomalous Hall Efect in CoNb3S6, thus highlighting potential routes for further study of unconventional electronic phenomena in metallic antiferromagnets.

P19. Electrical Switching of an unconventional odd-parity magnet

Qian Song, MIT, USA

Altermagnets with zero magnetization but non-relativistic spin splitting are outstanding candidates for the next generation of spintronic devices. Electrical control and readout of the spin polarization in altermagnets is of great interest for realizing ultrafast, energy-efficient and nanoscale compact devices for information storage and processing. Spin spiral (type-II) multiferroics are the ideal candidates on this front, as the non-relativistic odd-parity spin polarization strongly couples to the spin-induced ferroelectric polarization. Van der Waals material NiI2 exhibits such helical magnetic order below 60 K. We combine photocurrent measurements, first-principle calculation, magnetic and spin group symmetry analysis to provide direct evidence of voltage-based switching of spin polarization with predominantly non-relativistic origin.

P20. Optical Studies of RuO2 Thin Films

Benjamin Mead, Luka Mitrovic, Kyle Shen, Darrel Schlom, Liang Wu

UPenn, USA

Altermagnitism is a newly studied class of fully compensated collinear magnetic order where opposite spin sublattices are related by rotation instead of translation which appears in momentum space as non-trivial spin splitting. Rutile RuO2 is proposed as a textbook room temperature altermagnet because of the large spin splitting in the band structure. However, recent contradicting results on bulk crystals and epitaxial thin films suggest magnetic ordering isn’t intrinsic to RuO2. Instead, it’s proposed magnetic ordering is correlated with other degrees of freedom including strain and sample thickness. Here I’ll present our optical magnetic circular dichroism and second harmonic generation measurements on RuO2 thin films to study their magnetic and crystal structure. Our results suggest the properties of RuO2 thin films aren’t intricately linked to sample thickness further highlighting the need to establish consistency of RuO2 thin films.

Street view of the Workshop location.  The Workshop will take place behind the glass wall at the street level in Agile Lab L70, just next to the entrance. 

Street access for Taxi and Navigation:

355 Martin Luther King Jr. Blvd, Newark, NJ 07102 

PARKING

Personal or rental car parking is free upon filling out the online parking request form sent by Andrei Sirenko to registered participants. Entrance to the garage is permitted only during designated times and exit permitted at any time. Overnight parking permitted. For navigation: 

154  Summit Street, Newark, NJ 07102    

Walking time to the  Workshop is 2 min

Just in case, an ATM is located at the street level of the Parking Garage Building. More ATMs are in the Campus Center.

WiFi Internet access:

There are two wireless networks in the Workshop area:

1. NJITguest - OPEN network, no password, but a sign-on  is required meaning you will need to receive a text or email message sent for identification. The procedure is similar to that for free WiFis at the big International Airports. This network is OK for text messaging, but is sometimes too slow for video. 

https://ist.njit.edu/connecting-njitguest

2.               eduroam  -  the same WiFi as at the most Universities in USA and EU.  An account is needed, so if you are coming from a University, its a good idea to activate your account in advance.  

Preparation of a Backup copy of the talks/posters on a memory stick is strongly advised.

List of Hotels in Downtown Newark:

Robert Treat Hotel

Courtyard by Marriott Newark Downtown

TRYP by Wyndham Newark Downtown

DoubleTree by Hilton Newark Penn Station

List of Restaurants in Downtown Newark:

Fornos of Spain     

https://fornosofspain.com/ 
 

Free valet parking at the restaurant  ~$80/person  without drinks

Casa Vasca           
https://www.casavasca.net/
Limited free parking at the restaurant

~$50/person without drinks

Tony Da Caneca    
https://tonydacanecarestaurant.com/ 

~$60/person   without drinks

Spanish Tavern     
https://spanishtavernnewark.com/

~$60/person  without drinks

Fernandes 2  Steak House   
https://www.fernandessteakhouse.com/    Free parking at the restaurant, ~$60/person   without drinks

Sabor Unido   https://saborunido.com/  

~$40/person   without drinks

More info about local restaurants is here:

https://www.tripadvisor.com/Restaurants-g46671-zfn20483951-Newark_New_Jersey.html

Dinner will not be provided

by the workshop so we encourage small groups to arrange their own dinner

reservations. We can assist with coordinating dinner reservations for 4 - 6 people during the Workshop days  around lunch time. Participants with cars are encouraged to take a lead in making reservations and helping with transportation to the restaurants. Reservations can be made using restaurant websites and/or https://www.opentable.com/.  Usually restaurant bills can be split upon request between several guest's credit cards at the same table. Restaurants prefer "even splits" of the final bill. Preparing small bills in advance to settle the order differences is suggested. Cash always works. Also, "house wine" or/and "sangria pitchers", which cost much less compared to wine bottles, are available in all of the aforementioned restaurants. The main course dishes are usually large, so additional appetizers are not really necessary. Complimentary bread, olives, and even salads are provided for the table in most of the aforementioned restaurants.  

Participants are strongly encouraged to drive or use Uber / Lyft / Taxi between their Hotels, Restaurants  and the Workshop locations.

Dining options around NJIT Campus are limited due to the Winter Break.

Cultural events in the area:

MET Opera in NYC  

  https://www.metopera.org/calendar/#/on-stage?year=2025&month=0

Breakfast, Lunch, and 4pm Coffee breaks

Breakfast, lunch and coffee during breaks will be provided by the workshop. Below is the final MENU for the provided day-time meals/drinks.  

Additional healthy / vegetarian options are available for purchase in the "Farmer's Fridge" across the street:   350 Dr Martin Luther King Jr Blvd, Newark, NJ 07102   (open till 4 pm).

Lunch at the conclusion of the workshop on Friday Jan 10th at 12:30 pm will be offered only upon request using the google form