Controlled growths of polar multiferroic altermagnets

- Chemical vapour transport growth is optimized for polar altermagnets, Fe2Mo3O8 and NiZnMo3O8

- Finding the transport agent density as a primary factor

- Successful growth of large single crystals: i.e., 10 times larger than normal sizes

- Comprehensive characterizations using single-crystal XRD, Laue, magnetic susceptibility, Raman spectroscopy

- Enabling the detailed examinations of largely unexplored multiferroic altermagnets and allowing the manipulation of altermagnetic and multiferroic properties using nonmagnetic Zn ions in the honeycomb layer.

[14] H. Haruhiro, S. Choi* et al., Crystal Growth & Design 25, 4991 (2025).

Experimental finding on partial molecular orbital, a new type of correlated electrons, supported by DFT results 

- A new proposal for partial molecular orbital state [13], a new correlated electron state, by experiment and theory

- Optimized samples growths of new Ba4TaMn3O12 and improved quality of Ba4NbMn3O12 powder

- Magnetic susceptibility and heat capacity results propose an unusual partial molecular orbital state in the Mn-trimer, confirmed by DFT calculations

- Unusual dichotomy of localized and d-localized 3d electrons, potentially useful in developing novel electronic devices

[13] A. Ali, H.-S. Kim, S. Choi* et al., Physical Review Research 6, 013231 (2024).

Determination of a noncollinear magnetic order of a new magnetoelectric material and its anisotropy

- Via powder and single-crystal neutron diffraction, determination of an accurate noncollinear magnetic order of Ni2Mo3O8 [12] is made, whose anisotropy is confirmed by direction-dependent magnetic susceptibility. Based on the determined magnetic order, the local magnetoelectric tensor analysis predicts the unexplored in-plane polarization. Magnetic order indicates a possible mixture of the origin of the magnetoelectricity, the spin-current, and p-d hybridization mechanisms. 

- Accurate determination of the magnetic ground state of multiferroic Fe4Nb2O9 [15] in a very low symmetry via neutron diffraction and group theory, resolving the debate and explaining its unusual magnetoelectricity.

[12] P. Yadav, S. Choi* et al., Physical Review Research 5, 033099 (2023).

[15] R. Datta, S. Choi* et al., Physical Review B 112, 134439 (2025).

Revealing the p-d orbital hybridization on quantum phase transition in the frustrated cubic nickelate magnet

- Observation of a subtle change in the structure via the electronic insulator-metal transition in archetypal NiS2-xSex [10]

- Combined powder x-ray diffraction and single-crystal Raman scattering on a wide range of the Se doping at varying temperatures

- Confirmation of effect of p-d orbital coupling, verifying a rare case of a bandwidth-controlled electronic transition without a structural transition

[10] G. Han, S. Choi* et al., Physical Review B 98, 125114 (2018).

Determining a noncollinear magnetic order of the buckled honeycomb, explaining a new type of magnetoelectricity

- Discovery of the novel noncollinear magnetic order in a newly-interested magnetoelectric Co4Ta2O9 [9] in the new magnetic space group, explaining the main feature of unusual electric polarisation. It has a prospect for examining toroic moments and dichroism with the potential to develop multifunctional devices.

- The first observation of the P1 magnetic space group is made with nearly isotropic ferromagnetic moment at lower temperatures in high-quality Co4Ta2O9 [11]  single crystals, providing a tuning parameter to the coupled multiferroic state.

[9] S. Choi*, V. Kiryukhin  et al., Physical Review B 102, 214404 (2020) (Editors’ Suggestion): Highlighted by the ISIS Neutron and Muon Source.

[11] R. Goel, S. Choi* et al., Physical Review Research 7, 043196 (2025).

Testing the Kitaev interaction in the honeycomb Na2IrO3 magnet with neutrons and spin-wave calculations

- Direct observation of magnetic excitations from any iridates for the first time by inelastic neutron scattering on Na2IrO3 [1]

- Optimized experimental setup with the absorption simulation

- New insight on spin dynamics of iridates

- Parameterize exchanges by testing various proposed theoretical models

[1] S. Choi, R. Coldea et al., Physical Review Letter, 108, 127204 (2012). 

In-depth structural analysis of Kitaev materials

- Extensive structural analysis in prototypical Kitaev iridates

- Single-crystal x-ray diffraction with stacking faults analysis

- Na2IrO3 [1, 2], (Na,Li)2IrO3 [3], beta-Li2IrO3 [7], gamma-Li2IrO3 [4]

- Vital information to validate a strength of a Kitaev term in real materials that can give ideas to stabilize a Kitaev quantum spin liquid 

[2] V. M. Katukuri et al., New Journal of Physics, 16 013056 (2014).

[3] S. Manni, S. Choi et al., Physical Review B 89, 245113 (2014) (Editors’ Suggestion).

[4] K. A. Modic et al., Nature Communications 5, 4203 (2014).

Exploring spin dynamics and field-induced magnetic phase transition in the honeycomb Kitaev magnet alpha-Li2IrO3

- Observation of sharply dispersive low-energy spin dynamics on alpha-Li2IrO3 [6] with inelastic neutron scattering

- Goldstone and gapped excitations from a counterrotating noncoplanar magnetic order [5] by magnetic resonant x-ray diffraction and muon

- Gapped excitation confirmed by high-field magnetization, a novel field-induced magnetic phase transition

[5] S. C. Williams et al., Physical Review B 93, 195158 (2016).

[6] S. Choi, R. Coldea et al., Physical Review B 99, 054426 (2019). 

Probing a competition between intermetallic covalency and formation of moments in the hyperhoneycomb iridate

- A nontrivial counterrotating noncoplanar incommensurate magnetic order of beta-Li2IrO3, a rare three-dimensional Kitaev material, determined by neutron and magnetic resonant x-ray diffraction [7] at ambient pressure 

- Observation of a pressure-induced structural transition via high-pressure Raman [8] at room temperature

- The evidence of the competition between the covalency of the 5d orbitals and magnetism

- Implication as the strong connectivity of its electronic structure to an underlying crystal structure

[7] A. Biffin et al., Physical Review B 90, 205116 (2014) (Editors’ Suggestion).

[8] S. Choi* et al., Physical Review B 101, 054102 (2020).