Research
Unequivocal Identification of Spin-Triplet and Spin-Singlet Superconductors with Upper Critical Field and Flux Quantization
C. C. Chiang, H. C. Lee, S. C. Lin, D. Qu, M.W. Chu, C. D. Chen, C. L. Chien*, and S. Y. Huang* , “Unequivocal Identification of Spin-Triplet and Spin-Singlet Superconductors with Upper Critical Field and Flux Quantization,” Phys. Rev. Lett. 131, 236003 (2023).
Superconductors (SCs) hold spin-singlet or spin-triplet pairings. The majority of known SCs are the spin-singlet. Spin-triplet SCs, essential for Majorana Fermions and fault-tolerant quantum computing, are very rare and difficult to identify. This study is the first unequivocal demonstration of identifying singlet and triplet SCs via two methods–kink-point in the upper critical field and half-quantum flux. Chiang et al. demonstrated that spin-triplet SCs can be clearly and reliably identified with these two methods, which is beneficial for exploring triplet SCs and developing quantum computation.
Anomalous Nernst Effect Induced Terahertz Emission in a Single Ferromagnetic Film
Zheng Feng,* Wei Tan, Zuanming Jin,* Yi-Jia Chen, Zhangfeng Zhong, Liang Zhang, Song Sun,Jin Tang, Yexing Jiang, Po-Hsun Wu, Jun Cheng, Bingfeng Miao,* Haifeng Ding, Dacheng Wang, Yiming Zhu, Liang Guo, Sunmi Shin, Guo-Hong Ma, Dazhi Hou, and Ssu-Yen Huang*”, Nano Letters, 23, 8171 (2023)..
By developing a bidirectional pump-THz emission spectroscopy and associated symmetry analysis method, we set a benchmark for the experimental distinction of the THz emission induced by various mechanisms. Our results unambiguously unveil a new mechanism anomalous Nernst effect (ANE) induced THz emission due to the ultrafast temperature gradient created by a femtosecond laser. Our work not only clarifies the origin of the ferromagnetic-based THz emission but also offers a fertile platform for investigating the ultrafast optomagnetism and THz spintronics.
Neuromorphic computing devices based on the asymmetric temperature gradient
3. H. J. Chen, C. C. Chiang, C. Y. Cheng, D. Qu, and S. Y. Huang, Appl. Phys. Lett. 122, 264102 (2023).
In this study, we introduce a multi-layer-multi-terminal neuromorphic computing device based on the asymmetric temperature gradient. Our device exhibits a wide range of synaptic functions, including potentiation, depression, and both anti-symmetric and symmetric spike-timing-dependent plasticity. The thermal driving strategy offers an energy-efficient platform for future neuromorphic computing devices to achieve artificial intelligence
Anisotropic spin-to-charge conversion in bismuth
Hsia-Ling Liang, T. C. Chuang, Danru Qu, Ming-Hao Lee, Y. S. Chen, Jauyn Grace Lin, Ming-Wen Chu, C. L. Chien, and Ssu-Yen Huang*, “Anisotropic spin-to-charge conversion in bismuth”, Phys. Rev. B 106, L201304 (2022) (Letter).
Bismuth (Bi) is an important and intriguing element for many quantum materials. In this work, we show that the hexagonal crystal structure of Bi profoundly modifies the spin-to-charge conversion. We reveal the highly anisotropic spin-to-charge conversion between Bi(003) and Bi(012). Our results address the experimental criteria for the spin conversion in Bi, which is essential for Bi-based topological quantum materials and spintronics.
Observation of Vector Spin Seebeck Effect in a Noncollinear Antiferromagnet
Jinsong Xu, Jiaming He, J.-S. Zhou, Danru Qu, Ssu-Yen Huang, and C. L. Chien, “Observation of Vector Spin Seebeck Effect in a Noncollinear Antiferromagnet ”, Phys. Rev. Lett. 129, 117202 (2022).
In this work, we report the observation of a vector SSE in a noncollinear antiferromagnet (AF) LuFeO3, where temperature gradient along the out-of-plane and also the in-plane directions can both inject a pure spin current and generate a voltage in the heavy metal via the inverse spin Hall effect. The noncollinear AFs expand new realms for exploring spin phenomena and provide a new route to low-field antiferromagnetic spin caloritronics and magnonics.
Exploiting Spin Fluctuations for Enhanced Pure Spin Current
Po-Hsun Wu, Danru Qu, Yen-Chang Tu, Yin-Ze Lin, C. L. Chien, and Ssu-Yen Huang, “Exploiting spin fluctuations for enhanced pure spin current”, Phys. Rev. Lett. 128, 227203 (2022).
It is widely accepted that spin Hall angle, which measures the prowess to generate pure spin current, is intrinsic to the material. We show that the value of spin Hall angle can be greatly enhanced beyond their intrinsic values by exploiting spin fluctuations in 3d magnets. In this work, we demonstrate the interplay of pure spin current, spin-polarized current, and spin fluctuation in 3d NixCu1−x. By tuning the compositions of the NixCu1−x alloys, we separate the effects due to the pure spin current and spin-polarized current. By exploiting the interaction of spin current with spin fluctuation in suitable Ni-Cu alloys, we obtain an unprecedentedly high spin Hall angle of 46%, about 5 times larger than that in Pt, at room temperature. Furthermore, we show that spin-dependent thermal transport via anomalous Nernst effect can serve as a sensitive magnetometer to electrically probe the magnetic phase transitions in thin films with in-plane anisotropy. The enhancement of spin Hall angle by exploiting spin current fluctuation via composition control makes 3d magnets functional materials in charge-to-spin conversion for spintronic application.
Detection and manipulation of the antiferromagnetic Néel vector in Cr2O3
Yi-Hui Zhang, Tsao-Chi Chuang, Danru Qu, and Ssu-Yen Huang, “Detection and manipulation of the antiferromagnetic Néel vector in Cr2O3”, Phys. Rev. B 105, 094442 (2022)
Detection and manipulation of the Néel vector in antiferromagnetic materials are promising for more stable, faster, and much higher-density spintronic devices. However, the electrical manipulation of the antiferromagnetic Néel vector remains challenging and controversial due to the difficulty in the detection of the zero net magnetization and the unavoidable complications from thermal artifacts. Previously, we showed that, contrary to many prior claims, there is no evidence of spin current-induced spin-orbit torque switching in antiferromagnetic insulator NiO [Phys. Rev. Lett., 123, 227203 (2019)]. However, in this work, by utilizing the uniaxial antiferromagnet (AFM) Cr2O3, we demonstrate the detection and manipulation of the antiferromagnetic Néel vector. We reveal unambiguously the spin-dependent electrical responses of the coherent Cr2O3 Néel vector switching, where a symmetric Hall signal and a fourfold angular-dependent magnetoresistance are captured. We also demonstrate the in-plane arbitrary manipulation of the Cr2O3 Néel vector when the magnetocrystalline anisotropy energy is compensated. Our work for detecting and manipulating Néel vectors offers a critical guide for antiferromagnetic-based Néel vector switching exploration.
Effect of demagnetization factors on spin current transport
Po-Hsun Wu, Ying-Ting Chan, Tzu-Chao Hung, Yi-Hui Zhang, Danru Qu, Tien-Ming Chuang, C. L. Chien, and Ssu-Yen Huang
Phys. Rev. B 102, 174426 – Published 16 November 2020
In this study, through changing the shape of YIG slab, we demonstrated the strong influence of the demagnetizing field on magnetic properties in YIG slab and various spin current transports in Pt/YIG, including the spin Seebeck effect, spin Hall effect, spin Hall magnetoresistance, and planar Hall resistance. The thickness- and width-dependent unusual plateau behaviors of the spin-dependent electrical and thermal transports as well as magneto-optical measurements were manifested from the shape of the YIG sample and closely related to the calculated effective demagnetizing factors. Furthermore, we directly visualized the evolution of the magnetic domains with an abrupt 90° magnetic rotation through magnetic force microscopy and we further corroborated it by the 90° resistance phase shift in the angular-dependent planar Hall effect. We provide sufficient evidences to demonstrate the spin current transport is not only sensitive to the surface magnetic structure but also reflects the shape of bulk for a magnetic material.
Demonstration of Spin Current Switch across Ferro-Antiferromagnetic Transition
Yi-Hui Zhang,Ting-Wei Weng,Tsao-Chi Chuang,Danru Qu,Ssu-Yen Huang – Published 02 September 2020
The first-order magnetic phase transition between antiferromagnetic (AFM) and ferromagnetic (FM) states around room temperature is one of the unique features in FeRh. More fascinating is that the phase-transition temperature can be readily modulated by the magnetic field. In this work, we demonstrate the spin current valve by tuning temperature or magnetic field across the first-order phase transition. The spin-polarized current driven by the anomalous Nernst effect (ANE) can be readily excited in the FeRh FM state, but completely suppressed in the AFM state. As a result, the colossal magneto-spin polarized voltage in FeRh has been demonstrated, where the ratio of the thermally excited magnonic associated with the spin conductivity between the FM and AFM states is near infinity during the first-order phase transition. Furthermore, in isothermal conditions, the spin conducting and non-conducting states can be switched by magnetic field, and the critical magnetic field for the on/off switch can be significantly decreased with increasing temperature. With virtually infinite spin current on/off ratio and tunable critical field and temperature, FeRh is significant for spintronic devices.
Magnetization-dependent spin Hall effect in a perpendicular magnetized film
T. C. Chuang, D. Qu, S. Y. Huang, and S. F. Lee
Phys. Rev. Research 2, 032053(R) – Published 31 August 2020
In the spin Hall effect (SHE), a longitudinal charge-current induced transverse pure spin current has been extensively studied for the application of magnetization switching by spin-orbit torque. However, due to the strict right-hand rule among charge current, spin current, and spin polarization, this highly attractive switching scheme is not efficient and often accompanied by an unfavorable external magnetic field in a heterostructure with perpendicular magnetization. In this work, we lift the restriction by demonstrating the magnetization-dependent spin Hall effect (MDSHE) in a heterostructure of ferrimagnetic insulator YIG/perpendicular magnetized trilayer (PML) Pt(2)/Co(0.5)/Pt(2). Unlike the conventional SHE with the locked orientation of spin polarization, the spin current induced by the MDSHE can be arbitrarily and independently controlled by the magnetization direction of the PML. We reported about 3.6% of injected spins experience the effective spin rotation in PML. Our approach provides an explicit route in exploring the spin-to-charge conversion with controllable orientations of spin polarization, which offers significant advantages in developing energy-efficient spintronic devices.
Probing the spin-glass freezing transition in Cu1−xMnx alloy by spin current
Po-Hsun Wu, Yen-Chang Tu, Danru Qu, Hsia-Ling Liang, Shang-Fan Lee, and Ssu-Yen Huang
Phys. Rev. B 101, 104413 – Published 12 March 2020
In this study, we used the thermally driven spin current to investigate the spin frustrations and spin fluctuations in spin-glass (SG) Cu1−xMnx alloys. Tuning the Cu1−xMnx composition results in a transition of the alloys from the SG state to the antiferromagnetic state; these states have different spin-freezing temperatures (Tf). For each alloy composition, we obtained a temperature-dependent inverse spin Hall voltage with a peak at Tp. We demonstrated that Tp has a strong correlation with Tf because of almost identical composition dependence. Crucially, we provided direct evidence that the strongest spin fluctuation occurs at a temperature considerably higher than the magnetic critical temperature, which could be attributed to the long and complex spin-freezing process. The proposed approach utilizing the spin current is not only a promising alternative in studying the spin-freezing transition for SG but also may enable energy-efficient spintronic applications using SG materials.
This work is published in Physical Review B (101, 104413 (2020)).
Cr-induced Perpendicular Magnetic Anisotropy and Field-Free Spin-Orbit-Torque Switching
T.C. Chuang, C.F. Pai, and S.Y. Huang
Phys. Rev. Applied 11, 061005 – Published 20 June 2019
Current-induced spin-orbit torque (SOT) switching in a heterostructure with perpendicular magnetic anisotropy (PMA) has attracted great attention as a new writing method for spintronic devices. However, this highly attractive switching scheme is often accompanied by an unfavorable external magnetic field. In this work, we demonstrate polarity-controlled field-free SOT switching in 3d Cr. Moreover, we find that Cr metal can induce strong interfacial PMA, without either heavy metal or MgO layer. Most importantly, field-free SOT switching could be achieved without introducing asymmetrical geometrical pattern, heavy metal, additional ferromagnetic or antiferromagnetic layers. We show that the underlying cause for the deterministic field-free switching lies in the slanted columnar microstructure, whose tilting angle is only around 5°, for the otherwise uniform thin films. The direction of oblique columnar structure dictates the up and down orientations of the PMA layer. Our results uncover the significant role of 3d materials and shed light on field-free SOT magnetization switching.
Absence of Evidence of Electrical Switching of the Antiferromagnetic Néel Vector
C. C. Chiang, S. Y. Huang, D. Qu, P. H. Wu, and C. L. Chien
Phys. Rev. Lett. 123, 227203 – Published 27 November 2019
Antiferromagnet with zero net magnetization has several unique advantages, including ultrafast dynamics in the terahertz frequencies, robust against field perturbation, and generating no stray field. Recently, there have been numerous reports of electrical switching of antiferromagnetic (AFM) Néel vector via spin-orbit torque (SOT) attracting worldwide attention. By applying a write current in the AFM layer or the normal metal (NM)/AFM bilayer, in a patterned multiterminal structure, the measured resistance shows recurring signals due to supposedly electrical switching of the AFM Néel vector. However, the researchers including those at the Department of Physics (Professor Ssu-Yen Huang, Chih-Chieh Chiang) of the National Taiwan University and the Institute of Physics, Academia Sinica (Dr. Danru Qu) demonstrate that similar signals can be observed in such patterned structures, with and without the AFM layer. This widely held switching signal may not be conclusive evidence of SOT switching of AFM but the thermal artifacts of patterned metal structure on substrate. We show that under a large writing current density beyond the Ohmic regime, the multiterminal devices can generate unintended anisotropic thermal gradients and voltages. As a consequence, the strength of the signal is greatly affected by the thermal conductivity of the substrates. Our results seriously question the validity, and indeed the prospect, of SOT switching of AFM Néel vector. We indicate AFM switching requires unequivocal detection of the AFM Néel vector before and after the SOT switching.
This work is published in Physical Review Letter (123, 227203 (2019)) and is highlighted as an Editor’s suggestion and Featured in Physics.
Light-induced thermal spin current
Yi-Jia Chen and Ssu-Yen Huang
Phys. Rev. B 99, 094426 – Published 19 March 2019
The generation of magnonic spin current by the spin Seebeck effect (SSE) and that of spin-polarized current by the anomalous Nernst effect (ANE) are the most fascinating phenomena in the field of spin caloritronics. There are several methods to generate the temperature gradient (∇T), including Peltier heating, external heater, microwave heating, and light heating. Among them, light energy is one of the richest thermal energy harvesting sources. In this work, we systematically investigate the thermal spin current (JS) driven by light in the framework of the SSE and the ANE, respectively. Unlike JS induced by electric heating methods following ∇T, the JS can be enhanced, reduced, and even reversed via the SSE under certain light frequency and the thickness of magnetic layers. Most importantly, by flipping the direction of the incident light, we are able to qualitatively distinguish the interface and bulk contributions to the transverse spin accumulation for the first time. Interestingly, we find that the derived interfacial and bulk spin Seebeck coefficient is frequency independent. Thus, unlike the conventional electrical heating, light offers distinct heating mechanism to develop spintronic and spin caloritronic devices.
Robust spin current generated by the spin Seebeck effect
F. J. Chang, Jauyn Grace Lin, and S. Y. Huang, Robust spin current generated by the spin Seebeck effect, Phys. Rev. Materials. 1, 031401 (Rapid Communications) (2017), Editors’ suggestion
Spin pumping (SP) and the spin Seebeck effect (SSE), two of the most common methods for generating a pure spin current from ferromagnetic insulators, are considered to share similar physical mechanisms. However, a systematic study of the fundamental difference of their working principle is missing. In this work, we present experimental evidence of the contrast in a pure spin current generated by SP and SSE, based on results from yttrium iron garnet (YIG) with various crystalline properties. It is shown that while the SP-induced spin current could be two-orders-of-magnitude different between the polycrystalline and epitaxial films, the SSE excited spin current is surprisingly insensitive to the different crystal structures. Our results clearly distinguish the coherent mechanism of SP from the noncoherent mechanism of the SSE. Consequently, the robust SSE voltage against poor crystallinity proves that the SSE is a powerful tool to explore pure spin current physics, and suggests that polycrystalline YIG films are a promising candidate for spin caloritronic applications.
Enhancement of the anomalous Nernst effect in ferromagnetic thin films
T. C. Chuang, P. L. Su, P. H. Wu, and S. Y. Huang
Phys. Rev. B 96, 174406 – Published 7 November 2017
The anomalous Nernst effect (ANE), generating spin thermoelectric signal (E) through spin-orbit coupling, is an important mechanism to explore the interaction between charge, heat, and spin. In this work, we employ the longitudinal experimental setup with a uniform out-of-plane temperature gradient (∇ T) in various ferromagnetic materials (FMs), including Fe, Co, Ni, and Py (Ni80Fe20), with in-plane anisotropy to study the ANE. The magnitude and sign of the ANE exhibit nontrivial thickness dependent behaviors which do not simply follow the behaviors of the saturation magnetization (Ms) and resistivity (ρ). While the sign of the ANE of Fe is opposite to that of Co, Ni, and Py in thicker films, it can even be reversed via decreasing thickness. Most importantly, the anomalous Nernst angles (θANE), the conversion efficiency of the spin/charge signal, for these FMs can be significantly enhanced by up to one order of magnitude in ultrathin films. By systematically studying the thickness dependence of the electrical and thermal transport properties, we show that the enhanced ANE of FMs is dominated by spin-orbit coupling through the intrinsic and side-jump mechanisms in thin film.
