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1. Spin-generation in magnetic Weyl semimetal Co2MnGa across varying degree of chemical order

   https://doi.org/10.1063/5.0102039  

   Safi, Taqiyyah S.; Chou, Chung-Tao; Hou, Justin T.; Han, Jiahao; Liu, Luqiao (August 2022, Applied Physics Letters)

   Recently discovered magnetic Weyl semimetals (MWSM), with enhanced Berry curvature stemming from the topology of their electronic band structure, have gained much interest for spintronics applications. In this category, Co2MnGa, a room temperature ferromagnetic Heusler alloy, has garnered special interest as a promising material for topologically driven spintronic applications. However, until now, the structural-order dependence of spin current generation efficiency through the spin Hall effect has not been fully explored in this material. In this paper, we study the evolution of magnetic and transport properties of Co2MnGa thin films from the chemically disordered B2 to ordered L21 phase. We also report on the change in spin generation efficiency across these different phases, using heterostructures of Co2MnGa and ferrimagnet CoxTb1−x with perpendicular magnetic anisotropy. We measured large spin Hall angles in both the B2 and L21 phases, and within our experimental limits, we did not observe the advantage brought by the MWSM ordering in generating a strong spin Hall angle over the disordered phases, which suggests more complicated mechanisms over the intrinsic, Weyl-band structure-determined spin Hall effect in these material stacks. 

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2. Anomalous Hall effect and perpendicular magnetic anisotropy in ultrathin ferrimagnetic NiCo ₂ O ₄ films

   https://doi.org/10.1063/5.0097869  

   Chen, Xuegang; Wu, Qiuchen; Zhang, Le; Hao, Yifei; Han, Myung-Geun; Zhu, Yimei; Hong, Xia (June 2022, Applied Physics Letters)

   The inverse spinel ferrimagnetic NiCo 2 O 4 possesses high magnetic Curie temperature T C , high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo 2 O 4 thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo 2 O 4 films on MgAl 2 O 4 substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while T C for 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity [Formula: see text] and its scaling relation with the longitudinal conductivity ([Formula: see text]) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal–insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin–orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo 2 O 4 . 

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3. Direct and indirect spin current generation and spin-orbit torques in ferromagnet/nonmagnet/ferromagnet trilayers

   https://doi.org/10.1103/PhysRevB.110.214427  

   Amin, V P; Baez_Flores, G G; Kovalev, A A; Belashchenko, K D (December 2024, Physical Review B)

   Spin-orbit torques in ferromagnet/nonmagnet/ferromagnet trilayers are studied using a combination of symmetry analysis, circuit theory, semiclassical simulations, and first-principles calculations using the nonequilibrium Green's function method with supercell disorder averaging. We focus on unconventional processes involving the interplay between the two ferromagnetic layers, which are classified into direct and indirect mechanisms. The direct mechanism involves spin current generation by one ferromagnetic layer and its subsequent absorption by the other. In the indirect mechanism, the in-plane spin-polarized current from one ferromagnetic layer “leaks” into the other layer, where it is converted into an out-of-plane spin current and reabsorbed by the original layer. The direct mechanism results in a predominantly dampinglike torque, which damps the magnetization towards a certain direction 𝐬_𝑑. The indirect mechanism results in a predominantly fieldlike torque with respect to a generally different direction 𝐬_𝑓. Similarly to the current-in-plane giant magnetoresistance, the indirect mechanism is only active if the thickness of the nonmagnetic spacer is smaller than or comparable to the mean free path. Numerical calculations for a semiclassical model based on the Boltzmann equation confirm the presence of both direct and indirect mechanisms of spin current generation. First-principles calculations reveal sizable unconventional spin-orbit torques in Co/Cu/Co, Py/Cu/Py, and Co/Pt/Co trilayers and provide strong evidence of indirect spin current generation. 

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4. Efficient spin transport across a disordered interface in a low damping magnetic insulator/heavy metal bilayer

   https://doi.org/10.1063/5.0294600  

   Alaei, S P; Raj, R; Channa, S; Takana, L; O'Mahoney, D; Zheng, X Y; Fleck, E E; Chen, T-Y; Galazka, Z; Kent, A D; et al (December 2025, Applied Physics Letters)

   We demonstrate efficient spin transfer across a disordered interfacial layer that forms in low damping ferrimagnetic insulator lithium aluminum ferrite (LAFO) and tantalum bilayers. Despite the interfacial disorder, confirmed by transmission electron microscopy, we find a room temperature interfacial spin mixing conductance on the order of 1014 Ω−1m−2 similar to other LAFO-based bilayers with epitaxial interfaces. Broadband ferromagnetic resonance measurements confirm a linewidth broadening in LAFO following the addition of a Ta layer, consistent with the effects of spin pumping. Furthermore, the presence of spin current generated in the Ta layer by spin pumping is confirmed with inverse spin Hall effect measurements. Measurements of the Ta thickness dependence of the spin Hall magnetoresistance and the Gilbert damping enhancement indicate that the Ta spin diffusion length is on the order of 1 nm. This work not only provides a surprising example of efficient spin transport across a disordered interface but also demonstrates the potential for low damping spinel ferrites as a robust system for efficient spin wave spintronics. 

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5. Coexistence of unconventional spin Hall effect and antisymmetric planar Hall effect in IrO2

   https://doi.org/10.1063/5.0240538  

   Yang, Yifei; Nair, Sreejith; Fan, Yihong; Chen, Yu-Chia; Jia, Qi; Benally, Onri Jay; Lee, Seungjun; Jeong, Seung Gyo; Yang, Zhifei; Low, Tony; et al (March 2025, Applied Physics Letters)

   Crystal symmetry plays an important role in the Hall effects. Unconventional spin Hall effect (USHE), characterized by Dresselhaus and out-of-plane spins, has been observed in materials with low crystal symmetry. Recently, antisymmetric planar Hall effect (APHE) was discovered in rutile RuO2 and IrO2 (101) thin films, which also exhibit low crystal symmetry. In this study, we report the observation of both USHE and APHE in IrO2 (111) films, using spin-torque ferromagnetic resonance and harmonic Hall measurements, respectively. Notably, the unconventional spin-torque efficiency from Dresselhaus spin was more than double that of a previous report. Additionally, the temperature dependence of APHE suggests that it arises from the Lorentz force, constrained by crystal symmetry. Symmetry analysis supports the coexistence of USHE and APHE and demonstrates that both originate from the crystal symmetry of IrO2 (111), paving the way for a deeper understanding of Hall effects and related physical phenomena. 

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