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1. Angular dependence of spin-orbit torque in monolayer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$

   https://doi.org/10.1103/PhysRevB.108.144422  

   Xue, Fei; Stiles, Mark D.; Haney, Paul M. (October 2023, Physical Review B)

   In ferromagnetic systems lacking inversion symmetry, an applied electric field can control the ferromagnetic order parameters through the spin-orbit torque. The prototypical example is a bilayer heterostructure composed of a ferromagnet and a heavy metal that acts as a spin current source. In addition to such bilayers, spin-orbit coupling can mediate spin-orbit torques in ferromagnets that lack bulk inversion symmetry. A recently discovered example is the two-dimensional monolayer ferromagnet Fe3GeTe2. In this paper, we use first-principles calculations to study the spin-orbit torque and ensuing magnetic dynamics in this material. By expanding the torque versus magnetization direction as a series of vector spherical harmonics, we find that higher order terms (up to ℓ=4) are significant and play important roles in the magnetic dynamics. They give rise to deterministic, magnetic field-free electrical switching of perpendicular magnetization. 

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2. Pseudo-easy-axis anisotropy in antiferromagnetic $S=1$ diamond-lattice systems

   https://doi.org/10.1103/PhysRevB.110.174438  

   Vaidya, S; Hernández-Melián, A; Tidey, J P; Curley, S_P M; Sharma, S; Manuel, P; Wang, C; Hannaford, G L; Blundell, S J; Manson, Z E; et al (November 2024, Physical Review B)

   We investigate the magnetic properties of $S=1$antiferromagnetic diamond-lattice, $\mathrm{Ni}{X}_2{\left(\mathrm{pyrimidine}\right)}_2$ $(X=\mathrm{Cl}, \mathrm{Br})$, hosting a single-ion anisotropy (SIA) orientation which alternates between neighboring sites. Through neutron diffraction measurements of the $X=\mathrm{Cl}$compound, the ordered state spins are found to align collinearly along a pseudo-easy axis, a unique direction created by the intersection of two easy planes. Similarities in the magnetization, exhibiting spin-flop transitions, and the magnetic susceptibility in the two compounds imply that the same magnetic structure and a pseudo-easy axis is also present for $X=\mathrm{Br}$. We estimate the Hamiltonian parameters by combining analytical calculations and Monte Carlo (MC) simulations of the spin-flop and saturation field. The MC simulations also reveal that the spin-flop transition occurs when the applied field is parallel to the pseudo-easy axis. Contrary to conventional easy-axis systems, there exist field directions perpendicular to the pseudo-easy axis for which the magnetic saturation is approached asymptotically and no symmetry-breaking phase transition is observed at finite fields. Published by the American Physical Society2024 

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3. Temperature‐Dependent Surface Anisotropy in (110) Epitaxial Rare Earth Iron Garnet Films

   https://doi.org/10.1002/smll.202407381  

   Song, Yixuan; Lasinger, Katharina; Tang, Hao; Li, Ju; Beach, Geoffrey_S_D; Ross, Caroline_A (October 2024, Small)

   Abstract Ferrimagnetic oxide thin films are important material platforms for spintronic devices. Films grown on low symmetry orientations such as (110) exhibit complex anisotropy landscapes that can provide insight into novel phenomena such as spin‐torque auto‐oscillation and spin superfluidity. Using spin‐Hall magnetoresistance measurements, the in‐plane (IP) and out‐of‐plane (OOP) uniaxial anisotropy energies are determined for a thickness series (5–50 nm) of europium iron garnet (EuIG) and thulium iron garnet (TmIG) films epitaxially grown on a gadolinium gallium substrate with (110) orientation and capped with Pt. Pt/EuIG/GGG exhibits an (001) easy plane of magnetization perpendicular to the substrate, whereas Pt/TmIG/GGG exhibits an (001) hard plane of magnetization perpendicular to the substrate with an IP easy axis. Both IP and OOP surface anisotropy energies comparable in magnitude to the bulk anisotropy are observed. The temperature dependence of the surface anisotropies is consistent with first‐order predictions of a simplified Néel surface anisotropy model. By taking advantage of the thickness and temperature dependence demonstrated in these ferrimagnetic oxides grown on the low symmetry (110) orientations, the complex anisotropy landscapes can be tuned to act as a platform to explore rich spin textures and dynamics. 

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4. High-field/high-frequency electron spin resonances of Fe-doped $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ by terahertz generalized ellipsometry: Monoclinic symmetry effects

   https://doi.org/10.1103/PhysRevB.109.214106  

   Richter, Steffen; Knight, Sean; Bulancea-Lindvall, Oscar; Mu, Sai; Kühne, Philipp; Stokey, Megan; Ruder, Alexander; Rindert, Viktor; Ivády, Viktor; Abrikosov, Igor A; et al (June 2024, Physical Review B)

   We demonstrate detection and measurement of electron paramagnetic spin resonances (EPR) of iron defects in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$utilizing generalized ellipsometry at frequencies between 110 and 170 GHz. The experiments are performed on an Fe-doped single crystal in a free-beam configuration in reflection at ${45}^{\circ }$and magnetic fields between 3 and 7 T. In contrast with low-field, low-frequency EPR measurements, we observe all five transitions of the $s=5/2$high-spin state ${\mathrm{Fe}}^{3+}$simultaneously. We confirm that ferric ${\mathrm{Fe}}^{3+}$is predominantly found at octahedrally coordinated Ga sites. We obtain the full set of fourth-order monoclinic zero-field splitting parameters for both octahedrally and tetrahedrally coordinated sites by employing measurements at multiple sample azimuth rotations. The capability of high-field EPR allows us to demonstrate that simplified second-order orthorhombic spin Hamiltonians are insufficient, and fourth-order terms as well as consideration of the monoclinic symmetry are needed. These findings are supported by computational approaches based on density-functional theory for second-order and on ligand-field theory for fourth-order parameters of the spin Hamiltonian. Terahertz ellipsometry is a way to measure spin resonances in a cavity-free setup. Its possibility of varying the probe frequency arbitrarily without otherwise changing the experimental setup offers unique means of truly disentangling different components of highly anisotropic spin Hamiltonians. Published by the American Physical Society2024 

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5. Electronic structure of Co $3d$ states in the Kitaev material candidate honeycomb cobaltate ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ probed with x-ray dichroism

   https://doi.org/10.1103/PhysRevB.107.214443  

   van Veenendaal, M.; Poldi, E. H.; Veiga, L. S.; Bencok, P.; Fabbris, G.; Tartaglia, R.; McChesney, J. L.; Freeland, J. W.; Hemley, R. J.; Zheng, H.; et al (June 2023, Physical Review B)

   none (Ed.)

   The recent prediction that honeycomb lattices of Co2+ (3d7) ions could host dominant Kitaev interactions provides an exciting direction for exploration of new routes to stabilizing Kitaev’s quantum spin liquid in real materials. Na3Co2SbO6 has been singled out as a potential material candidate provided that spin and orbital moments couple into a Jeff = 1/2 ground state, and that the relative strength of trigonal crystal field and spin-orbit coupling acting on Co ions can be tailored. Using x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) experiments, alongside configuration interaction calculations, we confirm the counterintuitive positive sign of the trigonal crystal field acting on Co2+ ions and test the validity of the Jeff = 1/2 description of the electronic ground state. The results lend experimental support to recent theoretical predictions that a compression (elongation) of CoO6 octahedra along (perpendicular to) the trigonal axis would drive this cobaltate toward the Kitaev limit, assuming the Jeff = 1/2 character of the electronic ground state is preserved. 

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