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1. Spin-torque ferromagnetic resonance based on current-induced impedance

   https://doi.org/10.1063/5.0222114  

   Kobayashi, Yuta; Itoh, Tomoya; Hisatomi, Ryusuke; Moriyama, Takahiro; Shiota, Yoichi; Fan, Xin; Ono, Teruo (July 2024, Applied Physics Letters)

   Spin-torque ferromagnetic resonance (ST-FMR) has been widely used for measuring damping-like spin–orbit torques in magnetic bilayers. Typically, the ratio between the damping-like and field-like spin–orbit torques are extrapolated based on the ferromagnetic resonance line shapes. However, when the field-like spin–orbit torque is unknown, the line shape analysis may lead to errors in extrapolating the damping-like spin–orbit torque. Here, we propose a modified version of the ST-FMR that allows extrapolation of both damping-like and field-like torques independently. By introducing an alternating current to the sample, the RF impedance is modulated, allowing detection via the reflected microwave. We show that the extrapolated field-like and damping-like torques in Py/Pt samples are consistent with the technique measuring current-induced linewidth and resonance field change but have much better signal-to-noise ratio. Our proposed method paves a way for more accurate measurement of spin–orbit torques. 

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2. Spin–orbit torque nano-oscillator with giant magnetoresistance readout

   https://doi.org/10.1038/s42005-020-00454-7  

   Chen, Jen-Ru; Smith, Andrew; Montoya, Eric A.; Lu, Jia G.; Krivorotov, Ilya N. (October 2020, Communications Physics)

   Abstract Spin-orbit torque nano-oscillators based on bilayers of ferromagnetic and nonmagnetic metals are ultra-compact current-controlled microwave signal sources. They are attractive for practical applications such as microwave assisted magnetic recording, neuromorphic computing, and chip-to-chip wireless communications. However, a major drawback of these devices is low output microwave power arising from the relatively small anisotropic magnetoresistance of the ferromagnetic layer. Here we experimentally show that the output power of a spin-orbit torque nano-oscillator can be significantly enhanced without compromising its structural simplicity. Addition of a ferromagnetic reference layer to the oscillator allows us to employ current-in-plane giant magnetoresistance to boost the output power of the device. This enhancement of the output power is a result of both large magnitude of giant magnetoresistance compared to that of anisotropic magnetoresistance and their different angular dependencies. Our results hold promise for practical applications of spin-orbit torque nano-oscillators. 

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3. Photon Energy‐Dependent Optical Spin‐Orbit Torque in Heavy Metal–Ferromagnet Bilayers

   https://doi.org/10.1002/adfm.202307753  

   Wang, Chuangtang; Xu, Yihao; Liu, Yongmin (October 2023, Advanced Functional Materials)

   Abstract The manipulation of magnetization through optically generated ultrafast spin currents is a fascinating area that needs a thorough understanding for its potential future applications. In this work, a comprehensive investigation of helicity‐driven optical spin‐orbit torque in heavy metal/ferromagnetic metal heterostructures is presented, specifically cobalt capped with gold or platinum, subject to laser pumping at different wavelengths. The results demonstrate up to tenfold enhancement in optical spin‐orbit torque quantum efficiency for gold compared to platinum of the same thickness when pumped with a visible laser. Additionally, the study provides the first experimental analysis of the photon energy dependence of optical spin‐orbit torque and derives the optical spin orientation spectra for both gold/cobalt and platinum/cobalt heterostructures. A key insight gained from the study is the impact of photon energy‐dependent spin transport in the system, which suggests the use of a high photon energy pump for efficient spin transport. These findings highlight the potential of spin current generation and manipulation in gold/ferromagnet heterostructures for a wide range of applications such as all‐optical magnetization switching, spin‐wave generation and control, and spintronic terahertz emission. 

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4. 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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5. Dynamic Bloch Chirality and Enhanced Velocities from Spin-Orbit Torque Driven Domain Wall Motion in Thick Magnetic Films

   https://doi.org/10.3390/magnetochemistry8100119  

   Staggers, Trae Lawrence; Pollard, Shawn David (October 2022, Magnetochemistry)

   Spin-orbit torque (SOT) driven domain wall motion has attracted significant attention as the basis for a variety of spintronic devices due to its potential use as a high speed, low power means to manipulate the magnetic state of an object. While most previous attention has focused on ultrathin films wherein the material thickness is significantly less than the magnetic exchange length, recent reports have suggested unique dynamics may be achieved in intermediate and high thickness films. We used micromagnetic modelling to explore the role of the vertically non-uniform spin textures associated with the domain wall in nanowires of varying thickness on SOT driven domain wall motion. We found large velocity asymmetries between Bloch chiralities near the current density required for reversal of the Bloch component of the magnetization and linked these asymmetries to a gradual reorientation of the domain wall structure which drives a non-negligible, chiral Néel component of the domain wall. We further explored the influence of saturation magnetization, film thickness, the Dzyaloshinskii-Moriya interaction, and in-plane fields on domain wall dynamics. These results provide a framework for the development of SOT based devices based on domain wall motion in nanowires beyond the ultrathin film limit. 

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