Session G52: Spin-Orbit Torques and Spin-Torque Oscillators: I

Sagnac interferometry for high-sensitivity optical measurements of spin-orbit torque

We adapt Sagnac interferometry for magneto-optic Kerr effect measurements of spin-orbit-torque-induced magnetic tilting in thin-film magnetic samples. The high sensitivity of Sagnac interferometry permits for the first time optical quantification of spin-orbit torque from small-angle magnetic tilting of samples with perpendicular magnetic anisotropy (PMA). We find significant disagreement between Sagnac measurements and simultaneously-performed harmonic Hall (HH) measurements of spin-orbit torque on Pt/Co/MgO and Pd/Co/MgO samples with PMA. The Sagnac results for PMA samples are consistent with both HH and Sagnac measurements for the in-plane geometry, so we conclude that the conventional analysis framework for PMA HH measurements is flawed. We suggest that the explanation for this discrepancy is that although magnetic-field induced magnetic tilting in PMA samples can produce a strong planar Hall effect, when tilting is instead generated by spin-orbit torque it produces a negligible change in the planar Hall signal. This very surprising result demonstrates an error in the most-popular method for measuring spin-orbit torques in PMA samples, and represents an unsolved puzzle in understanding the planar Hall effect in magnetic thin films. Lastly, I will briefly talk about our effort to use Sagnac to probe heterostructures of vdW insulating magnets at cryogenic temperatures. (arXiv:2109.13759)   

Probing anisotropy in epitaxial Fe/Pt bilayers by spin-orbit torque ferromagnetic resonance

Spin-orbit torques (SOTs) in novel materials have attained much attention recently as they have the potential to revolutionize highly energy-efficient storage applications. From this end, spin-orbit torque ferromagnetic resonance (STFMR) is a prominent choice for studying spin-orbit torques in multilayers. However, an overlooked aspect of STFMR has been the dynamic response driven by oscillatory SOT.

Here, we report the generation and detection of STFMR in micropatterned epitaxial Fe/Pt bilayers grown by molecular beam epitaxy. The biasing magnetic field was applied in an in-plane geometry at an angle of 45° with respect to the microwave-current direction to maximize the signal strength. The STFMR measurement was compared with broadband ferromagnetic resonance spectroscopy of the unpatterned bilayer thin films. The results are interpreted using an analytical formalism which indicates the presence of strong magnetic anisotropy. This was further confirmed using micromagnetic modeling, which reveals the field-dependent magnetization alignment in the microstructures. Our results demonstrate a simple and efficient method for determining magnetic anisotropies in individual microstructures by means of on-chip rf spectroscopy.   

Highly efficient spin-orbit torque switching in Pt/Li0.5Al1.0Fe1.5O4 bilayers

Spin-wave-based electronics promise fast electrical switching of magnetization but require the isotropic excitation of spin waves and minimization of losses associated with spin-charge conversion. In this regard, spin-orbit torque (SOT) has been identified as a key phenomenon that may enable the coherent excitation of spin waves. Previously, we demonstrated the synthesis of the novel spinel oxide insulator Li_0.5Al_1.0Fe_1.5O₄ (LAFO) films on (001) oriented MgGa₂O₄ that possess PMA and a Gilbert damping parameter on the order of 10^-3. In this talk, we demonstrate current induced SOT switching of the magnetization in Pt/LAFO bilayers at current densities as low as 6 x 10⁵ A/cm^2, one of the lowest reported for a PMA ferrimagnetic insulator. Using second harmonic Hall measurements, we elucidate the origin of this low current density by quantifying a high damping-like SOT efficiency of 0.54. This is much larger than the typical value of < 0.1 in other Pt/ferromagnet systems. The combination of PMA, low damping, and efficient SOT switching make LAFO a promising system for future spintronics applications.   

Anisotropic spin-orbit torques in thin film bilayers of Li0.5(Al,Fe)2.5O4 and Pt studied with spin-torque ferromagnetic resonance

Low loss magnetic insulators with perpendicular magnetic anisotropy (PMA) are of great interest for exploring spin wave-based phenomena as well as integrating into higher density, more energy-efficient memory storage. However, there are very few materials that exhibit low loss, PMA and insulating behavior. Recently, we have stabilized epitaxial thin films of Li_0.5(Al,Fe)_2.5O₄ (LAFO) on single crystal (001) MgGa₂O₄ substrates which have strong PMA and are low loss, with a typical Gilbert damping parameter of ~7x10^-4. We have demonstrated spin pumping from LAFO into an adjacent Pt layer. Conversely, in this talk we demonstrate the excitation of LAFO magnetization by a microwave charge current in the Pt via spin-torque ferromagnetic resonance (ST-FMR). ST-FMR enables us to characterize charge to spin interconversion in bilayers with Pt. We find a highly anisotropic ST-FMR voltage signal depending on the relative orientation of the microwave charge current and the external in-plane magnetic field along the (110) direction, suggesting the presence of an out-of-plane anti-damping torque exerted by the Pt. The presence of this torque indicates a possibility of field-free SOT switching in this system.   

Spin-Orbit Torques beyond the Spin-Diffusion Model in Ferromagnet/Normal-Metal/Ferromagnet Trilayers

Spin-orbit torques in Co/Cu/Co, Co/Pt/Co, and Py/Cu/Py trilayers are studied using the first-principles non-equilibrium Green’s function method with supercell disorder averaging. Trilayers with a Cu spacer exhibit strong current-in-plane giant magnetoresistance, and the torques exhibit features that can not be captured by the spin-diffusion model. In the parallel configuration there is strong disorder-dependent dampinglike torque which can greatly exceed the torque in bilayers with Pt. This torque is strongly reduced in the antiparallel configuration. We also consider the case where the magnetizations in the two layers are orthogonal to each other. In addition to dampinglike and fieldlike torque components, we find a new torque with the angular dependence (s·m₂)m₁ where s=E×z while m₁ and m₂ are the magnetizations in the spin-orbit source and detector layer, respectively. For further insight, we develop a semiclassical model based on the Boltzmann equation. Numerical calculations show that spin torques on one ferromagnetic layer are modulated by the other through interlayer scattering. Thus, in contrast to ferromagnet/heavy-metal bilayers, ferromagnetic trilayers can exhibit unconventional torques that can not be captured by the spin-diffusion model.   

First-principles calculation of 3m spin-orbit torque and its effect on magnetization dynamics

Symmetry-dependent field-free switching of the perpendicular magnetization has been observed in an epitaxial L11-ordered CuPt/CoPt bilayer [1]. The switching is enabled by 3m spin-orbit torque (SOT) arising from the low-symmetry point group (3m1) at the CuPt/CoPt interface. Using a first-principles nonequilibrium Green’s function formalism combined with the Andersen disorder model, we calculate the damping-like (DL), fieldlike (FL), and 3m SOTs in such bilayer by expanding the SOT in the basis of vector spherical harmonics [2,3]. We find that the magnitude of 3m torque is appreciable compared to the DL SOT. We also study the magnetization dynamics using micromagnetics simulations in a perpendicularly magnetized circular nanodisk in the presence of 3m SOT and Dzyaloshinskii-Moriya interaction. We find that the 3m SOT can drive the domain wall motion, and if large enough, can enable the field-free switching of perpendicular magnetization.

Reference:

1. Liu, L., Zhou, C., Shu, X. et al. Nat. Nanotechnol. 16, 277–282 (2021).

2. K. D. Belashchenko, Alexey A. Kovalev, and M. van Schilfgaarde, Phys. Rev. Materials 3, 011401(R) (2019)

3. K. D. Belashchenko, Alexey A. Kovalev, and M. van Schilfgaarde, Phys. Rev. B 101, 020407(R) (2020)   

Alloying effect on auto-oscillation properties of W100-xTax/CoFeB/MgO spin Hall nano-oscillators

Spin current based devices have the potential for ultra-fast and energy-efficient operation as in spin Hall nano-oscillators (SHNOs)[1-3], SOT-MRAM, and spin logic [4,5]. However, key challenges remain of how to further reduce their high current densities (J_c) and energy consumption. We demonstrate alloying of W with Ta to substantially reduce the J_c in W_100-xTa_x(5nm)/CoFeB(t=1.4-2 nm)/MgO(2nm) SHNOs. Spin-torque ferromagnetic resonance (ST-FMR) measurements showed a substantial improvement in both spin Hall conductivity and spin-orbit torque efficiency from W-Ta alloying. As a direct outcome, we observed a 45% reduction in auto-oscillation current densities measured for different constriction widths resulting in a 65% reduction in threshold power consumption as compared to pure W based SHNOs. Our work reveals promising aspects of how W-Ta alloys can promote energy-efficient operation of emerging spintronic devices.

 

References:

1. Mazraati et al Appl. Phys. Lett. 109, 242402 (2016).

2. Fulara et al., Sci. Adv. 5, eaax8467 (2019).

3. Zahedinejad et al., Nat. Nanotechnol. 15, 47–52 (2020).

4. Garello et al., Appl. Phys. Lett. 105, 212402 (2014).

5. Cubukcu et al., Appl. Phys. Lett. 104, 042406 (2014).

    

Freezing and thawing magnetic droplet solitons

We use all-perpendicular nanocontact spin-torque oscillators to study the low-field behavior of magnetic droplets. These dissipative solitons are inherently dynamic and are characterized by a core of reversed spins surrounded by a precessing perimeter [1, 2, 3, 4]. The precession frequency lies between the ferromagnetic and Zeeman resonances, but the droplet is also prone to drift which gives additional dynamics [5, 6]. Electrical measurements reveal that the droplet transforms, freezes, into a static bubble at low fields. Once formed, the bubble is stable without a sustaining current. Furthermore, the droplet-to-bubble transition is fully reversible and the bubble can thaw back to a droplet at sufficient high field and current. The findings are corroborated by X-ray microscopy, which images the magnetic states during the freezing. Experimental data together with simulations identify pinning as the main mechanism behind the bubble stability.

 

[1] Hoefer et al., PRB 82, 054432 (2010)

[2] Mohseni et al., Science 339, 1295 (2013)

[3] Chung et al., J. Appl. Phys 115, 172612 (2014)

[4] Chung et al., PRL 120, 217204 (2018)

[5] Xiao et al., PRB 95, 024106 (2017)

[6] Chung et al, Nat. Com. 7, 11209 (2016)   

Enhancement of spin Hall effect in O-implanted Pt by side jump scattering

Materials exhibiting high damping-like spin-orbit torque efficiency, θ_DL owing to extrinsic spin Hall effect (SHE) are of interest in magnetization switching, auto oscillations, and domain wall motion. However, the value of θ_DL for heavy metals such as Pt is ∼0.06 [1]. Recent studies have shown enhancement in θ_DL by incorporating non-metallic elements in Pt such as S, N [2,3] but incorporation of these elements by sputtering [3] have shown non-monotonic θ_DL with varying concentrations. Here, we report a high θ_DL in modified Pt, designed by non-metallic O ion implantation [4], at an optimized low energy of 20 keV with varying dose from 2 ×10¹⁶-1 ×10¹⁷ ions cm^-2. We achieved a 3.5 times monotonic increase in θ​​​​​​​_DL from 0.064 to 0.230 with a smaller trade-off in resistivity, ρ_xx from 55.4 to 159.5 µΩ cm. We find that the spin Hall resistivity, ρ_SH, is proportional to the square of resistivity from impurities, ρ_imp, i.e., ρ_SH ∝ (ρ_imp)², indicating an extrinsic SHE mechanism of side jump scattering as the dominant origin of enhanced θ​​​​​​​_DL. These results encourage implantation-engineering as an effective tool to design materials for developing the next-generation devices that can exert high θ_DL for low energy manipulation of magnetization, indispensable in spintronics field.