Session K52: Spin-Orbit Torques and Spin-Torque Oscillators: II

Spin-orbit torque switching of metallic antiferromagnets and ferrimagnets

Spin-orbit torque (SOT) control of magnetic order in antiferromagnetic (AFM) and ferrimagnetic (FIM) materials is of great current interest, motivated by their exchange-dominated high-frequency dynamics and small (or absent) macroscopic magnetization, making them excellent candidates for high-speed and robust memory devices.

Here we present our recent results in SOT control of AFM and FIM order. First, we discuss SOT control of AFM order in two metallic antiferromagnets (PtMn and non-collinear IrMn₃) [1, 2]. We show that pillars of both AFMs, grown on a heavy metal (HM) layer, can be reversibly switched between different magnetic states by electric currents. We also present an experimental protocol to unambiguously distinguish current-induced magnetic and nonmagnetic switching signals in AFM/HM structures. A six-terminal double-cross device is constructed, with an IrMn₃ pillar placed on one cross. The differential voltage is measured between the two crosses after each switching attempt. For a wide range of current densities, reversible switching is observed only when write currents pass through the cross with the IrMn₃ pillar. This eliminates the possibility of non-magnetic switching artifacts, which complicated the interpretation of most previous AFM/HM switching experiments.

Next, we present a strategy for deterministic field-free switching of perpendicular ferrimagnetic films by using chiral symmetry-breaking to eliminate the need for an external magnetic field [3]. Bias-field-free SOT switching is demonstrated in a perpendicular CoTb film with an engineered vertical composition gradient. The vertical structural inversion asymmetry induces strong intrinsic SOTs and a gradient-driven Dzyaloshinskii–Moriya interaction (g-DMI), which dynamically breaks the in-plane symmetry during the switching process. This approach is scalable to large wafer size.

[1] Nature Elec. 3, 92, (2020)

[2] Nature Comm. 12, 3828, (2021)

[3] Nature Comm. 12, 4555, (2021)   

Exploring non-Hermitian topology with spin-torque oscillator arrays

Topological phases of matter have emerged as building blocks for new technologies due to their novel electronic and spin transport properties including robust conducting edge states. Recently, the theory of topological phases has been extended to open quantum systems which can be described by a non-Hermitian effective Hamiltonian. Magnonic systems are a natural platform in which to study non-Hermitian topology because they are inherently dissipative, and the injection of spin current into the system provides an external gain mechanism. Dissipation and gain are both described by non-Hermitian Hamiltonian terms. In this talk we show how the magnetization dynamics of a one-dimensional array of spin-torque oscillators can be mapped to a non-Hermitian Hamiltonian with topologically protected edge states. Changing the amount of spin current injected allows it to be tuned to the topologically nontrivial phase. The edge state manifests as an auto-oscillation of a single spin-torque oscillator on the edge of the system despite the uniform injection of spin current across the system. We will discuss the extension of these results to two-dimensional spin-torque oscillator arrays, which can exhibit one-dimensional edge states and present new possibilities for novel spintronic devices.   

Spin Orbit Torques from Magnetically Ordered Topological Semimetal CrPt3

Spin-orbit effects in some magnetically ordered materials have been shown to produce large spin torques with exotic geometries defined by the magnetic ordering of the material. In such materials, it may be possible to modify the magnetic ordering in order to optimize the geometry of the produced torques for applications to novel computing technologies including magnetic random access memory and neuromorphic computing. CrPt₃ is a topological semimetal [1] that offers a promising system for probing these effects, both for its large spin-orbit effects, large intrinsic Hall effect and its magnetic ordering with tunable anistropy. CrPt₃ shows ferrimagnetic behavior when chemically ordered, but paramagnetic behavior when disordered [2]. Thus, it is possible to compare CrPt₃ grown in each state and examine the role of magnetic ordering in its spin current generation properties. We present angular-dependent and DC biased spin torque ferromagnetic resonance measurements characterizing the size and geometry of spin currents generated in epitaxial CrPt₃ films. We additionally probe the dependence of the ordering direction on the resulting spin torques. Finally, we examine the effects of variation in the magnetic ordering on the generated spin currents.

[1]. A. Markou et al, Comm. Phys. 4, 104 (2021).

[2] O. Hellwig et al., Appl. Phys. Lett. 79, 1151 (2001).   

Spin-Orbit-Torque Oscillators Driven by Out-of-Plane Spin Currents

Spin torque oscillators are important candidates for many spintronic applications, ranging from neuromorphic computing to magnetic recording. Many conventional spin-torque oscillators suffer from small cone angle precession, susceptibility to thermal fluctuations, and expensive manufacturing, all of which make conventional spin-torque oscillators difficult for commercial uses. We explore the use of a new class of spin-torque oscillators that operate by running a current in-plane and utilizing spin-orbit effects to drive oscillations. The in-plane current through a ferromagnetic layer generates a spin-current where both the flow and spin direction are out-of-plane. The out-of-plane spin current tilts the magnetic moment of a ferromagnet out of plane, enabling sustained oscillations around the demagnetization field. In-plane spin currents are also generated in this process, via the spin Hall effect, which may reduce the performance of the oscillations resulting from the out-of-plane spin currents. To understand the conditions necessary to drive self-sustained oscillations, we vary the spin direction of the out-of-plane flowing spin current from 0° (out-of-plane) to 90° (in-plane) across a variety of spin current densities. Particular attention is being placed on small out of plane torques, resulting from low angled spin polarization, and their ability to enable self-sustained oscillations. Finally, we determine the temperature dependence of the amplitude and frequency of self-sustained oscillations in this novel spin-orbit torque oscillator. Our micromagnetic simulations of this easy plane, spin-orbit torque oscillator will help researchers determine how to employ unconventional spin currents in novel spintronic devices.   

Voltage-control of effective damping in spin Hall nano-oscillators

Constriction-based spin Hall nano-oscillators (SHNOs) [1] have attracted interest for their non-linear behavior [2] exhibiting ultra-wide microwave frequency tunability [3], mutual synchronization in chains [4] and 2D arrays [5], and voltage enabled frequency manipulation [6]. The latter provides an efficient path for the implementation of neuromorphic and quantum-like computing applications, such as Ising Machines [7]. In this work, we use micromagnetic simulations to explore voltage gate geometries for controlling SHNOs and obtain strong qualitative and quantitative changes in effective damping as a function of gate placement. We speculate that these effects are due to spin-wave localization and reflection at the voltage gate interfaces and the product of a change in magnetic anisotropy as a result of the applied electric field.

[1] T. Chen et al. Proc. IEEE,104(10):1919–1945, Oct 2016.

[2] M. Dvornik et al. Physical Review Applied,9(1):014017, Jan 2018.

[3] M. Zahedinejad et al. Applied Physics Letters, 112(13):132404, 2018.

[4] A. A. Awad et al. Nat. Phys., 13(Nov):292–299, 2017.1

[5] M. Zahedinejad et al. Nat. Nanotechnol. 15(1):47–52, Jan 2020.

[6] H. Fulara et al. Nature Communications, 11(1):1–7, Dec 2020.

[7] A. Houshang et al. arXiv:2006.02236. 2020   

Low-Damping Vertically Graded Ferromagnetic Films

Recent studies indicate that ~10-nm-thick ferromagnetic films with vertical compositional gradients can produce strong spin-orbit torques (SOTs). Such graded films are of technological interest since they are more thermally stable than ultrathin magnetic films in typical SOT devices. An important unanswered question is whether graded magnetic films can possess low damping, which is vital for minimizing the input power for precessional SOT devices.

We show that vertically graded ferromagnetic films exhibit low effective damping, comparable to homogeneous films of low-damping Co₂₅Fe₇₅[1] and Fe₈₀V₂₀[2]. Vertically graded films (thickness ~10 nm) of polycrystalline Co₂₅Fe₇₅-Fe₈₀V₂₀ were grown by continuously tuning the sputtering powers on Co₂₅Fe₇₅ and Fe₈₀V₂₀ targets. The FMR linewidths of graded Co₂₅Fe₇₅-Fe₈₀V₂₀ films are only <25% greater than those of homogeneous Co₂₅Fe₇₅. Thus, we find that the intentional inhomogeneity in the graded films does not lead to large linewidth broadening. Our results demonstrate vertically graded ferromagnetic films as promising candidates for power-efficient SOT devices.

 

[1] M. A. W. Schoen, et al. Nat. Phys. 12, 839 (2016)

[2] D. A. Smith, et al. Phys. Rev. Appl. 14, 034042 (2020)   

Spin-orbit torque driven multistate switching of canted GdCo moments without a symmetry breaking field

Rare earth-transition metal (RETM)-based ferrimagnets are known to exhibit strong bulk perpendicular magnetic anisotropy and bulk Dzyaloshinskii-Moriya interaction. These properties make RETM ferrimagnets an easily accessible system to study exciting magnetic phenomena such as magnetic skyrmion, chiral domain wall, all-optical switching, spin-orbit torque switching and canted magnetic moments. Here, we demonstrate a field-free spin-orbit torque-driven multistate switching of a nearly compensated GdCo ferrimagnet sandwiched between Ta and Pt. We show that our GdCo film exhibits a strong canting of magnetic moments, and the canted moments can be deterministically switched among three stable states using a current-driven spin-orbit torque. We further show that the three states persist in the presence of an in-plane magnetic field and the levels of the three states can be tuned with the magnitude of the magnetic field. We consider various possible origins of the canting, including concentration gradient, Dzyaloshinskii - Moriya interaction, and balanced pair-correlations of unlike atoms versus like atoms in the growth direction, and discuss various methods to differentiate among these mechanisms.   

Spin-orbit Torques in Magnetron-Sputtered MoTe2

Weyl semimetals can generate large current-induced spin-orbit torques (SOT) that can manipulate the magnetization dynamics in the ferromagnetic materials, and thus play an important role in spintronic devices. Due to their concomitant reduced symmetries, Weyl semimetals are also promising for generating novel SOT that may be able to efficiently switch perpendicular magnetic anisotropy thin films. Large spin-orbit torque efficiencies have already been reported for exfoliated and sputtered WTe₂ films[1]. Here we investigate magnetron-sputtered MoTe₂ films that are theoretically predicted to have even larger SOT efficiencies than WTe₂. We studied the effects of processing conditions on the stoichiometry and crystal structure of the magnetron-sputtered MoTe₂ films by using Rutherford backscattering and X-ray diffraction.  Furthermore we verified the presence of the 1T’ semimetal phase from Raman spectroscopy. SOT efficiencies of the MoTe₂ thin films were measured via angle-dependent Spin-Torque ferromagnetic resonance on MoTe₂/Ni₈₀Fe₂₀ heterostructures and we observed mixing voltage signals characteristic of large novel SOT in both the symmetric and anti-symmetric components of the lineshape.