Giant Damping-like Spin-Torque Conductivity in a GeTe/Py van der Waals Heterostructure

TL;DR

GeTe/Py vdW heterostructures exhibit a giant damping-like spin-torque conductivity, addressing a key limitation of FM/vdW devices. The authors combine spin-torque ferromagnetic resonance experiments with first-principles calculations to reveal that the large torque arises from a cooperative interaction of the spin Hall effect, orbital Hall effect, and orbital Rashba effect, aided by interfacial charge transfer that dopes GeTe/Py. The measured $σ_{DL}^{y}$ value is about $-(1.25 \\pm 0.11)\times 10^{5}$ ħ/(2e) Ω^{-1} m^{-1}, comparable to heavy metals and higher than other vdW/FM interfaces. This demonstrates the potential of engineered vdW interfaces to enable energy-efficient, room-temperature electrical control of magnetization for next-generation spintronic devices.

Abstract

Recent observations of large unconventional spin-orbit torques in van der Waals (vdW) materials are driving intense interest for energy-efficient spintronic applications. A key limitation of ferromagnet (FM)/vdW heterostructures is their lower value of damping-like torque conductivity ($σ{\rm_{DL}^{y}}$) compared to the conventional heavy metal-based systems, limiting their prospects for commercial spintronic devices. Here, we report both a giant $σ{\rm_{DL}^{y}}$ of $-(1.25 \pm 0.11)\times 10^{5}~\hbar/ 2e~Ω^{-1}$m$^{-1}$ and an unconventional spin-orbit torque in a heterostructure comprising an FM (Ni$_{80}$Fe$_{20}$) and the vdW material GeTe. The value of $σ{\rm_{DL}^{y}}$ represents the highest reported torque conductivity for any FM/vdW interface and is comparable to benchmark heavy metal heterostructures. First-principles calculations reveal that this substantial torque originates from the cooperative interplay of the spin Hall effect, orbital Hall effect, and orbital Rashba effect, assisted by interfacial charge transfer. These findings demonstrate the potential of carefully engineered vdW heterostructures to achieve highly efficient electrical manipulation of magnetization at room temperature, paving the way for next-generation low-power spintronic devices.

Figures (4)

• Figure 1: Crystal structure and characterization of GeTe thin films. (a) Crystal structure of $\alpha$-GeTe (rhombohedral structure, $R3m$). The lower panel illustrates the GeTe structure viewed along the $c-$axis, where the Ge and Te atoms are bonded by three shorter (2.83 $\textup{\AA}$) and three longer (3.15 $\textup{\AA}$) bonds due to Peierls distortion. Schematic illustration of the splitting of bulk bands in GeTe due to (b) spin Rashba effect (SRE) and (c) orbital Rashba effect (ORE). (d) Glancing angle x-ray diffraction pattern of a 23 nm thick GeTe thin film, confirming the phase purity and match with the $\alpha$-phase of GeTe. (e) The room-temperature Raman spectra before (black) and after (red) deposition of Py on top of GeTe, showing the characteristic $E$ and A$_1$ modes at 88.3 and 125.6 cm$^{-1}$. (f) Atomic force microscopy image of GeTe thin film, demonstrating a smooth surface morphology with an root mean square roughness of $\approx$ 0.2 nm. (g)-(h) X-ray photoelectron spectroscopy spectra showing the core level peaks for Ge 3d and Te 3d$_{5/2}$ peaks, respectively, for GeTe thin film capped with Al. The solid lines represent the fitted data.
• Figure 2: Spin-torque ferromagnetic resonance measurements in GeTe/Py heterostructures. (a) The schematic of the device consisting of a GeTe/Py bilayer, the setup for STFMR measurements, and a scanning electron microscopy image of the microstrip device with a co-planar waveguide. The top panel shows the schematic of the generation of both in-plane ($\tau _{\parallel}$) and out-of-plane ($\tau _{\bot}$) torque when the device is subject to electric current ($I_{\rm RF}$) flowing along the $x$-direction. (b) The frequency-dependent STFMR spectra (open symbols) and their fits (solid line) for the GeTe(15 nm)/Py(8 nm) sample. An example of decomposition of fitting into the symmetric (blue dotted curve) (V$_{\textrm{S}}$) and anti-symmetric (V$_{\textrm{A}}$) component (magenta dotted curve) is shown for 6.5 GHz. (c) Linewidth ($\Delta H$) vs. frequency ($f$) (open circle) and their corresponding fit (solid lines). (d) $f$ vs. $H_r$ curve (open circle) and their corresponding fit (solid lines) with Kittel's equation.
• Figure 3: Angle-dependent STFMR measurements and comparison of spin Hall conductivity in vdW Heterostructures. The angular dependence of (a) Symmetrical component, V$_{\textrm{S}}$ and (b) Asymmetrical component, V$_{\textrm{A}}$ of STFMR for the GeTe/Py sample. Experimental data (open circles) are fitted (solid lines) using $\text{Eq.}~\ref{['VS']}$ in panel (a) and $\text{Eq.}~\ref{['VA']}$ in panel (b). The dashed lines indicate the angular dependence of individual spin-torque contributions. (c) Extracted spin-torque conductivities ($\sigma_{\rm DL}^{y}$, $\sigma_{\rm FL}^{z}$, and $\sigma_{\rm DL}^{z}$) for the GeTe/Py together with a reference Py device without the GeTe layer. The spin-torque conductivities for the Py device are scaled by $\times 10$ to enhance their visualization relative to GeTe/Py. (d) Comparative analysis of the dominant damping-like spin-torque conductivity $\sigma_{\rm DL}^{y}$ for $\alpha$-GeTe (this work) against other vdW materials: MoS$_2$safeer2019room, MoTe$_2$stiehl2019layer, WTe$_2$macneill2017control, NbSe$_2$guimaraes2018spin, PtTe$_2$xu2020high, and TaIrTe$_4$liu2023field.
• Figure 4: Electronic structure, spin and orbital Hall conductivity calculations. All non zero components of the (a) spin and (b) orbital Hall conductivity tensors are plotted as a function of the Fermi energy. The conductivities are in the units of $10^5 (\hbar/2e) \Omega^{-1} m^{-1}$. The doped Fermi level induced by the Py overlayer is shown by the line $E_F^{het}$. The conductivities at $E_F^\prime$ listed in Table \ref{['tab1']}. (c) Rashba split near $\Gamma$ point in the band structure for the 4-layer GeTe, (d) energy and momentum shifts for the Rashba splitted bands, (e) orbital texture and (f) spin texture near the $\Gamma$ point. (g) top : Structure of the 4-layer GeTe with 4-layers of Ni. Note that here GeTe atoms for the top layer are replaced with the Ni atoms for simplicity; bottom : Band structure for the corresponding structure showing the shift of $\Gamma$ point close to Fermi.