Highly efficient spin-orbit torque and switching of layered ferromagnet Fe3GeTe2 (1903.00571v1)

Abstract: Among van der Waals (vdW) layered ferromagnets, Fe3GeTe2 (FGT) is an excellent candidate material to form FGT/heavy metal heterostructures for studying the effect of spin-orbit torques (SOT). Its metallicity, strong perpendicular magnetic anisotropy built in the single atomic layers, relatively high Curie temperature (Tc about 225 K) and electrostatic gate tunability offer a tantalizing possibility of achieving the ultimate high SOT limit in monolayer all-vdW nanodevices. The spin current generated in Pt exerts a damping-like SOT on FGT magnetization. At about 2.5x1011 A/m2 current density,SOT causes the FGT magnetization to switch, which is detected by the anomalous Hall effect of FGT. To quantify the SOT effect, we measure the second harmonic Hall responses as the applied magnetic field rotates the FGT magnetization in the plane. Our analysis shows that the SOT efficiency is comparable with that of the best heterostructures containing three-dimensional (3D) ferromagnetic metals and much larger than that of heterostructures containing 3D ferrimagnetic insulators. Such large efficiency is attributed to the atomically flat FGT/Pt interface, which demonstrates the great potential of exploiting vdW heterostructures for highly efficient spintronic nanodevices.

Spin-Orbit Torque and Magnetization Switching in Fe GeTe /Pt Heterostructures

This paper explores the spin-orbit torque (SOT) effects and magnetization switching in van der Waals (vdW) heterostructures composed of ferromagnetic Fe$_3$GeTe$_2$ (FGT) and a thin platinum (Pt) film. FGT emerges as a promising candidate within the vdW ferromagnet family due to its unique combination of metallic properties, substantial perpendicular magnetic anisotropy (PMA), elevated Curie temperatures, and the potential for electrostatic gate tuning. These attributes render FGT an ideal subject for studying the ultimate high SOT limit achievable in monolayer vdW nanodevices.

Researchers fabricated FGT/Pt heterostructures by sputtering 5 nm of Pt onto exfoliated FGT flakes, which ranged in thickness from approximately 15 to 23 nm. The paper investigates SOT effects by both pulsed current switching and second harmonic Hall measurements. These experimental approaches evaluate the efficiency of SOT-driven magnetization switching and contribute to quantifying the SOT efficiency of the heterostructure.

The analysis demonstrates that at a current density of approximately $2.5 \times 10^{11} \text{A/m}^2$, SOT can successfully switch the FGT magnetization. This switching process is observed through the anomalous Hall effect (AHE) in FGT. Comprehensive quantitative analysis reveals that the SOT efficiency within the FGT/Pt heterostructure is not only comparable to that of the state-of-the-art heterostructures using traditional three-dimensional ferromagnetic metals but also significantly higher than those employing 3D ferrimagnetic insulators.

This paper's numerical results are compelling, as evidenced by a high switching efficiency parameter exceeding those found in prevalent SOT devices. The damping-like torque efficiency ($\xi_{DL}$) was quantified by rigorous second harmonic Hall measurements that accounted for potential thermoelectric contributions, such as the anomalous Nernst effect. Remarkably, the lower constraint of $\xi_{DL}$ was measured to be 0.11 at 2.2 mA and 0.14 at 2.4 mA, comparable to the highest values recorded in the literature for CoFeB/Pt interfaces.

This paper emphasizes that the excellence of the atomically flat FGT/Pt interface plays a crucial role in the high efficiency of the SOT. Due to the vdW materials' inherently smooth interfaces, there is significant potential for reproducibly achieving high-quality device performance, even down to monolayer thicknesses. Consequently, this research suggests that monolayer FGT with strong PMA could potentially allow for magnetization switching at notably lower current densities, leading to the development of more efficient spintronic nanodevices.

The paper suggests that the persistent high SOT efficiency observed in FGT/Pt heterostructures could be extrapolated to similar vdW magnet-based devices, opening new avenues for spintronic applications. Future investigations may focus on the precise mechanisms underpinning the exceptionally high SOT efficiency and exploring the scalability of these heterostructures in practical device implementations.