Field-free deterministic switching of all-van der Waals spin-orbit torque system above room temperature

Two-dimensional van der Waals (vdW) magnetic materials hold promise for the development of high-density, energy-efficient spintronic devices for memory and computation. Recent breakthroughs in material discoveries and spin-orbit torque (SOT) control of vdW ferromagnets have opened a path for integration of vdW magnets in commercial spintronic devices. However, a solution for field-free electric control of perpendicular magnetic anisotropy (PMA) vdW magnets at room temperatures, essential for building compact and thermally stable spintronic devices, is still missing. Here, we report a solution for the field-free, deterministic and non-volatile switching of a PMA vdW ferromagnet, Fe3GaTe2 above room temperature (up to 320 K). We use the unconventional out-of-plane anti-damping torque from an adjacent WTe2 layer to enable such switching with a low current density of 2.23 × 106 A cm-2. This study exemplifies the efficacy of low-symmetry vdW materials for spin-orbit torque control of vdW ferromagnets and provides an all-vdW solution for the next generation of scalable and energy-efficient spintronic devices.

Methods
Device Morphology
Thicknesses of the constituent flakes were characterized after encapsulation using a Cypher VRS AFM. Polarized Raman spectra of WTe2 flakes was acquired using a 532 nm laser with a WITec Alpha300 Apyron Confocal Raman microscope, by rotating the polarizer and analyzer while the sample was static.

Transport Measurements
All transport measurements were performed in a 9 T PPMS DynaCool system. Measurements were performed by sourcing current using a Keithley 6221 current source and measuring the transverse voltage across the devices, using a Keithley 2182A nanovoltmeter. Anomalous Hall effect measurements with field sweeps were performed using a drive current of 50 – 200 A. For the current-induced switching measurements, a 1 ms pulse of write-current was followed by 999 ms of read pulses ( 200 A). Field could be applied in and out of the sample plane using the PPMS’ horizontal rotator module.

First Principles Calculations
Electronic properties calculation and structural optimizations were performed using density functional theory (DFT) with the Quantum ESPRESSO package. We use the optimized norm-conserving Vanderbilt (ONCV) pseudopotentials with Perdew-Burke-Ernzerhof (PBE) functionals to account for the exchange-correlation interaction. To accurately describe the structural properties of layered FGaT structure, we use the nonlocal vdW-DF2 functional for the van der Waals interaction. A large plane wave cut-off energy of 60 Ry (~ 816 eV) is used for all calculations. The FGaT monolayer and bilayer are modeled by the slab supercells, with the separations between the neighboring slabs being about 20 Å. A 16×16×1 k-point mesh is used for monolayer and bilayer FGaT, while 14×14×2 mesh is used for the bulk FGaT. These parameters are selected based on the convergence test of the total energy. The atomic positions and lattice constants are optimized by the BFGS quasi-newton algorithm, in which the convergence values for the forces and stress components are 0.0001 Ry/a.u.3 and 0.005 GPa, respectively. To determine the magnetic anisotropy energy (MAE) of FGaT, we first perform the total energy calculations for an in-plane magnetization (along the x-axis) and then out-of-plane magnetization (along the z-axis), including spin-orbit coupling (SOC). Then, the MAE is given by the difference in total energy for the two systems.