Uniaxial Magnetic Anisotropy and Type-X/Y Current-Induced Magnetization Switching in Oblique-Angle-Deposited Ta/CoFeB/Pt and W/CoFeB/Pt Heterostructures

TL;DR

This work addresses field-free, current-induced magnetization switching in heavy-metal/CoFeB/Pt trilayers by engineering in-plane uniaxial anisotropy through oblique-angle deposition. It employs USMR (type Y) and DC planar Hall effect (types X and XY) to electrically detect switching in sub-microsecond pulses, achieving low switching currents, notably $j_c \approx 2\times10^{11}$ A/m$^2$ in W/CoFeB/Pt. The results show macrospin-like switching for type Y, while type X and XY switching involve nucleation and domain-wall propagation, respectively, enabling efficient, field-free operation. The findings highlight the potential of combining oblique deposition with easy-axis engineering for low-power, fast spintronic devices and MRAM/Circuits. The work also emphasizes the importance of accounting for magnetic dead layers and provides quantitative SOT efficiencies in these trilayer systems.

Abstract

Planar current-induced magnetization switching (CIMS) driven by spin-orbit torque (SOT) requires an in-plane uniaxial magnetic anisotropy (UMA), which can be induced by oblique-angle sputter deposition of the heavy-metal underlayer in heavy-metal/ferromagnet heterostructures. To enhance the SOT efficiency, we employ trilayer heterostructures of (Ta or W)/CoFeB/Pt, where the CoFeB layer exhibits a UMA of 50 mT at 2 nm thickness of Ta or W. The magnetization reversal in Hall-bar devices is detected through unidirectional spin Hall magnetoresistance (USMR) for the type Y geometry (easy-axis transverse to current) and planar Hall measurements for the type X geometry (easy-axis parallel to current). Both configurations exhibit CIMS with sub-microsecond current pulses, reaching switching current densities as low as $2 \times 10^{11}$ A/m$^2$ for a W (4 nm)/CoFeB (1.4 nm)/Pt (2 nm) stack with a UMA of 146 mT. Macrospin simulations reproduce the type Y switching as coherent magnetization rotation, whereas the type X devices switch at much lower currents than predicted, indicating that nucleation and domain-wall propagation dominate reversal in this geometry. Our results show that combining oblique-angle deposition with easy-axis engineering enables deterministic, field-free switching, paving the way for future low-power spintronic devices.

Figures (8)

• Figure 1: Sample deposition geometry and CIMS measurement setup. (a) W/Ta underlayer deposited at $\theta = 60^{\circ}$. (b) Samples were marked along the HA direction. The EA is parallel to the microscopic ripple structureMcMichael2000. (c) In the Hall cross, write current is injected with opposite polarities ($\pm V_P/2$), generating a virtual ground at the device center. This configuration effectively avoids current shunting into the transverse voltage probes and enforces write current propagation along the longitudinal current channel. (d) Schematic illustration of type Y, type X and type XY switching scheme with current channel width $5\,\mu \text{m}$. In-plane magnetic field $H_{\mathrm{y}}$ is applied transverse to the current channel in type Y, $H_{\mathrm{x}}$ is applied parallel to current channel in type X, and at $45^{\circ}$ in type XY. The measurement sequence of $\Delta R_{\mathrm{xx}}$ and $\Delta R_{\mathrm{yx}}$ is also shown.
• Figure 2: AMR and L-MOKE magnetic hyteresis loops measurement. (a) Field sweep AMR measurements for device Ta/CFB(2)/Pt by applying the sense current ($I_{\mathrm{sense}} = \pm 3\text{\,mA}$) transverse to the in-plane applied field ($I_{\mathrm{sense}}\parallel \text{EA}$). (b) Magnetic hysteresis loop recorded using L-MOKE while applying the in-plane external magnetic field along the hard axis. The residual hysteresis indicates imperfect alignment of the sample in the L-MOKE setup.
• Figure 3: Type Y, type X and type XY SOT switching behaviours in Ta/CFB(2)/Pt based on DC USMR and PHE: (a,b) field sweep scan and field-free current-induced switching for type Y (DC USMR), (c,d) Field sweep scan and field ($H_{\mathrm{z}}$) assisted current-induced switching for type X (DC PHE), (e,f) Field sweep scan and field-free current induced switching in type XY (DC PHE).
• Figure 4: Type Y, type X and type XY SOT switching behaviours in Ta/CFB(t$_\mathrm{FM}$)/Pt based on DC USMR and PHE: Thickness dependence of (a) $R^{\mathrm{USMR}}$ for type Y at $I_{\mathrm{sense}}=\pm3\,\text{mA}$, (b) average critical switching current ($I_c$) for type Y at $I_{\mathrm{sense}}=\pm3\,\text{mA}$, (c) $I_c$ for type X at $I_{\mathrm{sense}}=\pm2\,\text{mA}$, (d) $I_c$ for type XY at $I_{\mathrm{sense}}=\pm2\,\text{mA}$, The MDL thicknesses are indicated by the arrows.
• Figure 5: Type Y, type X and type XY SOT switching behaviours in $\mathrm{W (4)/CFB(1.4)/Pt}$ based on USMR and DC PHE: (a) Field sweep AMR measurements by applying the sense current ($I_{\mathrm{sense}}=\pm3\,\text{mA}$) transverse to the in-plane applied field ($I_{\mathrm{sense}}\parallel \text{EA}$), (b,c) field sweep scan and field free current induced switching for type Y (USMR), (d,e) Field sweep scan and field free current-induced switching for type X (DC PHE), (f,g) Field sweep and field-free current-induced switching in type XY (DC PHE) for different sense ($I_{\mathrm{sense}}$) currents.