Generating unconventional spin-orbit torques with patterned phase gradients in tungsten thin films

Technical Summary: Generating Unconventional Spin-Orbit Torques with Patterned Phase Gradients in Tungsten Thin Films

Problem Statement
A primary objective in spintronics is achieving current-induced magnetization switching via spin-orbit torques (SOT) without the need for an external magnetic field. While SOT-based magnetic random access memory (SOT-MRAM) offers high speed and endurance, current implementations are hindered by the requirement of an applied field for deterministic switching, which complicates industrial integration. Breaking the mirror symmetry of the device along the current direction is necessary for field-free switching. Previous approaches, such as oblique deposition, vertical composition gradients, or ion irradiation, often suffer from limited scalability or difficulty in implementation. Furthermore, existing methods for creating lateral gradients in ferromagnet/heavy-metal devices typically rely on growth techniques that produce small gradients, limiting both the impact of the approach and the fundamental understanding of the underlying physical mechanisms.

Methodology
The authors utilize direct-write laser annealing (DWLA) to locally pattern the crystalline phase of tungsten (W) thin films on a mesoscopic length scale. Tungsten exists in two distinct phases: the metastable, high-resistivity, high spin-orbit coupling $\beta$-W phase (A15 structure), and the stable, low-resistivity, low spin-orbit coupling $\alpha$-W phase (body-centered cubic).

1. Fabrication: Metastable $\beta$-W films (5–15 nm thick) were grown via magnetron sputtering.
2. Phase Transformation: The films were subjected to DWLA with varying laser fluences (ranging from 0 to 31 J cm$^{-2}$ depending on the substrate). This process induces a local, continuous transformation from the $\beta$-phase to the $\alpha$-phase.
3. Characterization:
   • Structural: Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) were used to track grain size evolution and phase composition along the laser fluence gradient.
   • Electronic: Resistivity measurements were performed on Hall bar devices to quantify the transition from $\beta$-W to $\alpha$-W.
   • Spintronic: Second harmonic Hall resistance measurements were conducted on W/CoFeB bilayers to determine the damping-like torque efficiency ($\xi^j_{DL}$) as a function of the $\alpha$-W fraction.
   • Thermal/Current Mapping: Spatially resolved thermoreflectance measurements were used to visualize current density distributions in devices with phase gradients.
4. Switching Experiments: Field-free SOT switching was tested in W/CoFeB bilayers where a lateral phase gradient (transverse to the current direction) was introduced. Hysteresis loop shift measurements were performed to detect unconventional torques.

Key Results

• Microstructural Control: TEM analysis revealed that increasing laser fluence transforms the film from a uniform $\beta$-phase with small grains (5–20 nm) to a mixed phase, and finally to a dominant $\alpha$-phase with large grains (80–120 nm). SAED confirmed a smooth transition from $\beta$-W diffraction peaks to $\alpha$-W peaks, indicating a mixed-phase region with bimodal grain distribution.
• Electronic Properties: The film resistivity ($\rho_W$) decreased significantly from $\sim$230 $\mu\Omega$-cm (pure $\beta$-W) to $\sim$30 $\mu\Omega$-cm (pure $\alpha$-W) as the laser fluence increased. Devices with phase gradients created by DWLA demonstrated long-term stability over several months, unlike sputtered mixed-phase films which tend to transform rapidly.
• Spin-Orbit Torque Efficiency: The damping-like torque efficiency per current ($\xi^j_{DL}$) decreased from $-0.45 \pm 0.01$ for pure $\beta$-W to $-0.07 \pm 0.01$ for 95% $\alpha$-W. This confirms that the local spin-orbit torque efficiency can be precisely tuned by controlling the phase fraction.
• Current Density Modulation: Thermoreflectance measurements showed that in devices with a phase gradient, the current flow is highly non-uniform. Due to the lower resistivity of $\alpha$-W, the majority of the current is channeled through regions with a higher $\alpha$-W fraction.
• Field-Free Switching: W/CoFeB devices with a transverse phase gradient (0% to 20% $\alpha$-W over 10 $\mu$m) achieved deterministic magnetization switching without an applied magnetic field at a current density of $1.4 \times 10^{11}$ A/m$^2$. In contrast, uniform $\beta$-W devices required an in-plane magnetic field to switch.
• Unconventional Torques: Hysteresis loop shift measurements revealed a non-zero effective field ($\mu_0 H^{\text{eff}}_z$) in gradient devices, which increased with current density. This indicates the generation of unconventional torques (likely $z$-polarized spins) that break the mirror symmetry. Devices with steeper gradients exhibited lower current thresholds for this effect.

Significance and Claims
The paper claims that exploiting the unique microstructure of mixed-phase tungsten allows for the precise control of local electronic current density, direction, and spin-orbit torque efficiency. By patterning phase gradients, the authors demonstrate a new avenue for designing efficient spintronic devices that can switch magnetization without external magnetic fields.

The authors posit that the field-free switching is driven by unconventional torques arising from broken electronic symmetries or microstructural asymmetries (specifically, lateral interfaces between $\alpha$ and $\beta$ grains with different spin Hall conductivities), rather than solely from current density or anisotropy gradients, as micromagnetic simulations incorporating only the latter failed to reproduce deterministic switching.

The work provides a tunable platform to study the physical mechanisms of field-free switching in polycrystalline systems with broken structural symmetry. The authors suggest that combining field-free switching with non-uniform current directions could lead to ultrahigh efficiency in SOT switching, offering a pathway toward energy-efficient spintronic devices. Additionally, the ability to locally tune the crystalline phase of tungsten offers potential applications beyond spintronics, given tungsten's relevance in superconductivity, solar energy, and microelectronics.