Generating Unconventional Spin-Orbit Torques With Patterned Phase Gradients in Tungsten Thin Films

Area of Science:

• Spintronics
• Materials Science
• Condensed Matter Physics

Background:

• Current-induced magnetization switching via spin-orbit torques is a key goal in spintronics.
• Achieving this without external magnetic fields typically relies on lateral gradients in ferromagnet/heavy-metal devices.
• Existing methods using growth techniques yield shallow gradients, limiting effectiveness and understanding.

Purpose of the Study:

• To develop a novel method for creating controlled crystalline phase gradients in tungsten thin films.
• To investigate the transformation between tungsten phases (beta and alpha) induced by laser annealing.
• To demonstrate current-induced magnetization switching using these patterned gradients without external magnetic fields.

Main Methods:

• Direct-write laser annealing to pattern crystalline phase gradients in tungsten (W) thin films.
• Transmission electron microscopy (TEM) to analyze film microstructure.
• Resistivity and second harmonic measurements to characterize W phase transformation and properties.
• Integration with Cobalt-Iron-Boron (CoFeB) for spintronic device testing.

Main Results:

• Continuous transformation from beta-phase (high spin-orbit coupling, high resistivity) to alpha-phase (low spin-orbit coupling, low resistivity) tungsten was achieved with varying laser fluence.
• Gradients of varying steepness and arbitrary shapes were successfully patterned in the tungsten phase.
• Magnetization switching in CoFeB was demonstrated using current-induced spin-orbit torques in W films with sufficiently steep phase gradients, without applied magnetic field.

Conclusions:

• Exploiting mixed-phase tungsten microstructures enables precise control over local electronic current density, direction, and spin-orbit torque efficiency.
• This approach provides a new pathway for designing highly efficient spintronic devices.
• Direct-write laser annealing offers a versatile tool for fabricating tailored spin-orbit torque heterostructures.