Intrinsic Torques Emerging from Anomalous Velocity in Magnetic Textures

Area of Science:

• Condensed matter physics
• Materials science
• Spintronics

Background:

• Electron momentum-space topology, particularly under strong spin-orbit coupling, is crucial for understanding electrically induced torques on magnetic textures.
• These torques exhibit robustness against disorder and thermal fluctuations, suggesting a fundamental origin.
• Existing theories often link torques to transport currents, but intrinsic mechanisms are also significant.

Purpose of the Study:

• To establish a direct phenomenological connection between electron band topology and electrically induced torques.
• To identify and characterize a novel intrinsic torque mechanism arising from anomalous electron velocity.
• To investigate the role of this new torque in magnetic materials, particularly ferromagnetic SrRuO3.

Main Methods:

• Phenomenological classification of electrically induced torques.
• Theoretical analysis of anomalous electron velocity and its contribution to torques.
• Numerical simulations to compare the novel torque with conventional spin-transfer torque.

Main Results:

• A direct link between band topology and torques is established.
• Torques intrinsically emerge from anomalous electron velocity, independent of nonequilibrium transport currents.
• A new intrinsic contribution, termed "topological Hall torque" (THT), is identified, originating solely from magnetic textures in bulk crystals.
• Numerical simulations show THT enhancement over conventional spin-transfer torque in bulk metallic ferromagnets.

Conclusions:

• The topological Hall torque (THT) offers a new paradigm for understanding electrically induced torques in magnetic materials.
• THT's intrinsic nature and enhancement in bulk ferromagnets, like SrRuO3, explain experimentally observed giant torques.
• This finding opens new avenues for spintronic device applications leveraging band topology.