Long-Range Orbital Torque by Momentum-Space Hotspots.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Spintronics
• Quantum Mechanics

Background:

• Orbital response in ferromagnets is typically assumed to be short-ranged due to crystal field effects and orbital quenching.
• Spin injection in ferromagnet/nonmagnet bilayers leads to spin accumulation and torque, which decay rapidly due to spin dephasing.

Purpose of the Study:

• To investigate the range and nature of orbital response in ferromagnets, particularly when an electric field is applied to an adjacent nonmagnetic layer.
• To explore the underlying mechanisms responsible for long-range orbital effects and their potential implications for device applications.

Main Methods:

• Theoretical investigation of a ferromagnet/nonmagnet bilayer system.
• Analysis of induced orbital angular momentum and torque under external electric fields applied to the nonmagnetic layer.
• Examination of the role of crystal symmetry and electronic band structure in mediating orbital transport.

Main Results:

• Demonstrated substantially long-ranged induced orbital angular momentum in the ferromagnet, exceeding the spin dephasing length.
• Attributed this long-range behavior to nearly degenerate orbital characters imposed by crystal symmetry, creating 'hotspots' for orbital response.
• Identified a distinct type of orbital torque that increases with ferromagnet thickness and does not suffer from destructive interference.

Conclusions:

• Orbital response in ferromagnets can be remarkably long-ranged, contrary to prevailing assumptions.
• The findings provide critical evidence for orbital transport and suggest a new mechanism for generating orbital torque.
• This long-range orbital response holds significant potential for future orbitronic device applications.