Absence of orbital current torque in Ta/ferromagnet bilayers

Area of Science:

• Spintronics
• Condensed Matter Physics
• Materials Science

Background:

• The orbital Hall effect (OHE) is theorized to induce non-local orbital currents and spin-orbit torques (SOTs) in adjacent magnetic layers.
• Investigating the OHE's contribution to SOT is crucial for understanding spintronic device mechanisms.

Purpose of the Study:

• To determine if the OHE in tantalum (Ta) generates non-local orbital currents and SOTs in adjacent ferromagnets (FMs).
• To clarify the dominant mechanism responsible for interfacial torque in Ta/FM heterostructures.

Main Methods:

• Utilized spin-torque ferromagnetic resonance (ST-FMR) to analyze SOT efficiency in various Ta/FM (Ni, NiFe, Fe, FeCoB, FePt) systems.
• Investigated the influence of FM layer thickness on the ST-FMR signal and torque generation.

Main Results:

• Observed a consistently negative SOT efficiency across all tested Ta/FM systems, irrespective of the FM material.
• Identified that the spin Hall effect (SHE) of Ta, not the OHE, is the primary contributor to the interfacial torque.
• Found that previously reported positive torque estimates in some studies were artifacts due to neglecting thickness-dependent self-induced ST-FMR signals in the FM layer.

Conclusions:

• The OHE does not induce significant interfacial torque in Ta/FM systems.
• The SHE of Ta is the dominant source of SOT in these heterostructures.
• Accurate characterization of SOT requires careful consideration of self-induced signals within the FM layer.