PhD defense – Benjamin Bony

Benjamin Bony will defend his thesis entitled “Orbital torques in transition metals using light metal Cu oxides” on June 11th 2025, at 3:30pm, at the TRT auditorium.

Abstract:

The study of spin currents in materials with strong spin-orbit coupling (SOC), such as 5d transition metals, gave rise to the spin-orbit torque (SOT) research field, used for the switching of SOT-MRAM magnetic memories. Recent advances have shown substantial enhancements of SOT efficiency with the polarization of orbital angular momentum (OAM). This emerging field, known as Orbitronics, holds significant industrial promises for improving the performance spintronic devices. The first experimental demonstrations of orbital polarization were achieved using naturally oxidized copper thin films, revealing an orbital Rashba-Edelstein effect (OREE) at the Cu|CuOx interface.

In this thesis, we experimentally and theoretically investigate the contribution of OAM to magnetic torques. The first study focuses on polycrsystalline Cu|CuOx multilayers fabricated by magnetron sputtering. Second harmonic and magnetoresistance measurements reveal an enhancement of magnetic torque in Co|Pt|Cu|CuOx systems with a maximum for a Pt thickness of 4 nm, and the generation of orbital currents in Co|Cu|CuOx systems. These results are corroborated by spin-pumping measurements demonstrating orbital-to-charge conversion in Cu|CuOx systems. By measuring an increased spin dephasing length in Co, we demonstrate that the enhanced torque in Co|Pt|Cu|CuOx originates from a pure orbital current rather than its conversion into spin current in Pt. To support this, we developed a 5d multi-orbital diffusive model, in which each OAM value is treated as a conduction channel with SOC-assisted scattering, enabling the conversion of the orbital current into spin current as observed experimentally. The second study focuses on the growth of Al₂O₃||Co|Cu|CuOx samples by molecular beam epitaxy. Magnetic and X-ray characterizations were used to determine the hexagonal (0001) structure in the stacks. Second harmonic measurements reveal an anisotropic orbital torque along the crystalline directions. This work opens several promising research perspectives, particularly regarding the mechanisms of orbital-to-spin current conversion, which are still only partially understood.