Bulk and Interface Effects Based on Rashba-like States in Ti and Ru Nanoscale-Thick Films: Implications for Orbital-Charge Conversion in Spintronic Devices

The interplay between spin currents and orbital momentum currents is fundamental in the emerging field of orbitronics, particularly when considering nanoscale features. Unlike spin currents, orbital currents have the potential for long-range propagation through a wide range of materials. Despite being extensively studied, direct observation and manipulation of orbital currents remain a challenge. In this work, employing spin-pumping techniques driven by both ferromagnetic resonance (SP-FMR) and longitudinal spin Seebeck effect (LSSE), we investigated, at nanometric scale, the volume conversion of spin and orbital currents into charge-current in YIG(100 nm)/Pt(2 nm)/NM2 structures, where YIG (yttrium iron garnet, Y 3 Fe 5 O 12 ) is an ferrimagnetic insulator and NM2 represents Ti or Ru. While the YIG/Ti bilayer displayed a negligible SP-FMR signal, the YIG/Pt/Ti structure exhibited a significantly stronger signal, which was attributed to the orbital Hall effect of Ti. Substituting the Ti layer with Ru revealed a similar phenomenon, wherein the effect is ascribed to the combined action of both the spin and orbital Hall effects. Furthermore, we measured the SP-FMR signal in the YIG/Pt(2 nm)/Ru(6 nm)/Ti(6 nm) and YIG/Pt(2 nm)/Ti(6 nm)/Ru(6 nm) heterostructures by simply altering the stack order of Ti and Ru layers, where the peak value of the spin pumping signal is larger for the first sample. We also investigated the surface conversion of spin and orbital currents into charge current, when the NM2 layer is capped by a thin layer of CuO x, and measured giant increases in the SP-FMR and LSSE signals compared to YIG/Pt(2 nm) sample. We investigated our samples using X-ray absorption spectroscopy at the nanometric scale and concluded that samples with natural oxidation of Cu exhibit more significant results than those when CuO x is obtained by reactive sputtering. In particular, samples in which the Cu layer is naturally oxidized exhibit a Cu 2 O-rich phase. Our findings help elucidate the mechanisms underlying the inverse orbital Hall and inverse orbital Rashba-Edelstein-like effects and offer a way to further enhances efficiency in spintronics devices, in particular by utilizing the orbital current.