Quantifying Ultrafast Orbital Pumping and Nanometre-Scale Orbital Diffusion in Magnetic Heterostructures

Abstract

Quantifying ultrafast orbital pumping and transport remains challenging because spin and orbital currents contribute coherently to terahertz emission. Here, we combine spintronic terahertz emission spectroscopy with wedge-sample thickness control to disentangle these contributions in Fe, Co and Ni heterostructures with Ta, W and Nb. Their opposite spin Hall and orbital Hall polarities enable quantitative determination of relative pumping strengths and propagation lengths. We find that orbital pumping increases from Fe to Ni, with inferred orbital-to-spin current ratios reaching several tens of percent in Ni, exceeding current microscopic predictions. Remarkably, Nb exhibits an orbital diffusion length of ~1.5 nm, substantially longer than in Ta and W and beyond the sub-nanometre scale predicted by recent orbital-relaxation calculations. These findings provide quantitative experimental benchmarks for ultrafast orbital pumping and transport.