Quantiative X-ray Imaging of Spin Accumulation due to the Spin Hall Effect in Ni<i>_x</i>Cu_1-<i>x</i> Nanowires

We recently reported a spin Hall ratio θ_SH = 1.05 ± 0.18 in Ni₆₀Cu₄₀, an alloy that is nonmagnetic at room temperature, based on FMR measurements of unpatterned Ni₈₀Fe₂₀ films deposited in a bilayer with the alloy [1]. Like previous studies of spin Hall effect (SHE) in heavy metals such as Pt and W, this work inferred spin accumulation indirectly, in this case from its effect on the FMR resonance. An alloy containing Ni enables scanning transmission x-ray microscopy (STXM) to directly image spin accumulation using circularly polarized x-rays tuned to the Ni L3 absorption edge. Importantly, this technique can quantify the spin accumulation without resorting to a theoretical model of spin transport. In wires of Ni₆₀Cu₃₅Al₅ with a cross section ≈ 80 nm x 30 nm and driven at a current density ≈ 5 x 10¹¹ A/m², fabricated on Si₃N₄ membranes, we observed spin polarization along each wire edge in normal-incidence STXM images. The amount of spin accumulation is consistent with θ_SH ≈ 1 and it obeys three symmetries expected for SHE measured via STXM: 1) opposite sign on opposite edges of the wire, 2) opposite sign with reversal of the driving current, and 3) opposite sign with opposite x-ray helicity.
[1] M.W. Keller et al., Phys. Rev. B 99, 214411 (2019).