Spin Hall Effect in Ni_xCu_1-x Alloys

We report a large spin Hall conductivity, σ_SH, in high resistivity, paramagnetic Ni₆₀Cu₄₀. The value is comparable to what we reported for Pt [1]. Broadband FMR measurements of ferromagnet/nonmagnet bilayers provide an accurate measure of σ_SH, and simultaneously the damping due to spin pumping, when the various contributions to the inductively detected signal are analyzed as described in [2]. These measurements do not require patterning of the bilayer, nor perpendicular anisotropy of the ferromagnet. In a series of samples with the layer structure substrate/Ta(3)/Py(3.5)/Ni_xCu_1-x(d)/Ta(3) (thickness in nm), we varied Ni fraction x for d = 10 nm and varied thickness d for x = 0.6, a composition with a magnetic ordering critical temperature T_C = 140 K. Room temperature FMR measurements with a saturating out-of-plane field showed a substantial σ_SH for all compositions we deposited, 0.3 ≤ x ≤ 0.75, with a maximum at x = 0.7 where T_C = 270 K. For the thickness series at x = 0.6, we use the approach detailed in [1] to extract a spin diffusion length of (7.8 ± 0.5) nm and a spin Hall ratio of 0.81 ± 0.14. Both values are about twice as large as we reported for Pt [1].
[1] Berger, A. J., et al., Phys. Rev. B 98, 024402 (2018).
[2] Berger, A. J., et al., Phys. Rev. B 97, 094407 (2018).