Characterization of spin relaxation anisotropy in Co using spin pumping

TL;DR

This study investigates the anisotropy of spin relaxation in cobalt using spin pumping, revealing nearly isotropic absorption and a short relaxation length of less than 2 nm, challenging previous measurements.

Contribution

It provides the first characterization of spin relaxation anisotropy in Co using spin pumping, showing nearly isotropic behavior and quantifying the relaxation length.

Findings

Longitudinal spin relaxation length in Co is less than 2 nm.

Spin pumping absorption in Co is nearly isotropic.

Relaxation lengths are much shorter than GMR measurements suggest.

Abstract

Ferromagnets are believed to exhibit strongly anisotropic spin relaxation, with relaxation lengths for spin longitudinal to magnetization significantly longer than those for spin transverse to magnetization. Here we characterize the anisotropy of spin relaxation in Co using the spin pumping contribution to Gilbert damping in noncollinearly magnetized Py$_{1-x}$Cu$_{x}$/Cu/Co trilayer structures. The static magnetization angle between Py$_{1-x}$Cu$_{x}$ and Co, adjusted under field bias perpendicular to film planes, controls the projections of longitudinal and transverse spin current pumped from Py$_{1-x}$Cu$_{x}$ into Co. We find nearly isotropic absorption of pure spin current in Co using this technique; fits to a diffusive transport model yield the longitudinal spin relaxation length $< 2$ nm in Co. The longitudinal spin relaxation lengths found are an order of magnitude smaller than those determined by current-perpendicular-to-planes giant magnetoresistance measurements, but comparable with transverse spin relaxation lengths in Co determined by spin pumping.