Ferromagnets are believed to exhibit strongly anisotropic spin relaxation, with relaxation lengths for spin longitudinal to the magnetization significantly longer than those for spin transverse to the magnetization. Here, we characterize the anisotropy of spin relaxation in Co using the spin pumping contribution to Gilbert damping in noncollinearly magnetized ${\mathrm{Py}}_{1-x}{\mathrm{Cu}}_x$/Cu/Co trilayer structures. The static magnetization angle between ${\mathrm{Py}}_{1-x}{\mathrm{Cu}}_x$ and Co, adjusted under field bias perpendicular to film planes, controls the projections of longitudinal and transverse spin current pumped from ${\mathrm{Py}}_{1-x}{\mathrm{Cu}}_x$ 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.

• Received 12 February 2015
• Revised 23 October 2016

DOI:https://doi.org/10.1103/PhysRevB.94.174439