Decoherence of transverse spin currents in ferrimagnetic CoGd

It has been predicted that the coherence length λ_c of transverse spin current in antiferromagnetic and ferrimagnetic metals can be much longer than λ_c ≈ 1 nm in ferromagnets. A recent experiment reports λ_c > 10 nm in ferrimagnetic CoTb alloys [1], although this is rather surprising given the strong spin-orbit coupling in CoTb. Here, we determine λ_c in ferrimagnetic Co_1-xGd_x alloys with much weaker spin-orbit coupling than CoTb. We perform spin pumping measurements on NiFe/Cu/CoGd trilayers, where a coherent spin current pumped from NiFe decoheres in the CoGd spin sink. The spin sink thickness dependence of Gilbert damping enhancement of the NiFe layer allows us to quantify λ_c. We demonstrate a maximum of λ_c ≈ 4 nm for the composition of CoGd close to magnetic compensation, indicating partial enhancement of spin coherence due to compensated antiferromagnetic order. We also observe a monotonically decreasing effective spin-mixing conductance with increasing Gd content, suggesting the dominance of Co 3d bands in spin current transport across the interface of the ferrimagnet.

[1] J. Yu et al., Nat. Mater. 18, 29 (2019)