Absorption of Transverse Spin Current in Ferromagnetic NiCu: Dominance of Bulk Dephasing over Spin-Flip Scattering

TL;DR

This study demonstrates that in ferromagnetic NiCu alloys, transverse spin currents are mainly absorbed through bulk dephasing rather than interface spin-flip scattering, with the coherence length increasing as the exchange field weakens.

Contribution

The paper provides experimental evidence that bulk dephasing dominates transverse spin-current absorption in ferromagnetic NiCu alloys, clarifying the underlying mechanism.

Findings

Coherence length increases with decreasing Curie temperature.

Cu impurities do not reduce spin-transfer torque efficiency.

Bulk dephasing is the primary absorption mechanism in NiCu alloys.

Abstract

In ferromagnetic metals, transverse spin currents are thought to be absorbed via dephasing -- i.e., destructive interference of spins precessing about the strong exchange field. Yet, due to the ultrashort coherence length of $\approx$1 nm in typical ferromagnetic thin films, it is difficult to distinguish dephasing in the bulk from spin-flip scattering at the interface. Here, to assess which mechanism dominates, we examine transverse spin-current absorption in ferromagnetic NiCu alloy films with reduced exchange fields. We observe that the coherence length increases with decreasing Curie temperature, as weaker dephasing in the film bulk slows down spin absorption. Moreover, nonmagnetic Cu impurities do not diminish the efficiency of spin-transfer torque from the absorbed spin current. Our findings affirm that transverse spin current is predominantly absorbed by dephasing inside the nanometer-thick ferromagnetic metals, even with high impurity contents.