Field-like spin orbit torque in ultra-thin polycrystalline FeMn films

TL;DR

This study investigates the field-like spin orbit torque in ultra-thin FeMn/Pt bilayers, revealing a large effective field consistent with a spin Hall origin and quantifying a spin diffusion length of 2 nm.

Contribution

It provides the first detailed characterization of field-like spin orbit torque in ultra-thin polycrystalline FeMn films and models the effect using a macro-spin approach.

Findings

Large effective field observed in 2-5 nm FeMn layers.

Spin current absorption confirmed through trilayer experiments.

Estimated spin diffusion length of 2 nm in FeMn.

Abstract

Field-like spin orbit torque in FeMn/Pt bilayers with ultra-thin polycrystalline FeMn has been characterized through planar Hall effect measurements. A large effective field is obtained for FeMn in the thickness range of 2 to 5 nm. The experimental observations can be reasonably accounted for by using a macro-spin model under the assumption that the FeMn layer is composed of two spin sublattices with unequal magnetizations. The large effective field corroborates the spin Hall origin of the effective field considering the much smaller uncompensated net moments in FeMn as compared to NiFe. The effective absorption of spin current by FeMn is further confirmed by the fact that spin current generated by Pt in NiFe/FeMn/Pt trilayers can only travel through the FeMn layer with a thickness of 1 to 4 nm. By quantifying the field-like effective field induced in NiFe, a spin diffusion length of 2 nm is estimated in FeMn, in consistence with values reported in literature by ferromagnetic resonance and spin-pumping experiments.