The accurate measurement of spin orbit torque by utilizing the harmonic longitudinal voltage with Wheatstone bridge structure

Abstract

The microscopic mechanism for the current-induced spin–orbit torque (SOT) in magnetic heterostructures is still under debate. The accurate measurement of SOT effective fields and their thickness dependence is the basis for understanding this issue. In this work, we measured the SOT effective fields for Pt/NiFe bilayers by utilizing the harmonic longitudinal voltage (HLV) method with a Wheatstone bridge structure. Benefiting from the elimination of the linear resistance in the bridge structure and the large magnetoresistance difference resulting from the large length–width ratio of the bridge element, we achieved very high measurement accuracy for both field-like and damping-like effective fields in the Pt/NiFe bilayers. On this basis, we demonstrated the possibility of the SOT measurement with a relatively low current density (∼109 A/m2) by utilizing the HLV method with a Wheatstone bridge structure and found that the method we proposed is also applicable to the Pt/CoFeB system with a low anisotropic magnetoresistance ratio.