Measurement of the Spin-Transfer-Torque Vector in Magnetic Tunnel Junctions

TL;DR

This paper presents direct measurements of the magnitude and direction of spin-transfer torque in magnetic tunnel junctions, revealing in-plane and out-of-plane components and their bias dependence, crucial for spintronic device development.

Contribution

It provides the first direct measurement of both the magnitude and direction of spin torque in MTJs, including the bias-dependent out-of-plane component.

Findings

In-plane torque aligns with spin-polarized tunneling predictions.

Out-of-plane torque component increases with bias, reaching 30% of in-plane torque.

Differential torque remains large even as magnetoresistance decreases.

Abstract

Spin-polarized currents can transfer spin angular momentum to a ferromagnet, generating a torque that can efficiently reorient its magnetization. Achieving quantitative measurements of the spin-transfer-torque vector in magnetic tunnel junctions (MTJs) is important for understanding fundamental mechanisms affecting spin-dependent tunneling, and for developing magnetic memories and nanoscale microwave oscillators. Here we present direct measurements of both the magnitude and direction of the spin torque in Co60Fe20B20/MgO/Co60Fe20B20 MTJs. At low bias V, the differential torque vector d{tau}/dV lies in the plane defined by the electrode magnetizations, and its magnitude is in excellent agreement with a prediction for highly-spin-polarized tunneling. With increasing bias, the in-plane component d{tau}_{parallel}/dV remains large, in striking contrast to the decreasing magnetoresistance ratio. The differential torque vector also rotates out of the plane under bias; we measure a perpendicular component tau_{perp}(V) with bias dependence proportional to V^2 for low V, that becomes as large as 30% of the in-plane torque.