Spin transfer torque induced paramagnetic resonance

TL;DR

This paper demonstrates how ac voltage-induced spin-transfer torque can excite paramagnetic resonance in atomic spins on surfaces, providing a universal explanation for observed phenomena via a nonlinear coupling mechanism.

Contribution

It introduces a model using a time-dependent Redfield master equation to explain paramagnetic resonance driven by spin-transfer torque, independent of atomic environment.

Findings

Resonance arises from nonlinear coupling between magnetic moment and spin-polarized current.

Large second-harmonic amplitude detectable in current signal.

Mechanism explains ubiquity of paramagnetic resonance observations.

Abstract

We show how the spin-transfer torque generated by an ac voltage may be used to excite a paramagnetic resonance of an atomic spin deposited on a metallic surface. This mechanism is independent of the environment of the atom and may explain the ubiquity of the paramagnetic resonance reported by Baumann $\textit{et al.}$ [$\href{http://dx.doi.org/10.1126/science.aac8703}{Science \textbf{350}, 417 (2015)}$]. The current and spin dynamics are modeled by a time-dependent Redfield master equation generalized to account for the periodic driven voltage. Our approach shows that the resonance effect is a consequence of the nonlinearity of the coupling between the magnetic moment and the spin-polarized current which generates a large second-harmonic amplitude that can be measured in the current signal.