Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach

TL;DR

This paper investigates spin and charge currents generated by precessing magnetization in nanoscale magnetic tunnel junctions using a nonequilibrium Green function approach, revealing how these currents depend on device configuration and resonance conditions.

Contribution

It introduces a nonequilibrium Green function method to analyze spin and charge pumping in magnetic tunnel junctions with precessing magnetization, including both simple models and realistic devices.

Findings

Pumped spin current can be converted into a measurable dc charge voltage.

The magnitude of the dc voltage is around 1 microvolt at 10 GHz FMR frequency.

Charge currents and voltages are significantly affected by device asymmetry and barrier properties.

Abstract

We study spin and charge currents pumped by precessing magnetization of a single ferromagnetic layer within F|I|N or F|I|F (F-ferromagnet; I-insulator; N-normal-metal) multilayers of nanoscale thickness attached to two normal metal electrodes with no applied bias voltage between them. Both simple one-dimensional model, consisting of a single precessing spin and a potential barrier as the "sample," and realistic three-dimensional devices are investigated. In the rotating reference frame, where the magnetization appears to be static, these junctions are mapped onto a four-terminal dc circuit whose effectively half-metallic ferromagnetic electrodes are biased by the frequency $\hbar \omega/e$ of microwave radiation driving magnetization precession at the ferromagnetic resonance (FMR) conditions. We show that pumped spin current in F|I|F junctions, diminished behind the tunnel barrier and increased in the opposite direction, is filtered into charge current by the second $F$ layer to generate dc pumping voltage of the order of $\sim 1$ $\mu$V (at FMR frequency $\sim 10$ GHz) in an open circuit. In F|I|N devices, several orders of magnitude smaller charge current and the corresponding dc voltage appear concomitantly with the pumped spin current due to barrier induced asymmetry in the transmission coefficients connecting the four electrodes in the rotating frame picture of pumping.