Time-Dependent Spintronic Transport and Current-Induced Spin Transfer Torque in Magnetic Tunnel Junctions

TL;DR

This paper investigates how alternating current (AC) influences spintronic transport and spin transfer torque in magnetic tunnel junctions using a time-dependent Green function approach, revealing frequency-dependent effects and conditions for torque enhancement.

Contribution

It introduces a theoretical framework for analyzing AC effects on spin transfer torque in magnetic tunnel junctions, highlighting the frequency-dependent behavior and conditions for torque amplification.

Findings

Electrons tunnel via photonic sidebands under AC bias.

Spin transfer torque can be tuned by varying AC frequency and amplitude.

High frequencies facilitate electron tunneling, low frequencies hinder it.

Abstract

The responses of the electrical current and the current-induced spin transfer torque (CISTT) to an ac bias in addition to a dc bias in a magnetic tunnel junction are investigated by means of the time-dependent nonquilibrium Green function technique. The time-averaged current (time-averaged CISTT) is formulated in the form of a summation of dc current (dc CISTT) multiplied by products of Bessel functions with the energy levels shifted by $m\hbar \omega _{0}$. The tunneling current can be viewed as to happen between the photonic sidebands of the two ferromagnets. The electrons can pass through the barrier easily under high frequencies but difficultly under low frequencies. The tunnel magnetoresistance almost does not vary with an ac field. It is found that the spin transfer torque, still being proportional to the electrical current under an ac bias, can be changed by varying frequency. Low frequencies could yield a rapid decrease of the spin transfer torque, while a large ac signal leads to both decrease of the electrical current and the spin torque. If only an ac bias is present, the spin transfer torque is sharply enhanced at the particular amplitude and frequency of the ac bias. A nearly linear relation between such an amplitude and frequency is observed.