Tunnel magnetoresistance in organic spin valves in the regime of multi-step tunneling

TL;DR

This paper models multistep tunneling in organic spin valves, revealing conditions for negative tunnel magnetoresistance due to interference effects and analyzing how bouncing influences spin memory loss.

Contribution

It introduces a novel interference-based explanation for negative TMR in multistep tunneling and highlights the impact of bouncing on spin memory loss, advancing understanding of organic spin valves.

Findings

Negative TMR occurs in specific three-site configurations.

Interference of spin-flip amplitudes causes TMR sign reversal.

Bouncing between sites significantly affects spin memory loss.

Abstract

A model of a spin valve in which electron transport between the magnetized electrodes is due to multistep tunneling is analyzed. Motivated by recent experiments on organic spin valves, we assume that spin memory loss in the course of transport is due to random hyperfine fields acting on electron while it waits for the next tunneling step. Amazingly, we identify the three-step configurations of sites, for which the tunnel magnetoresistance (TMR) is negative, suggesting that the resistance for antiparallel magnetizations of the electrodes is smaller than for parallel magnetizations. We analyze the phase volume of these configurations with respect to magnitudes and relative orientations of the on-site hyperfine fields. The effect of sign reversal of TMR is exclusively due to interference of the spin-flip amplitudes on each site, it does not emerge within commonly accepted probabilistic description of spin transport. Another feature specific to multistep inelastic tunneling is bouncing of electron between nearest neighbors while awaiting a "hard" hop. We demonstrate that this bouncing, being absolutely insignificant for conduction of current, can strongly affect the spin memory loss. This effect is also of interference origin.