Spin effects in transport through single-molecule magnets in the sequential and cotunneling regimes

TL;DR

This paper investigates spin-dependent electron transport in single-molecule magnets, revealing how cotunneling and magnetic interactions influence tunnel magnetoresistance and shot noise in different regimes.

Contribution

It provides a detailed analysis of cotunneling effects and magnetic interactions on transport properties in single-molecule magnets using the real-time diagrammatic technique.

Findings

Cotunneling enhances TMR above the Julliere value in the blockade regime.

Super-Poissonian shot noise occurs due to inelastic cotunneling bunching.

Exchange interaction type significantly affects TMR, with ferromagnetic coupling increasing TMR and antiferromagnetic coupling causing negative TMR.

Abstract

We analyze the stationary spin-dependent transport through a single-molecule magnet weakly coupled to external ferromagnetic leads. Using the real-time diagrammatic technique, we calculate the sequential and cotunneling contributions to current, tunnel magnetoresistance and Fano factor in both linear and nonlinear response regimes. We show that the effects of cotunneling are predominantly visible in the blockade regime and lead to enhancement of tunnel magnetoresistance (TMR) above the Julliere value, which is accompanied with super-Poissonian shot noise due to bunching of inelastic cotunneling processes through different virtual spin states of the molecule. The effects of external magnetic field and the role of type and strength of exchange interaction between the LUMO level and the molecule's spin are also considered. When the exchange coupling is ferromagnetic, we find an enhanced TMR, while in the case of antiferromagnetic coupling we predict a large negative TMR effect.