Magnetic-anisotropy induced spin blockade in a single-molecule transistor

TL;DR

This paper introduces a novel spin blockade mechanism in a single-molecule transistor caused by a change in magnetic anisotropy type upon charging, leading to current suppression due to spin selection rules.

Contribution

It demonstrates how magnetic anisotropy switching in a molecule induces spin blockade, providing new insights into controlling molecular spin states with electrical and magnetic fields.

Findings

Current is suppressed at low bias due to magnetic anisotropy change.

Magnetic levels differ by | M| > 1/2, forbidding transitions.

A magnetic field can lift the blockade depending on its strength and direction.

Abstract

We present a new mechanism for a spin blockade effect associated with a change in the type of magnetic anisotropy over oxidation state in a single molecule transistor, by taking an example of an individual Eu$_{2}$(C$_{8}$H$_{8}$)$_{3}$ molecule weakly coupled to non-magnetic electrodes without linker groups. The molecule switches its magnetization direction from in-plane to out-of-plane when it is charged. In other words, the magnetic anisotropy of the molecule changes from easy plane to easy axis when the molecule is charged. By solving the master equation based on model Hamiltonian, we find that current through the molecule is highly suppressed at low bias independently of gate voltage due to the interplay between spin selection rules and the change in the type of magnetic anisotropy. Transitions between the lowest magnetic levels in successive charge states are forbidden because the magnetic levels differ by $|\Delta M| > 1/2$ due to the change in the type of magnetic anisotropy, although the total spins differ by $|\Delta S|=1/2$. This current suppression can be lifted by significant B field, and the threshold B field varies as a function of the field direction and the strength of magnetic anisotropy. The spin blockade effect shed light on switching the magnetization direction by non-spin-polarized current and on exploring effects of this property coupled to other molecular degrees of freedom.