Magnetoresistance in organic spintronic devices: the role of nonlinear effects

TL;DR

This paper develops a kinetic model for organic spintronic devices showing that nonlinear charge transport causes voltage-dependent variations in magnetoresistance, revealing three distinct regimes including a novel high-voltage behavior.

Contribution

It introduces a kinetic equation framework that predicts how nonlinear effects influence magnetoresistance in organic semiconductors, highlighting a new high-voltage regime.

Findings

Magnetoresistance varies strongly with voltage in organic devices.

Three regimes of magnetoresistance behavior are identified.

Nonlinear charge transport can enhance or suppress the spin valve effect.

Abstract

We derive kinetic equations describing injection and transport of spin polarized carriers in organic semiconductors with hopping conductivity via an impurity level. The model predicts a strongly voltage dependent magnetoresistance, defined as resistance variation between devices with parallel and antiparallel electrode magnetizations (spin valve effect). The voltage dependence of the magnetoresistance splits into three distinct regimes. The first regime matches well known inorganic spintronic regimes, corresponding to barrier controlled spin injection or the well known conductivity mismatch case. The second regime at intermediate voltages corresponds to strongly suppressed magnetoresistance. The third regime develops at higher voltages and accounts for a novel paradigm. It is promoted by the strong non-linearity in the charge transport which strength is characterized by the dimensionless parameter $eU/k_BT$. This nonlinearity, depending on device conditions, can lead to both significant enhancement or to exponential suppression of the spin valve effect in organic devices. We believe that these predictions are valid beyond the case of organic semiconductors and should be considered for any material characterized by strongly non-linear charge transport.