Vertical organic spin valves in perpendicular magnetic fields

TL;DR

This study investigates magnetoresistance in vertical organic spin valves under perpendicular magnetic fields, finding no Hanle precession effects, suggesting the magnetoresistance arises from tunneling mechanisms rather than spin injection.

Contribution

It provides experimental evidence that magnetoresistance in these devices is dominated by tunneling effects, challenging the assumption of spin injection as the primary mechanism.

Findings

No Hanle precession observed in perpendicular fields.

Magnetoresistance attributed to tunneling through pinholes.

Results suggest tunneling, not spin injection, governs magnetoresistance.

Abstract

We report the results of magnetoresistance measurements in vertical organic spin valves with the magnetic field oriented perpendicular to the layer stack. The magnetoresistance measurements were performed after carefully preparing either parallel or antiparallel in-plane magnetization states of the magnetic electrodes in order to observe traces of Hanle precession. Due to the low mobility in organic semiconductors the transit time of spin polarized carriers should allow for precession of the spins in perpendicular fields which in statistical average would quench the magnetoresistance. However, in none of the experiments we do observe any change in resistance while sweeping the perpendicular field, up to the point where the electrode's magnetization starts to reorient. This absence of Hanle type effects indicates that the magnetoresistance is not based on the injection of spin polarized electrons into the organic semiconductor but rather on tunneling through pinholes superimposed with tunneling anisotropic magnetoresistance.