Low temperature tunneling magnetoresistance on $(La,Sr)MnO3∕Co$ junctions with organic spacer layers

This paper concerns with giant magnetoresistance (MR) effects in organic spin valves, which are realized as layered $(La,Sr)MnO3$ (LSMO)-based junctions with tris-(8, hydroxyquinoline) aluminum $(Alq3)$-spacer and ferromagnetic top layers. The experimental work was focused on the understanding of the transport behavior in this type of magnetic switching elements. The device preparation was carried out in an ultrahigh vacuum chamber equipped with a mask changer by evaporation and sputtering on $SrTiO3$ substrates with LSMO stripes deposited by pulsed laser technique. The field and temperature dependences of the MR of the prepared elements are studied. Spin-valve effects at $4.2K$ have been observed in a broad resistance interval from $50Ω$ to $MΩ$ range, however, without systematic dependence on spacer layer thickness and device area. In some samples, the MR changes sign as a function of the bias voltage. The observed similarity in the bias voltages dependences of the MR in comparison with conventional magnetic tunnel junctions with oxide barriers suggests a description of the found effects within the classical tunneling concept. This assumption is also confirmed by a similar switching behavior observed on ferromagnetically contacted carbon nanotube devices. The proposed model implies the realization of the transport via local Co chains embedded in the $Alq3$ layer and spin dependent tunneling over barriers at the interface Co grains∕$Alq3$∕LSMO. The existence of conducting Co chains within the organics is supported by transmission electron microscopic∕electron energy loss spectroscopic studies on cross-sectional samples from analogous layer stacks.