Role of Interfacial States in Alq3-based Organic Spin Valves

Valentin A. Dediu 1Ilaria Bergenti 1Alberto Riminucci 1Mirko Prezioso 1Patrizio Graziosi 1Donald MacLaren 2John Chapman 2Francesca Casoli 3

1. Istituto per lo Studio dei Materiali Nanostrutturati (ISMN-CNR), Gobetti 101, Bologna 40129, Italy
2. University of Glasgow, Department of Physics and Astronomy, Glasgow, United Kingdom
3. IMEM- CNR (IMEM), Parma, Italy

Abstract

Applications of organic semiconductors into spintronic domain, motivated by unquestionable advantage of weak spin-orbit coupling, have been recently successfully demonstrated; in particular magnetoresistive spin valves (SV) and magnetic tunnel junction (MTJ) based on archetypal organic materials Alq3 have been experimentally obtained.In spite of the different mechanism, injection/transport for SV and tunnelling for MTJ, performances of such devices are affected not just by the properties of its constituent but also by their metal/organic interfaces.Considerable interest has been devoted to the investigation of Alq3 based vertical spin valves containing half metallic manganite (LSMO) as bottom electrode and Cobalt as the top one.While the deposition of Alq3 on LSMO give rise to a well defined interface which energetics has been investigated, it has been claimed by several authors that the Co/alq3 interface is intrinsically “ill defined” and transport properties are strongly affected by the Co clusters that can be found inside the organic material down to 50 nm from the surface.The introduction of a thin (1-2 nm) Al2O3 barrier between the Alq3 and the Co results in a sharper definition of the metal/organic interface. From X-ray reflectivity measurements, the intermixing region at the interface between Alq3 and Co can be estimated to be around 2.5 nm, while the direct interface suffers from a thicker degradation layer. These results have been confirmed by cross Sectional TEM and High Energy Photoemission Spectroscopy (HAXPES) measurements, which indicate that the tunnel barrier prevents also the reaction of Co with the organic molecules. Scarce control of interfacial layer for the direct deposition of Co on top of Alq3 gives rise to a variety of spurious effect on the device operation such as high closure field and not uniform and defined thickness of organic layer. In this latter condition, the estimation of length over which coherent spin transport is achieved is hardly reproducible. We present a full characterization of LSMO/Alq3 and Alq3/Co interfaces in presence of buffer insulating layer and we correlate the results with the operation of spin valve device. The presented results clearly indicate that extension of hybrid spintronic devices to practical application requires the engineering of interfaces between organic and the spin polarized materials.

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Presentation: Poster at E-MRS Fall Meeting 2009, Symposium E, by Ilaria Bergenti
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-22 21:36
Revised:   2009-09-04 13:54
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