Nanosized perpendicular organic spin-valves

Abstract

A fabrication process for vertical organic spin-valve devices has been developed which offers the possibility to achieve active device areas of less than 500x500 nm² and is flexible in terms of material choice for the active layers. Characterization of the resulting devices shows a large magnetoresistance of sometimes more than 100 %, however with equally large variation from device to device. Comparison with large-area spin-valves indicates that the magnetoresistance of both, large and small devices most likely originates from tunneling through pinholes and tunneling magnetoresistance.Organic spin-valve devices are promising candidates for low cost non-volatile electronics, for example for RF-ID tags.1 Nevertheless, a deeper understanding of the underlying device physics is still necessary, especially of the charge transport mechanisms through the organic material and the origin of the magnetoresistance. In vertical organic spin-valves (OSV) an organic semiconductor (OSC) thin film is sandwiched between two ferromagnetic electrodes and the device is operated by a current flow perpendicular through the layers.2 Similar to giant magnetoresistance (GMR) 2-5 and tunneling magnetoresistance (TMR) 6-9 the device resistance depends on the relative magnetization of the two electrodes. However, in contrast to GMR in all-metal structures or TMR in metal/oxide structures it is still an open question whether the origin of the magnetoresistance (MR) effects in OSVs published so far is dominated by spin polarized charge transport through the organic spacer layer or by tunneling processes. 10,11 There is, however, a possible approach to gain deeper insight into this problem which takes into account the scaling of the MR effect with device size. In the case of spin injection and charge transport (GMR) the complete device area is expected to contribute to the current path through the OSC. Reducing the size of the device in this case should keep the resistance area product and the relative magnetoresistance constant. Tunneling processes, however, most likely occur in just a small part of the active area.12 Most probable candidates for tunneling sites would be pinholes at which the thickness of the OSC is reduced to such an extent that tunneling and thus TMR becomes possible. [8][9][10][11][12][13] Pinholes, however, follow certain statistics resulting in a different resistance change upon downscaling. They vary in size and also exhibit a varying thickness of the remaining OSC layer. Each pinhole thus contributes to the total device characteristics by a different resistance and MR contribution. In order to appreciate the necessary density of pinholes we can use results published by Barraud and coworkers.14 They create artificial pinholes in an OSC layer using conducting tip atomic force microscopy (AFM) and study the magnetoresistance after the pinhole is filled up with a ferromagnetic metal. The resistance of the pinholes is typically in the range of 100 MΩ while the magnetoresistance can be as high as 300%. In addition, they ob...

Resistive switching and voltage induced modulation of tunneling magnetoresistance in nanosized perpendicular organic spin valves

“…In recently published experiments we have already identified the origin of the magnetoresistance in our devices as tunneling via pinholes through the Alq3 layer resulting in TMR. 6,7 Also with respect to the interplay of RS and MR they differ considerably from the OSVs by Prezioso et al as we do not only observe a change in magnitude but also a sign change of the MR.All samples are fabricated using our recently reported technique 7 which allows to define perpendicular OSVs with nanosized active device area. The samples consist of 7 to 14 devices, respectively, with a layer stack of LSMO/Alq3/MgO/Co/Ru (thicknesses 20/12/3/30/10 nm, respectively).…”

Tuning the effective spin-orbit coupling in molecular semiconductors

“…In addition, organic spin valve measurements can be affected by tunnelling magneto resistance through pin hole defects10, which complicates an accurate determintion of λ S .…”

“…The comparison of spin diffusion lengths λ S on the other hand has indeed revealed significant variations between different organic semiconductors 5 7 8 but separating effects of the charge carrier mobility and density 9 from the spin's coupling to its environment remains challenging. In addition, organic spin valve measurements can be affected by tunnelling magneto resistance through pin hole defects 10 , which complicates an accurate determintion of λ S .…”

Spin Transport in Organic Semiconductors

“…The observation of GMR in Alq3 based vertical spin valve in 2004 triggered intense research into spin transport in a variety of organic semiconductors using two terminal spin valve approach. However, much scepticism arises in recent years as to whether this measurement approach is an adequate method to probe spin transport in organic semiconductors since such magnetoresistance response may originate from pin-holes, tunnelling through hot spots and tunnelling anisotropic magnetoresistance associated with LSMO ferromagnetic contacts [39], [131], [132], [74], [40]. Therefore, it is necessary to find an artefact-free technique to probe spin transport in organic semiconductors.…”