Organic materials are very interesting for spintronics, since the low spin-orbit coupling and small hyperfine interactions in these materials lead to potentially very long spin relaxation times [1]. The drawback of using organic materials for spintronic applications is the low mobility in this kind of materials. As a solution to this problem, we investigated rubrene single-crystal, with very large mobilities compared to other organic materials. For the first time, we have measured FET structures with ferromagnetic electrodes and a rubrene single-crystal spacer [2]. Our electrodes consist of an aluminum oxide tunnel barrier on top of cobalt. The tunnel barrier can be used to overcome the conductivity mismatch between the metal electrodes and the semiconducting organic spacer and prevents the oxidation of cobalt. Another important feature of this device is that it explores the possibility of fabricating devices with both FET and spin-valve functionality. Such a device has not been realized yet.

We have fabricated several devices with Co/Al₂O₃ electrodes. With our electrode configuration, which is shown in the inset of the figure with electrodes numbered from 1 to 12, we are able to perform 2 and 4-point measurements in the same device. Using the 2-point measurement setup we observe clean FET behaviour in our devices, as can be seen in the figure. The current is contact dominated, as we expect from the tunnel barrier in our contacts. With the 4-point measurement setup we are able to study the length dependence of the resistance. From these measurements, we extract a mobility of 0.281 cm²(Vs)^-1 and a contact resistance of 2800 Wm.

Small electrode separations will be used to perform magnetoresistance measurements to study the spin injection efficiency and spin relaxation in rubrene single-crystal. Our devices are suitable to perform non-local spin-dependent measurements [3].

[1] W.J.M. Naber, S. Faez and W.G. van der Wiel, Organic Spintronics, J. Phys. D: Appl. Phys. 40, R205-R228 (2007).

[2] W.J.M. Naber et al., to be submitted

[3] M. Johnson and R.H. Silsbee, Phys. Rev. Lett. 55, 1790 (1985).

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