Magnetoresistance in CuPc Based Organic Magnetic Tunnel Junctions

We investigate the magnetoresistance of exchange biased spin valves with hybrid barrier layers, MgO(2 nm)/CuPc (t = 0, 1, 2, nm) where CuPc is the organic semiconductor copper phthalocyanine, which are patterned into micron size tunnel junctions by UV lithography. The stacks are produced by sputtering in a Shamrock tool and the organic layer is subsequently deposited by thermal evaporation. Devices in the first set showed 22 %, 10 % and 5% MR at 300 K for t = 0, 1 and 2 nm, respectively. These values increased up to 33 %, 28 % and 23% for the same junctions at 50 K. From these data we infer that the spin polarization is reduced by CuPc in the barrier and the spin scattering by Cu2+ is strongly temperature-dependent. By comparison, if an Alq3 layer is used, where there are no paramagnetic scattering centres, the magnetoresistance is independent of the thickness of the organic layer, at least up to 8 nm.