Transition of Magnetoresistance in Co/Alq₃ Granular Film on Silicon Substrate

A Co_0.38(Alq₃)_0.62 granular film is prepared using a co-evaporating technique on a silicon substrate with a native oxide layer. A crossover of magnetoresistance (MR) from positive to negative is observed. The positive MR ratio reaches 17.5% at room temperature (H=50 kOe), and the negative MR ratio reaches -1.35% at 15K (H=10 kOe). Furthermore, a metal-insulator transition is also observed. The transition of resistance and MR results from the channel switching of electron transport between the upper Co-Alq₃
granular film and the inversion layer underneath. The negative MR originates from the tunneling magnetoresistance effect due to the tunneling conducting between adjacent Co granules, and the positive MR may be attributed to the transport of high mobility carriers in the SiO₂/Si inversion layer.