Abstract
Longitudinal, transverse, and perpendicular magnetoresistances were investigated at temperatures 1.2<T<300K in fields of up to 20 T for four ZnO films: one undoped, two with 4% Co (one paramagnetic, the other exhibiting anhysteretic ferromagnetism at room temperature), and one with 25% Co which exhibits hysteretic ferromagnetism, butterfly magnetoresistance, and presence of Co clusters. The magnetoresistance becomes negligibly small above 50–100 K, and magnetic terms are only evident below 20 K, where the mobility is sufficiently high. Quantum oscillations observed below 2 K in the paramagnetic sample with 4% Co give two different extremal Fermi surface cross sections. The data for dilute ferromagnetic samples are consistent with coherent spin transport in a ferromagnetic matrix; data for the x=25% films suggest that spin-polarized electrons can tunnel coherently between well-separated cobalt clusters.
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