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Abstract:
Longitudinal, transverse, and perpendicular magnetoresistances were
investigated at temperatures 1.2 < T < 300 K 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. (C) 2006 American Institute of Physics.
