Magnetoresistance of Co-doped ZnO thin films

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.