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.