Origin of large positive magnetoresistance in the hard-gap regime of epitaxial Co-doped ZnO ferromagnetic semiconductors

We studied the electrical transport properties of single-crystal epitaxial Co-doped ZnO films grown by molecular-beam epitaxy. Upon increasing temperature, transport mechanisms changed from hard-gap resistance to Efros variable range hopping. The magnetic field and the temperature dependence of magnetoresistance revealed that large positive magnetoresistance observed in the hard-gap regime originated from the shrinkage of electron wave functions. However, no sign of spin-dependent transport behavior was found.