Influence of the developing magnetic order on the transport properties of epitaxial Zn_1-xCo_xO films

A series of high quality Zn_1-xCo_xO films prepared by reactive magnetron sputtering displays altered magnetic properties ranging from paramagnetic (PM) to ferromagnetic (FM)-like at room temperature. With tuning the properties of the Zn_1-xCo_xO films from PM- to FM-like, the temperature dependent resistivity of the films changes from a Mott variable-range hopping to an Efros variable-range hopping process. This indicates the emergence of a Coulomb gap, which can be explained by the presence of nanoclusters. Moreover, the anhysteretic M (H)-curves above the blocking temperature for superparamagnetic (SPM) films can be described by a Langevin function, which confirms the existence of FM nanoclusters. The investigation on the magneto-transport of these films shows no spin-dependent transport behaviour associated with the observed FM-like/SPM properties.