Transformation of the electrical transport from the Efros and Shklovskii ͓J. Phys. C 8, L49 ͑1975͔͒ variable range hopping to the "hard gap" resistance was experimentally observed in a low temperature range as the Fe compositions in Zn 1−x Fe x O 1−v ferromagnetic semiconductor films increase. A universal form of the resistance versus temperature, i.e., ϰ exp͓T H / T + ͑T ES / T͒ 1/2 ͔, was theoretically established to describe the experimental transport phenomena by taking into account the electron-electron Coulomb interaction, spin-spin exchange interaction, and hard gap energy. The spin polarization ratio, hard gap energy, and ratio of exchange interaction to Coulomb interaction were obtained by fitting the theoretical model to the experimental results. Moreover, the experimental magnetoresistance was also explained by the electrical transport model.

The polar Kerr rotation and ellipticity spectra of the as-deposited and annealed Zn 1−x Co x O inhomogeneous magnetic semiconductors were measured. The Kerr rotation spectra versus the photon energy can be greatly modulated by adjusting the Co concentration or annealing the samples. Moreover, the observed maximal Kerr rotation, 0.72°, in an annealed sample is higher than that of pure Co films, Pt/ Co multilayers, and Pt x Co 1−x alloys. The enhanced Kerr rotation in the annealed samples can be explained by the fact that the annealed samples became a nanocomposite system consisting of Co clusters and Zn 1−x Co x O magnetic semiconductor.

( Co x Al 1 − x ) 2 O 3 − v oxide magnetic semiconductor films were synthesized by introducing an impurity band in the insulating Al2O3 band gap, and the corresponding granular films were obtained by annealing. For both kinds of films, their electrical transport properties are well described by spin dependent variable range hopping mechanism instead of the usually expected intergrain tunnelling. The magnetoresistance was also discussed.

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