Abstract
Magnetic and magneto-transport properties of granular ferromagnetic Cox-(Al2O3)1-x films with a large range of cobalt concentrations x are investigated. The films with x below the percolation threshold xp demonstrate the presence of oriented uniaxial anisotropy of the granules with the anisotropy field Ha ∼ 2 kOe and easy anisotropy axis perpendicular to the film plane. It is shown that for the values of x close but below xp, field dependencies of the magnetoresistance in the magnetic fields applied in the film plane, possess an anomalous field dependence with a positive magnetoresistance and a maximum in the range of field magnitudes 0 < H < Ha. The positive magnetoresistance effect is isotropic in the film plane and reduces as the field direction is rotated out of the film plane up to the complete quenching for the field perpendicular to the film plane. The effect is not related to an anisotropic magnetoresistance manifestation and is temperature-dependent. We propose a model, which explains the phenomenon and bases on an accounting of thermal fluctuations of uniaxial particle magnetic moments in the fields perpendicular to the particle easy anisotropy axis in the presence of the ferromagnetic interaction between the particles.
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