It is experimentally established that the magnetoresistance of a Fe_x(SiO_2)_(1-x) nanocomposite (x ≈ 0.6) in a strong magnetic field is described by a logarithmic function of the field strength. This field dependence is inconsistent with the well-known theory of the giant magnetoresistance in ferromagnetic nanocomposites. A model is developed according to which the unusual behavior of the magnetoresistance is explained by nonsphericity of the material grains, exhibiting a broad variety of shapes. The experimental results agree with conclusions and predictions of the proposed model.
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