We calculate the magnetoresistance and magnetothermopower of magnetic granular solids. Contrary to previous theories of the giant magnetoresistance (GMR), we demonstrate that the unfilled d bands of magnetic grains play an essential role in the transport properties of these systems. Our results relate GMR and magnetothermopower to microscopic and geometric quantities, and provide a natural explanation for many experimentally observed features, such as the (M/${\mathit{M}}_{\mathit{s}}$${)}^2$ dependence of the GMR, the giant magnetothermopower and its 1/ρ scaling behavior, and the absence of negative GMR in rare-earth–nonmagnetic structures.

• Received 16 June 1993

DOI:https://doi.org/10.1103/PhysRevB.48.6728