Characteristic features of the extrinsic electric resistance in ferromagnets with low carrier density

Switching from simple semiconductors to more complicated chemical compositi ons, we encounter mainly nonstoichiometric or undoped compounds. Combined w ith other characteristic features of d(f) compounds, this can lead, togethe r with the ordinary scattering by spin disorder in magnetic semiconductors, to an unusual impurity contribution to the total scattering of carriers ev en in intrinsic semiconductors. A unique scheme for calculating the energy structure of the conduction-band bottom of a ferromagnetic semiconductor an d the temperature and field dependences of the impurity contribution to the resistivity is proposed on the basis of a model Hamiltonian. The computed magnetoresistance ratio is negative and has a maximum near T-c. A qualitati ve comparison is made between the results and the experimental temperature dependences of the Hall mobility and magnetoresistance ratio in the ternary semiconductor n-HgCr2Se4, which is nonstoichiometric with respect to the c halcogen. To identify previously unobserved temperature oscillations of the resistance, a careful analysis is made of the low-temperature part of the resistance using the relations obtained. (C) 1999 American Institute of Phy sics. [S1063-7834(99)01701-3].