Static and dynamic properties of the insulator-metal transition in magnetic semiconductors, including the perovskites

All magnetic semiconductors experience an insulator-metal transition with d ecreasing temperature near and below the magnetic ordering temperature. T-c , as long as the carrier concentration, whether electrons or holes, is low enough. For somewhat higher carrier concentrations, a resistivity anomaly o rdinarily occurs near T-c, which cannot be explained in terms of scattering from the magnetic fluctuations alone. The thrust of this paper is to revie w the magnetotransport properties of various magnetic semiconductors and to present arguments that magnetic polarons, a many body state involving char ge carriers and the localized magnetic spins in their immediate neighborhoo ds, are involved in the physical processes leading to the insulator-metal t ransition. Magnetic polarons have been observed directly by a variety of ph ysical techniques including, for example, magnetic measurements, neutron di ffraction, scanning tunneling microscopy. and, most recently, by noise meas urements. This article will review these physical manifestations of the exi stence of magnetic polarons and relate them to the transition.