Delocalizing Effect of Polarons in Colossal MagnetoresistanceMaterial FeCr₂S₄

The electrical transport and magnetic properties of colossal
magnetoresistance material FeCr₂S₄ are studied. The temperature dependence of resistivity ρ and thermoelectric power S are investigated from 5 to 300 K and from 77 to 300 K, respectively. The activation energy E_ρ obtained from ρ-T is larger than the activation energy E_S, obtained from S-T. This indicates that magnetic polarons dominate the conduction behaviour at temperatures above T_c. The M-T curve and electron spin resonance spectra show that the paramagnetic-ferromagnetic transition is incomplete, and that a paramagnetic phase coexists with a ferromagnetic phase in a certain temperature range below T_c. It is proposed that the insulator-metal transition near T_c could be attributed to the gradual delocalizing effect from magnetic polarons to naked carriers. The resistivity in the presented temperature range can be described by considering the coexistence of magnetic polarons and naked carriers.