Spin correlations and colossal magnetoresistance in HgCr2Se4

This study aims to unravel the mechanism of colossal magnetoresistance (CMR) observed in n-type HgCr2Se4, in which low-density conduction electrons are exchange-coupled to a three-dimensional Heisenberg ferromagnet with a Curie temperature T-C approximate to 105 K. Near room temperature the electron transport exhibits an ordinary semiconducting behavior. As temperature drops below T * similar or equal to 2.1T(C), the magnetic susceptibility deviates from the Curie-Weiss law, and concomitantly the transport enters an intermediate regime exhibiting a pronounced CMR effect before a transition to metallic conduction occurs at T < T-C. Our results suggest an important role of spin correlations not only near the critical point but also for a wide range of temperatures (T-C < T < T*) in the paramagnetic phase. In this intermediate temperature regime the transport undergoes a percolation type of transition from isolated magnetic polarons to a continuous network when temperature is lowered or magnetic field becomes stronger.