A facile method is developed to efficiently prepare metamagnetic
mercury thiodichromite (HgCr2S4, HCS) polycrystals
and single crystals, and their transport properties are studied. The
resistivity of the as-prepared HCS polycrystal shows a semiconducting
behavior and no magnetic field dependence in the whole temperature
range. In contrast, the annealing treatment of the HCS polycrystal
induces gigantic changes: an insulator–metal transition is
driven by a magnetic field of 5 T, leading to colossal magnetoresistance
(CMR) as high as ∼104. The HCS single crystal grown
by a newly developed facile method displays similar properties with
a larger CMR up to 106–107. First-principles
calculation demonstrates a large spin splitting of band structures,
providing the possibility of magnetic polaron existence, which is
further evidenced by electron spin resonance spectra. Thus, the insulator–metal
transition and CMR can be explained in a magnetic polaronic scenario.
This work opens a new window for CMR-based spintronics.