  The antiferromagnetic ground state and the metamagnetic transition to the
ferromagnetic state of CsCl-ordered FeRh epilayers have been characterized
using Hall and magnetoresistance measure- ments. On cooling into the ground
state, the metamagnetic transition is found to coincide with a suppression in
carrier density of at least an order of magnitude below the typical metallic
level shown by the ferromagnetic state. The data reveal that this sub-metallic
density of electron-like majority carriers in the antiferromagnetic phase are
attributable to intrinsic doping from Fe/Rh substitution defects, with
approximately two electrons per pair of atoms swapped. Based on these
observations it is suggested that an orbital selective Mott transition,
selective to the Fe 3d electrons drives the metamagnetic transition.
