Crossover from Kondo semiconductor to metallic antiferromagnet with -electron doping in

We report a systematic study of the -electron-doped system . With increasing , the orthorhombic axis decreases slightly while accompanying changes in and leave the unit cell volume almost unchanged. Inelastic neutron scattering, along with thermal and transport measurements, reveal that for the Kondo semiconductor , the low-temperature energy gap, which is proposed to be a consequence of strong hybridization, is suppressed by a small amount of Ir substitution for Fe and that the system adopts a metallic ground state with an increase in the density of states at the Fermi level. The charge or transport gap collapses (at ) faster than the spin gap with Ir substitution. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements demonstrate that the system undergoes long-range antiferromagnetic order below a Néel temperature of 3.1(2) K for . The ordered moment is estimated to be smaller than 0.07(1) /Ce, although the trivalent state of Ce is confirmed by Ce -edge x-ray absorption near edge spectroscopy. It is suggested that the hybridization gap, which plays an important role in the unusually high ordering temperatures observed in ( = Ru and Os), may not be necessary for the onset of magnetic order with a low seen here in .