Superzone gap formation evidenced by specific heat in UNiGa

To investigate the effect of the superzone gap on the Fermi surface, we have measured the field dependence of specific heat of UNiGa in the field H parallel to c-axis. The electronic specific heat coefficient gamma, being 48 mJ/K-2 mole in the high-field ferromagnetic (F) phase, decreases by 10% in the low-field antiferromagnetic (AF) phase. This is the first direct evidence of the superzone-gap formation, which should play an important role in the giant magnetoresistance in this compound. A clear nuclear contribution to the specific heat is found below 0.4 K, which is mainly due to Ga sites sandwiched by the ferromagnetically coupled U layers, and the estimated hyperfine field is similar to 8 Tesla.