Effect of Se-Substitution on the Magnetic and Electrical Transition in the NiAs-Type NiS

Hexagonal lattice parameters a and c of the NiAs-type NiS_1−ySe_y expand with increasing y below the solubility limit y_L=0.20. The first-order transition temperature T_t(y) [between the antiferromagnetic less-conductive and the Pauli-paramagnetic high-conductive states] decreases with increasing y, and is suppressed down to liq. He temperature at the critical concentration y_c=0.133. Just above T_t(y) the temperature slope of the susceptibility has positive sign at low y, and shows a y-dependence. The isothermal susceptibility of the Pauli-paramagnetic state decreases with increasing y. These experimental results are discussed on the basis of the strongly correlated features of d and p bands in NiS.