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We report Hall-eff'ect measurements on single crystals of antiferromagnetic pyrite-type NiS2 (T~= 40 K), in which the resistivity appears to show s metal-insulator transition at T -100 K.Our transport measurements demonstrate that this transition is not intrinsic to the bulk: At low temperatures the transport is dominated by a metalliclike conduction at the surface, with carrier density ns 5 x 10 cm and mobility ass -1.3 cm /Vs at T = 0. For T ) 100 K the sample conductance is dominated by the bulk, which behaves like a doped semiconductor; the acceptorlike impurity states have binding energy E& = 80 meV. The valence-band mobility is y, = 0.002 cm /V s.

In pyrite-structure antiferromagnetic NiS2, the magnetic susceptibility y(T) shows downward curvature above the Neel transition (T~i --39.2 K), suggesting that magnetic correlations persist to T = 400 K. However, neutron-scattering measurements indicate that the bulk magnetic behavior above Typal is conventional. We present evidence that the unusual temperature dependence of y(T) arises from a surface contribution. At T~2 --29.75 K a transition occurs to a weakly ferromagnetic state, accompanied by a small structural distortion. We show that this distortion is not tetragonal as suggested previously, but is most likely rhombohedral. Below T~2, y(T) is anisotropic; our data suggest that in this range y(T) is dominated by next-nearest-neighbor exchange interactions.

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