Neutron Diffraction Study of NiS

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Abstract
A powder neutron diffraction study of stoichiometric NiS has been made in the range 85–770°K. The NiAs-type structure can be obtained at room temperature by quenching from above 700°K. This NiAs form exhibits a paramagnetic-antiferromagnetic transition at 263°K. In the antiferromagnetic state the moments in a hexagonal layer are coupled ferromagnetically and point in the direction of the c axis. The coupling between adjacent hexagonal layers is antiferromagnetic. The sublattice magnetization goes from zero to full saturation on cooling through 263°K. The measured moment just below the transition is 2.1 μB. The a- and c-axis lattice parameters are also observed to increase abruptly at this temperature by ⅓% and 1%, respectively. The transition is 3–4°K wide and exhibits thermal hysteresis. This transition thus has several characteristics of a first-order transition.