Antiferromagnetic phase transition and spin correlations in NiO

We have investigated the antiferromagnetic (AF) phase transition and spin correlations in NiO by high-temperature neutron diffraction below and above T-N. We show that AF phase transition is a continuous second-order transition within our experimental resolution. The spin correlations manifested by the strong diffuse magnetic scattering persist well above T-N approximate to 530 K and could still be observed at T=800 K which is about 1.5T(N). We argue that the strong spin correlations above T-N are due to the topological frustration of the spins on a fcc lattice. The Neel temperature is substantially reduced by this process. We determined the critical exponents beta=0.328 +/- 0.002 and nu=0.64 +/- 0.03 and the Neel temperature T-N=530 +/- 1 K. These critical exponents suggest that NiO should be regarded as a 3dXY system