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 TN. 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 TN≈530 K and could still be observed at T=800 K which is about 1.5TN. We argue that the strong spin correlations above TN are due to the topological frustration of the spins on a fcc lattice. The N\'eel temperature is substantially reduced by this process. We determined the critical exponents β=0.328±0.002 and ν=0.64±0.03 and the N\'eel temperature TN=530±1 K. These critical exponents suggest that NiO should be regarded as a $3dXY$ system.