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 ${T}_{N}\ensuremath{\approx}530\text{ }\text{K}$ and could still be observed at $T=800\text{ }\text{K}$ which is about $1.5{T}_{N}$. We argue that the strong spin correlations above ${T}_{N}$ are due to the topological frustration of the spins on a fcc lattice. The N\'eel temperature is substantially reduced by this process. We determined the critical exponents $\ensuremath{\beta}=0.328\ifmmode\pm\else\textpm\fi{}0.002$ and $\ensuremath{\nu}=0.64\ifmmode\pm\else\textpm\fi{}0.03$ and the N\'eel temperature ${T}_{N}=530\ifmmode\pm\else\textpm\fi{}1$ K. These critical exponents suggest that NiO should be regarded as a $3dXY$ system.