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 $T_N\approx 530\text{K}$ and could still be observed at $T=800\text{K}$ 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 Néel temperature is substantially reduced by this process. We determined the critical exponents $\beta =0.328\pm 0.002$ and $\nu =0.64\pm 0.03$ and the Néel temperature $T_N=530\pm 1$ K. These critical exponents suggest that NiO should be regarded as a $3dXY$ system.

Figure 1

(Color online) (a) Temperature dependence of the $\frac{1}{2}\frac{1}{2}\frac{1}{2}$ magnetic reflection of NiO. (b) Log-log plot of the intensity as a function of the reduced temperature $t=\left(T_N-T\right)/T_N$. From the slop of the linear plot the critical exponent is determined to be $\beta =0.329\pm 0.002$.

Figure 2

$Q$ scans from NiO through $Q=\left(\frac{1}{2}\frac{1}{2}\frac{1}{2}\right)$ along [111] at temperatures below and above $T_N$. The continuous curves are results of fits with a Gaussian below $T_N$ and convoluted Lorentzian above $T_N$.

Figure 3

(a) Log-log plot of the HWHM (inverse correlation length) vs reduced temperature. (b) Log-log plot of the intensity of the diffuse scattering (susceptibility) vs reduced temperature.