Magnetic order induced birefringence and critical behaviour of the long range order parameter in NiO

The optical linear magnetic birefringence of single-domain NiO crystals was measured at temperatures above and below the antiferromagnetic phase transition. The analysis of the experimental data yield a Néel temperature of T_N = (523.7 ± 0.2)K and a critical exponent β = 0.325^+0.01_-0.02 which determines the temperature behaviour of the magnetic long range order parameter $\text{S}\text{?}\text{～ (}\text{1?T}\text{T}{}_{\mathrm{N}}\text{)}{}^{\mathrm{\beta }}$. This critical behaviour of S? is found to be continued to temperatures well below T_N.