Influence of Lattice Contraction on Long-Range Order in CoO NearTN

Just below the Néel temperature of CoO, the intensity of neutrons scattered by the anti-ferromagnetic lattice of ${\mathrm{Co}}^{2+}$ ionic spins should vary as $I\propto {(T_{\mathrm{N}}-T)}^{2\beta }$. We measure $\beta =0.244\mathrm{}\pm 0.015$, or substantially lower than the Ising prediction of 0.3125 which CoO should assume. By applying a phenomenological correction for the measured tetragonal lattice contraction below $T_{\mathrm{N}}$, similar in form to Heller's correction for the contraction in Mn${\mathrm{F}}_2$, we make $T_{\mathrm{N}}=T_{\mathrm{N}}\mathrm{}\left(T\right)\mathrm{}$ which raises $\beta$ to a value of 0.290±0.025. By thus adjusting for nonrigidity of the lattice we bring $\beta$ into a range which includes the rigid-lattice Ising prediction.