Critical Magnetic Neutron Scattering from CoO

The critical magnetic neutron scattering from CoO has been measured near the Néel temperature $T_{\mathrm{N}}$. The diffuse intensity at various wave vectors was assumed proportional to $A_{+}{\left|\frac{T}{T_{\mathrm{N}}-1\mathrm{}}\right|}^{-\gamma }$ above $T_{\mathrm{N}}$ and $A_{-}{\left|\frac{T}{T_N-1\mathrm{}}\right|}^{{-\gamma }^{\prime }}$ below $T_{\mathrm{N}}$. Using the method of Als-Nielsen, we measured the temperature dependence of the critical scattering at several values of $q$ [the displacement from the (111) position in reciprocal space] and used these data to calculate the relative magnitudes of $\gamma$ and ${\gamma }^{\prime }$. In order to satisfy scaling law requirements we referred our measurements to a rigid lattice by correcting for the large tetragonal contraction below $T_{\mathrm{N}}$. We found that ${\gamma }^{\prime }=(1.02\mathrm{}\pm 0.02)\gamma$, and this suggests that static scaling laws may apply to CoO. In addition we ascertained that $\frac{A_{+}}{A_{-}}\cong 7.6\pm 1.5$ in agreement with the linear theory of Schofield, where $\frac{A_{+}}{A_{-}}=6.0\mathrm{}\pm 0.8$ for CoO.