Short-Range Magnetic Order in CoO

Abstract

The local spin correlation in CoO has been investigated above the N\'eel temperature (${T}_{N}\ensuremath{\cong}289$\ifmmode^\circ\else\textdegree\fi{}K) by means of magnetic-neutron-scattering measurements of the diffuse intensity from a single crystal near the (111) position in reciprocal space. Spin-correlation parameters were determined out to 50 neighbors for $\frac{{T}_{N}}{T}=0.93 \mathrm{and} 0.97$. The measurements indicate that the fundamental antiferromagnetic spin order, which involves antiparallel layers of spins on alternate {111} planes, is retained above ${T}_{N}$. One notable exception is that the nearest spin-correlation value is approximately -0.01, and not zero as in the completely ordered state. The topological features of the (111) diffuse peak were utilized to ascertain the relative values of ${V}_{1}$, ${V}_{2}$, and ${V}_{3}$, the first-three neighbor potentials. Assuming ${V}_{3}$ to be zero, we found $\frac{{V}_{2}}{{V}_{1}}=7$. When we included a nonzero ${V}_{3}$, it was only possible to restrict the set ($\frac{{V}_{2}}{{V}_{1}}$, $\frac{{V}_{3}}{{V}_{1}}$) to a range of values. On applying a qualitative correction for the spin-orbit coupling, we concluded that the most probable set of potential ratios was $\frac{{V}_{2}}{{V}_{1}}=5.0$ and $\frac{{V}_{3}}{{V}_{1}}=+0.1$. From these ratios we obtain ${J}_{1}\ensuremath{\cong}\ensuremath{-}3.3$\ifmmode^\circ\else\textdegree\fi{}K, ${J}_{2}\ensuremath{\cong}\ensuremath{-}16.6$\ifmmode^\circ\else\textdegree\fi{}K, and ${J}_{3}\ensuremath{\cong}\ensuremath{-}0.3$\ifmmode^\circ\else\textdegree\fi{}K.