Short-Range Magnetic Order in CoO

The local spin correlation in CoO has been investigated above the Néel temperature ($T_N\cong 289$°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 $\frac{T_N}{T}=0.\mathrm{}93 \mathrm{and} 0.97$. 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 $\frac{V_2}{V_1}=7\mathrm{}$. 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 $\frac{V_2}{V_1}=5.\mathrm{}0$ and $\frac{V_3}{V_1}=+0.\mathrm{}1$. From these ratios we obtain $J_1\cong -3.\mathrm{}3$°K, $J_2\cong -16.\mathrm{}6$°K, and $J_3\cong -0.\mathrm{}3$°K.