ANISOTROPIC SPIN CORRELATIONS IN CUO ABOVE THE NEEL TEMPERATURE

A single-crystal neutron scattering study of the spin correlations in the a-c plane of cupric oxide above its Néel temperature of 229 K has shown them to be anisotropic: at 230 K the correlation length parallel to (1, 0, -1) is estimated to be at least 300 Å, which is much larger than the correlation length of 50-60 Å in the approximately perpendicular direction parallel to (1, 0, 1). This correlation length falls approximately linearly with increasing temperature in the interval 230 to 234 K, being reduced by a factor of two at the latter temperature. The full-width at half-maximum of Q-scans along (1, 0, -1) is resolution-limited close to TN, so the corresponding correlation lengths could not be determined accurately. However, no significant reduction is seen until the temperature is higher than 237 K. These observations, together with the results of susceptibility and specific heat measurements, clearly establish that CuO behaves as a low, probably quasi-one-dimensional antiferromagnet. © 1990.