Magnetism in cupric oxide

Single-crystal neutron diffraction measurements have been used to study the long-range magnetic ordering in cupric oxide, CuO. An incommensurate antiferromagnetic structure forms below the Neel temperature on 230(1) K, with a propagation vector (0.506(1)a*-0.483(1)c*) which remains constant down to a magnetic phase transition at 213(1) K. Below the latter temperature, the structure remains antiferromagnetic with a commensurate propagation vector (1/2 0-1/2), and this structure persists to the lowest temperature reached in the investigation (20 K). The arrangement of the copper moments in both phases is such that the n-glide perpendicular to the b axis of the monoclinic cell, space group C2/c, does not reverse the direction of the spin. The two magnetic sublattices related to the C-face-centring scatter in phase quadrature and the relative directions of the spin on them could not be determined. Good agreement is obtained for the commensurate phase with a multipole model for the copper magnetisation density and spins of 0.65(3) mu _B directed parallel to b. The lower sublattice magnetisation in the incommensurate phase precluded a meaningful multipole fit, but a reasonable agreement is obtained with a model in which the spins rotate in the a-c plane following an elliptical envelope with major axis directed 33(2) degrees to c in beta obtuse and a maximum moment of 0.38(2) mu _B at 215 K. The paramagnetic scattering at ambient temperature and 550 K was measured to try to find the origin of the peak in the susceptibility. No significant paramagnetic scattering could be obtained from a powdered sample although the sensitivity of detection was some five times that required to observe the scattering from a Cu²⁺ ion in an ideal paramagnet.