THE SPATIAL-DISTRIBUTION OF MAGNETIZATION IN COBALT TUNGSTATE, COWO4

Single-crystal neutron-diffraction measurements on cobalt tungstate, C oWO4, at 15 K and powder measurements in the temperature interval 5-55 K have confirmed that it orders antiferromagnetically below its Neel temperature of 55 K with a magnetic propagation vector (1/2 0 0). The chemical unit cell is monoclinic, space group P2/c, Z = 2 with a = 4.6 59(1), b = 5.667(1), c = 4.940(1) angstrom, beta = 89.94(1)-degrees at 15 K. The magnetic moments on the two equivalent Co2+ ions within the cell are coupled ferromagnetically and lie in the a-c plane at an ang le 43.4(8)-degrees from [001] towards -a. The magnetization density of the Co2+ ion is not spherically symmetric, but it can be modelled as a multipolar expansion on axes Z taken parallel to the moment directio n and Y parallel to b. The moment on the Co2+ ion is 3.42(3) mu(B) wit h the largest coefficients, apart from Y00, being Y41+ = -0.20(2) and Y42+ = -0.21(2). In addition, the radial form factor is expanded over that appropriate to a spin-only moment of a free ion as should be expe cted when there is a significant orbital moment. These observations ar e related to the ground-state wavefunction of the ion. There is also e vidence for a small covalent transfer of moment to O amounting to 0.04 (1) mu(B) per ligand. The development of magnetic order is accompanied by magnetostriction, which mimics the magnitude of the ordered Co2+ m oment and amounts to -3.4 x 10(-3) DELTAV/V between 5 and 55 K.