The spatial distribution of magnetization in cobalt tungstate, CoWO₄

Single-crystal neutron-diffraction measurements on cobalt tungstate. CoWO₄, 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=4.659(1), b=5.667(1), c=4.940(1) AA, 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 angle 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 direction and Y parallel to b. The moment on the Co²⁺ ion is 3.42(3) mu B with 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 expected when there is a significant orbital moment. These observations are related to the ground-state wavefunction of the ion. There is also evidence 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⁺ moment and amounts to -3.4*10^-3 Delta V/V between 5 and 55 K.