ANTIFERROMAGNETISM IN CUO STUDIED BY NEUTRON POLARIMETRY

Single-crystal neutron diffraction measurements on cupric oxide, CuO, using a zero-field neutron polarimeter have confirmed that the Cu2+ moments are parallel to the monoclinic b axis in the commensurate antiferromagnetic phase stable below 213 K. Any component of moment in the a-c plane must have a magnitude less than 0.03 of the total. The incommensurate antiferromagnetic phase, stable between 213 K and the Neel temperature of 230 K, has a propagation vector (0.506, 0, -0.483) as previously reported, but the elliptical envelope of the helical modulation at 215.5 K is shown to have its axes parallel to [010] and in the a-c plane making an angle of 28.2(8)-degrees to [001] in beta obtuse. The envelope is almost circular with the ratio of the b to a-c components of 1.03(1) and the plane of the moments makes an angle of 73.0(5)-degrees to the propagation vector. The new magnetic model produces an equally good fit to the original unpolarized neutron diffraction intensities as the previous model, in which the Cu2+ moments were confined to the a-c plane with a more elliptical envelope, and gives a root mean square Cu2+ effective moment of 0.27(1)-mu-B.