Antiferromagnetism in CuO studied by neutron polarimetry

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Abstract
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) 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.

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