The accurate magnetic structure of CeAl2 at various temperatures in the ordered state

The magnetic structure of the cubic compound CeAl2 is incommensurate and double-k. The moments on the two Ce sites describe two elliptical helices of opposed chiralities and lie in the () plane, with their Fourier components mk close to the [111] direction. Recent symmetry considerations, including for the first time the inversion center of the crystal, have reduced the number of parameters of this structure and have underlined the existence of a phase difference between the projections mxk, myk and mzk of mk. Up to now, although many neutron investigations have been carried out on CeAl2 single crystals, no set of magnetic intensities was available which was large and good enough to check whether this phase difference exists or not. We have measured such a set of data, taking great care of the instrumental resolution in order to avoid unwanted contributions to the intensities from other domains. As the magnetic form factor of the Γ7 ground state of CeAl2 depends very much on the direction of the applied field, it was necessary to go through a self-consistent determination of the magnetic form factor in the direction of the Fourier components mk to obtain good agreement between the refined magnetic structure and the experimental data. The resulting Fourier components are close but not exactly along the [111] direction and their departure from this direction at low temperature reduces the ellipticity of the helices. The phase difference between the projections mxk, myk and mzk is small but undoubtedly exists and is temperature dependent. It leads to a small distortion of the elliptical helices, with magnetic moments slightly out of the () plane.