Analysis of the magnetic properties of Ce 3 Pt 23 Si 11 : orthorhombic crystal field and mean-field approximation
Résumé
We present here a quantitative analysis of the ground state and magnetic properties
of Ce3Pt23Si11, based on a crystalline electric field description within the mean-field
approximation. In this face-centered cubic compound, the point group symmetry at the
Ce site is orthorhombic. One main difficulty in this low symmetry case is that the CEF
potential for Ce3+ ions is determined by five independent parameters, while only two
magnetic excitations are observed by inelastic neutron scattering. Moreover the anisotropy
of the magnetic susceptibility of the Ce ion, that permits an independent determination of
the second-order CEF parameters is hidden by the cubic symmetry of the compound. A
specific procedure is developed for this purpose that combines genetic algorithms and more
conventional optimization methods. A set of CEF parameters is found that best reproduces
the different experimental observations in both the paramagnetic and ferromagnetic phases of
Ce3Pt23Si11. The analysis accounts for two seemingly contradictory properties: a strong local
anisotropy that aligns the moment along a fourfold axis and a rather weak anisotropy of the
bulk magnetization with an easy threefold magnetization axis. Ce3Pt23Si11 is shown to be a
model system where single site anisotropies compete within a crystal structure of overall high
symmetry.