Magnetic Structure and Excitations in CeCuAl System.

CeCuAl crystallizing in the tetragonal BaNiSn-type structure and CeCuAl solid solutions were investigated by means of elastic and inelastic neutron scattering. Powder neutron diffraction brought information on both temperature evolution of crystallographic parameters and magnetic order at low temperatures. No structural change was observed in the investigated temperature range from 1.5 to 300 K. Weak magnetic peaks outside nuclear Bragg positions observed in solid solutions with 0.90 ≤ x ≤ 1.10 were described by the propagation vector k = (0.40 + δ, 0.60 + δ, 0), where δ ≈ 0.02 and δ ≈ 0.01. The magnetic structure of CeCuAl consists of two components: an anti-ferromagnetic one described by the same k and a ferromagnetic one with k = (0, 0, 0) and magnetic moments lying within the tetragonal basal plane. The evolution of magnetic excitations as a function of Cu-Al concentration in CeCuAl was studied by inelastic neutron scattering. The measured spectra of CeCuAl and the solution with x = 0.95 point to a three-magnetic-peak energy scheme, while only two excitations are expected from the local symmetry conditions on Ce atoms. The standard two-peak spectrum of crystal electric field excitations was observed for Cu-Al substitutions further from the 1:1:3 stoichiometry (x = 0.75 and 1.10). The intermediate concentrations (x = 0.90 and 1.05) exhibit spectra on the border between the former cases with a less clear pronounced first inelastic magnetic peak. The observed behavior is discussed considering the evolution of structural parameters in the CeCuAl system and the coupling between the lattice vibrations and the crystal electric field excitations.