Effect of Fe site distribution on the magnetic properties of UFe4Al8−xGax (x=1.0 and 1.5) and UFexAl12−x (4.5⩽x⩽5)

The semi-ordered alloys with compositions UFexAl12−x (4.5⩽x⩽5.0) and UFe4Al8−xGax (x=1.0 and 1.5) were prepared by induction melting and found to crystallize in the tetragonal ThMn12-type structure (space-group I4/mmm). The comparative study of magnetic properties showed that the magnetic behaviour in UFe4Al8−xGax as Al is replaced by Ga is similar to that observed in UFexAl12−x with the increase of Fe concentration. In both cases an increase of the ordering temperature, Tord, and saturation magnetization, Msat, as well as an enhancement of the ferromagnetic character are detected. The presence of a second magnetic transition below Tord is also observed both in UFexAl12−x and in the Ga-containing compounds. The Rietveld refinement of X-ray powder diffraction data and the analysis of the Mössbauer spectra are consistent with the occupation of the 8f and 8j sites by Fe atoms in both series. The similarities between the magnetic behaviour of the UFe4Al8−xGax and UFexAl12−x are related rather to the fraction of 8j sites occupied by Fe than to the total Fe content of the samples.