Magnetic phase diagram of theUFe1−xNixAl system

Measurements of the lattice parameter (a,c), magnetic susceptibility, magnetization, electrical resistivity, and Mössbauer effect were performed on polycrystalline samples of ${\mathrm{UFe}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ni}}_{\mathit{x}}$Al solid solutions in the 0<x≤1.0 composition range. The lattice-parameter variation with x was found to be quite unusual. It appears that at x≊0.3 the a(x) and c(x) functions go through a sharp cusp; with a maximum value for a(x) and a minimum value for c(x). The susceptibility and magnetization show a systematic increase in magnetic correlation effects with growing Ni substitution, which for the 0.35<x<0.8 samples leads finally to the occurrence of spontaneous ferromagnetism. The maximum Curie temperature and saturation moment are achieved for x=0.5. This behavior has been confirmed by the Mössbauer-effect study performed in the temperature range 13–300 K. Moreover, this study undoubtedly indicates that the Ni substitution is preferential and initially takes place mainly at the (1b) sites. This finding and the increase in the total electron concentration by about 1 electron/f.u. caused by the Ni substitution are primarily responsible for inducing the ferromagnetic properties around x=0.5 in the ${\mathrm{UFe}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ni}}_{\mathit{x}}$Al alloys. Starting with the x=0.9 composition, an antiferromagnetic correlation develops and a stable antiferromagnetic state is reached for UNiAl with ${\mathit{T}}_{\mathit{N}}$=19–23 K. The Debye temperature for the selected x=0.5 sample, being ${\mathit{FTHETA}}_{\mathit{D}}$=300 K, was also estimated from the temperature-dependent Mössbauer-effect data.