Magnetoelastic phase transitions and critical behaviour of (Fe1-xNix)(2)P system

This report focuses on magnetoelastic properties and critical behaviour of (Fe1-xNix)(2)P system. For low Ni content (x = 0.01) an isolated critical p oint was found. Analysis of this critical behaviour in the frame of molecul ar field approximation was carried out. Moreover, thermal variations of the lattice parameters, as measured by X-ray diffraction techniques, were stud ied. The (P,T) phase diagrams obtained for several Ni contents show the evi dence that the Curie temperature decreases with pressure. The Mossbauer spe ctra collected for x = 0.025, 0.1, 0.2, 0.25 at different temperatures are also analysed. The existence of two nonequivalent crystallographic sites oc cupied by iron atoms as well as local magnetic structure is discussed.