Magnetic-phase transitions and magnetocaloric effects

We have studied the magnetocaloric properties of a variety of compounds, like Gd-5(Si1-xGex)(4) with x = 0.576 and 0.5875, MnFeP1-xAsx with x between 0.25 and 0.65, RTiGe with R = Tb, Dy, Ho, Er and Tm, Ni53Mn22Ga25, Mn5Si3, and Mn1.95Cr0.05Sb. These compounds have in common that they exhibit either temperature- or field-induced first-order magnetic-phase transitions. Gd-5(Si1-xGex)(4) exhibits simultaneously a magnetic and a structural transition, which is accompanied by a huge magnetic-entropy change. A temperature-induced ferromagnetic (FM) to paramagnetic (PM) transition and a magnetic-field-induced PM to FM transition which are both of first order are observed in MnFeP1-xAsx compounds. Here the magnetic-phase transitions are not accompanied by structural transitions. Nevertheless, a large magnetic-entropy change, comparable with that in Gd-5(Si1-xGex)(4), is observed in the MnFeP1-xAsx compounds. In several of the RTiGe compounds, an applied magnetic field induces an anti ferromagnetic (AF) to FM phase transition. Here, we observed a magnetic anisotropy dependence of the magnetic-entropy change. The Hensler alloy Ni53Mn22Ga25 exhibits a first-order martensitic transformation accompanied by a magnetic-phase transition around 220 K. The magnitude and the shape of the magnetic-entropy changes observed for this compound are quite different. Mn5Si3 compound exhibits two successive first-order magnetic-phase transitions and shows a different type of magnetocaloric effect (MCE). Mn1.95Cr0.05Sb exhibits an AF to FM phase transition and a negative MCE. The relationship between the magnetic-phase transitions and the MCE is discussed, based on the comparison of the observed MCEs and the exchange interactions in these materials. (C) 2002 Published by Elsevier Science B.V.