Magneto-volume effects in rare-earth transition metal intermetallics

The pressure derivative of the Curie temperature, the compressibility, and the thermal expansion of several representative members of the group ofR-Co andR-Fe intermetallics (Rstands for rare-earth elements Y, Th, and in one case Zr) has been studied. The results derived for the volume dependence of Tcagree rather well with the relation (d \ln T_{c})/(d \ln V) = a+b/T_{c}^{2}in which, for both series of compounds (Co and Fe), the constant a equals about -5/3. This result favors an interpretation of the magnetic properties of these compounds in terms of the collective electron model rather than one in terms of localized moments. For some compounds the thermal expansion above room temperature has been studied. A considerable reduction of the thermal expansion has been observed near Tcfor those compounds for whichdT_{c}/dpis large and negative (invar effect).