Heat capacity in RFexMn12-x compounds

We have measured the heat capacity of a series of RFexMn12-x compounds (R = Y, Gd, Tb, Lu; x = 2, 4, 6, 8) from 3 to 350 K, using adiabatic calorimetr y. Magnetic ordering is observed at 204 K for x = 2, and 240 K for x = 4 an d 6; no defined anomaly has been found for x = 8. In the low-temperature ra nge there is a smooth Schottky-like behaviour coming from low-energy levels of the rare-earth ions. No lambda-type ordering anomaly of the rare-earth sublattice has been detected. In the case of Gd compounds the magnitude of the exchange-held parameter has been deduced fitting the magnetic contribut ion to the heat capacity with a simple molecular-field model. From this ana lysis we found that the exchange molecular held on the rare earth increases with the iron concentration. On the other hand the values of the electroni c coefficient gamma are quite enhanced, gamma = 245 mJ/mol K-2 for YFe8Mn4. (C) 1999 Elsevier Science B.V. All rights reserved.