Heat capacity of RFexMn12−x (R = Gd,Tb and Dy) compounds: wiping out a cooperative 4f–4f exchange interaction by breakingthe 3d–4f magnetic symmetry

Abstract

Using adiabatic calorimetry the heat capacity of a series ofRFexMn12−x (R = Gd,Tb and Dy) compounds has been measured from 3 to 350 K. The substitution of Fe for Mn inRFexMn12−x influences both the magnetic interactions on the 3d sublattice andthe magnetism of R (the Néel temperature doubles on going fromx = 0 to 6 and the compounds become ferromagnetic forx = 8 with Curie temperatures of around 300 K). In pureTbMn12 the heat-capacitydata shows a λ-type anomaly associated with the independent cooperative magnetic ordering of the R sublattice(∼5 K), while the anomaly related to the Mn magnetic ordering(∼100 K) is rather smooth, as observed in other itinerant magnetic systems such asYMn12. In contrast, the substitution of Fe for Mn leads, on the one hand, to a more localizedmagnetic behaviour of the 3d sublattice, and, on the other, to magnetic polarization effectsbetween the 3d and 4f sublattices, together with the disappearance of the cooperativemagnetic ordering of the R sublattice due to the breaking of the antiferromagneticsymmetry in the 3d sublattice. This is reflected in the heat-capacity curve through asmooth Schottky-like anomaly. In the case of Gd compounds the magnitude of theexchange molecular-field parameter has been deduced by fitting the magneticcontribution to the heat capacity within a simple mean-field model. From this analysiswe found that this molecular field acting on the rare-earth site increases withthe iron concentration, reaching values as large as 48 T for the concentrationx = 6. A similar analysis of the heat capacity in the ordered phase on the Tb compoundsalso leads to an enhancement of the molecular field with increasing Fe content.These results allow checking the possible crystal-field parameters for theseRFexMn12−x compounds.