Existence and Structure of Rare Earth Monocarbides: Study of Their Magnetic Properties at Low Temperatures

摘要

We investigated the physical properties of the rare earth monocarbides. These carbides have fcc structures and have many carbon gaps. We proposed a rare earth-carbon equilibrium phase diagram whose essential characteristic is the existence of an M sub 2 C phase arising from the face centered cubic phase through ordering of the carbon atoms. X-ray diffraction crystallographic studies and the neutron diffraction work on the M sub 2 C phase were used to determine the structure. M sub 2 C is rhomohedral, and the order of the carbon atoms can be described as an alternation of full and empty planes perpendicular to the ternary axis. All the carbides exhibit magnetic ordering phenomena of the M exp 3 /sup +/ ions at low temperatures. The magnetic properties are fairly complicated. The carbides can be said to be ferromagnetic, but their magnetic structure is not simple. Assuming the trivalent ions couple through the conduction electrons, the coupling function being the Ruderman-Kittel oscillating function, it is possible to interpret the transition temperatures, the magnetic resistivities, and the variation in the transition temperature as a function of the carbon content. The calculations lead us to assume that the number of almost free conduction electrons which couple the ions together is slightly larger than the number of electrons in the rare earth metals, as if the carbon provides about one electron per atom to the metal conduction band. Interpretation of the phenomena of local order produced around the transition temperature leads to analogous conclusions. In the series, keeping similar electronic structures for the carbide and the metal, assuming that the carbon given up a conduction electron to the metal, and applying classical theories of coupling for metal, we can account for most of the phenomena which we observed. Considering the uniqueness of the properties measured, the rare earth monocarbides are as good metals as the rare earths themselves. (ERA citation 09:050171)