The order of the magnetic phase transitions in RCo₂(R = rare earth) intermetallic compounds

It has been found experimentally that the order of the magnetic phase transitions in RCo 2 compounds (R standing for rare-earth metals) at T c changes from second order for the light-rare-earth series up to TbCo 2 to first order for the heavier-rare-earth compounds DyCo 2 , HoCo 2 and ErCo 2 . On the basis of results of fixed-spin-moment band-structure calculations for the isostructural compound YCo 2 at different lattice constants, we propose an explanation for this behaviour. In contrast to the widely accepted Inoue-Shimizu theory for this class of compounds, our explanation also includes Pr, Nd which were thought to behave differently due to the influence of crystal-field effects. We show that an itinerant-electron metamagnetic transition in these compounds can occur only over a certain range of lattice constants and that the possibility of a first-order phase transition is connected to features of the electronic structure rather than to the magnitude of the transition temperature as conjectured earlier. The influence of the latter is only important if the transition takes place at elevated temperatures, where effects of spin fluctuations can suppress a first-order transition.