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

Abstract

It has been found experimentally that the order of the magneticphase transitions in RCo2 compounds (R standing for rare-earthmetals) at Tc changes from second order for thelight-rare-earth series up to TbCo2 to first order for theheavier-rare-earth compounds DyCo2, HoCo2 and ErCo2. On the basis ofresults of fixed-spin-moment band-structure calculations for theisostructural compound YCo2 at different lattice constants, wepropose an explanation for this behaviour. In contrast to thewidely accepted Inoue-Shimizu theory for this class of compounds,our explanation also includes Pr, Nd which were thought to behavedifferently due to the influence of crystal-field effects. We showthat an itinerant-electron metamagnetic transition in thesecompounds can occur only over a certain range of lattice constantsand that the possibility of a first-order phase transition isconnected to features of the electronic structure rather than to themagnitude of the transition temperature as conjectured earlier. Theinfluence of the latter is only important if the transition takesplace at elevated temperatures, where effects of spin fluctuationscan suppress a first-order transition.