Volume dependence of the first-order transition temperature for RCo2compounds

Abstract

The magnetic free energy in the so-called s-d model where the itinerant d electrons and localised spins coexist is expanded in a power series of the total magnetisation M. The first-order transition observed for DyCo₂, HoCo₂ and ErCo₂ is explained from the negative values of the coefficient of the M⁴ term at T_C where the paramagnetic susceptibility diverges in the expansion of the free energy. Volume dependence of the first-order transition temperature T_C' is studied by using the expansion of the free energy up to the M⁶ term. It is shown that large experimental values of d ln T_C'/d ln V for DyCo₂, HoCo₂ and ErCo₂ can be explained by this model.