Volume dependence of the first-order transition temperature for RCo2compounds

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 TC where the paramagnetic susceptibility diverges in the expansion of the free energy. Volume dependence of the first-order transition temperature TC' 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 TC'/d ln V for DyCo2, HoCo2 and ErCo2 can be explained by this model.