First order transitions in ACo2compounds

Abstract

Some magnetic properties of ACo₂ compounds, where A is a heavy rare earth metal, are shown to be related to properties of YCo₂. In particular the existence of first order transitions for A=Er, Ho, Dy, but second order transitions for A=Tb, Gd, implies a special temperature dependence of the Landau parameter B which appears in the expansion of the free energy of YCo₂ in powers of the magnetization. B changes from negative to positive with increasing temperature and this is verified experimentally by measurements of the field dependence of the susceptibility of YCo₂ at 4.2, 77 and 300K in field up to 150 kOe. It is estimated that an applied field of about 1400 kOe would be required to produce itinerant electron metamagnetism in YCo₂ and the magnetization of Gd_xY_1-xCo₂, where the 'applied' field is the molecular field due to Gd moments, is discussed from this point of view. Stoner theory is used to relate the behaviour of B to the existence of a maximum in the susceptibility of YCo₂ as a function of temperature.