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 GdxY1-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.