MAGNETIC-PROPERTIES OF (Y,LU)(CO,AL)(2)

A drastic change in the magnetic ground state observed for pseudobinary compounds (Y1-tLut)(Co0.88Al0.12)(2) with cubic Laves phase structure is discussed by making use of the phenomenological Landau energy expanded up to the eighth power of magnetization. The dependence of Landau coefficients on Lu concentration is determined from observed results of the spontaneous magnetization and the critical field of a metamagnetic transition. It is explicitly shown that small changes in the Landau coefficients leads to the transition from a weakly to strongly ferromagnetic state. The 'lutetium paradox', that the observed Curie temperature of Lu(Co, Al), is higher than that of analogous compounds with some heavy rare-earth elements, is also discussed in the present theory.