FERROMAGNETISM IN THE UCO1-XRUXAL QUASI-TERNARY INTERMETALLICS

We report the structure and magnetic investigations of the quasiternar y UCo1-xRuxAl compounds crystallizing in the hexagonal structure of th e ZrNiAl type. A large deviation from linearity is observed in the con centration dependence of lattice parameters, positive for a and negati ve for c, whereas the unit-cell volume V obeys Vegard's law satisfacto rily. The nonlinearity is explained by a preferential occupation by th e Co and Ru atoms of the two non-equivalent positions of the transitio n metal in the lattice. Although UCoAl and URuAl are paramagnetic down to low temperatures, ferromagnetism is observed in a wide concentrati on range of quasiternary solid solutions UCo1-xRuxAl. Both the Curie t emperature and the spontaneous magnetization reach their maximum value s of 60 K and 0.6 eta(B) (formula unit)(-1) respectively in UCo0.7Ru0. 3Al. The samples in the Ru concentration range 0.5 less than or equal to x less than or equal to 0.7 exhibit a second phase transition in th e ordered state, indicated by anomalies in temperature dependence of b oth the spontaneous magnetization and the ac susceptibility. The devel opment of magnetism is discussed in the framework of the anisotropic 5 f-ligand hybridization and the direction dependent exchange interactio ns mediated by it.