Alloying and pressure-induced transitions between 5f-band metamagnetism and ferromagnetism

Effects of external hydrostatic pressure up to 1.2 GPa and the Al-Ga alloying on the magnetism in UCoAl1-xGax compounds have been studied. Ga substitutions for Al in UCoAl lead to gradual transformation of a 5f-band metamagnetism towards ferromagnetism (for x greater than or equal to 0.2). The application of pressure yields reverse effects clearly seen in compounds with x less than or equal to 0.4. The spontaneous magnetization mu(s), and the Curie temperature T-C of ferromagnetic compounds are gradually suppressed with increasing pressure. The values of d In T-C/dp and d ln mu(s)/dp slowly increase with x decreasing down to 0.4, whereas for x<0.4 an abrupt growth of pressure parameters is observed with further lowering Ga content. This is a fingerprint of a pressure-induced collapse of ferromagnetism and a simultaneous reentrance of the "UCoAl-type" metamagnetism in compounds with lower Ga content (x less than or equal to 0.3). The observed evolution of magnetism is discussed in terms of pressure and alloying-induced variation of the 5f-ligand hybridization that determines both the stability of the U magnetic moment and the strength of U-U exchange interactions.