Alloying and pressure-induced transitions between5f-band metamagnetism and ferromagnetism

Effects of external hydrostatic pressure up to 1.2 GPa and the Al-Ga alloying on the magnetism in ${\mathrm{UCoAl}}_{1-x}{\mathrm{Ga}}_x$ compounds have been studied. Ga substitutions for Al in UCoAl lead to gradual transformation of a $5f$-band metamagnetism towards ferromagnetism (for $x>~0.2$). The application of pressure yields reverse effects clearly seen in compounds with $x<~\mathrm{0.4.}$ The spontaneous magnetization ${\mu }_s$ and the Curie temperature $T_C$ of ferromagnetic compounds are gradually suppressed with increasing pressure. The values of $d\ln T_C/dp$ and $d\ln {\mu }_s/dp$ slowly increase with x decreasing down to 0.4, whereas for $x<\mathrm{}0.4\mathrm{}$ 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 $(x<~0.3).$ 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.