We report an observation of metamagnetism in the $5f$-electron heavy-fermion antiferromagnets ${\mathrm{UNi}}_2{\mathrm{Al}}_3$ at 78 T and $\mathrm{U}{\left({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Al}}_3$ at 30 T (42 T) for $x=0.5$ ($x=0.75$) using single-crystalline samples. The magnetization curves for $\mathrm{U}{\left({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Al}}_3$ ($x=0.5$ and 0.75) show a sharp increase at the metamagnetic transition (MMT) field $B_{\mathrm{m}}$, strongly indicating their first-order nature. The obtained $B-T$ phase diagram for $x=0.5$ suggests the possible presence of the tricritical point, while the MMT still occurs at high temperatures above $T_{\mathrm{N}}$ from paramagnetic to polarized-paramagnetic states as a crossover. The results indicate that the antiferromagnetic order is closely connected to the MMT in $\mathrm{U}{\left({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Al}}_3$, but the metamagnetic behavior cannot be explained by the conventional spin flip of antiferromagnetic ordered moments. Our data for $\mathrm{U}{\left({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Al}}_3$ have revealed an empirical linear relation of $B_{\mathrm{m}}\left(x\right)\propto T_{{\chi }_{\max }}\left(x\right)$ and a scaling behavior in magnetic susceptibility of $\chi (T/T_{\chi \max })/{\chi }_{\max }$, where $T_{\chi \max }$ is the temperature of the susceptibility maximum [${\chi }_{\max }\equiv \chi \left(T_{\chi \max }\right)$]. Importantly, the critical value of the magnetic polarization at the MMT $M_{\mathrm{cr}}$ is roughly unchanged, although the value of susceptibility is notably suppressed with increasing $x$.

• Received 2 July 2018
• Revised 6 September 2019

DOI:https://doi.org/10.1103/PhysRevB.100.165137