Observation of a metamagnetic transition in the 5f heavy-fermion compound UNi2Al3: Magnetization studies up to 90 T for single-crystalline U(Pd1−xNix)2Al3

We report an observation of metamagnetism in the 5f-electron heavy-fermion antiferromagnets UNi2Al3 at 78 T and U(Pd1−xNix)2Al3 at 30 T (42 T) for x=0.5 (x=0.75) using single-crystalline samples. The magnetization curves for U(Pd1−xNix)2Al3 (x=0.5 and 0.75) show a sharp increase at the metamagnetic transition (MMT) field Bm, 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 TN from paramagnetic to polarized-paramagnetic states as a crossover. The results indicate that the antiferromagnetic order is closely connected to the MMT in U(Pd1−xNix)2Al3, but the metamagnetic behavior cannot be explained by the conventional spin flip of antiferromagnetic ordered moments. Our data for U(Pd1−xNix)2Al3 have revealed an empirical linear relation of Bm(x)∝Tχmax(x) and a scaling behavior in magnetic susceptibility of χ(T/Tχmax)/χmax, where Tχmax is the temperature of the susceptibility maximum [χmax≡χ(Tχmax)]. Importantly, the critical value of the magnetic polarization at the MMT Mcr is roughly unchanged, although the value of susceptibility is notably suppressed with increasing x.