Magnetic and Electrical Properties in NpAl₄and UAl₄

We grew high-quality single crystals of NpAl4. together with UAl4, by the Al-flux method and measured the electrical resistivity. specific heat, magnetic susceptibility, magnetization and de Haas-van Alphen (dHvA) effect. NpAl4 is an antiferromagnet with the Neel temperature T-N = 49 K. The corresponding magnetic moment is most likely directed along the [010] direction in the orthorhornbic crystal structure. The large magnetic entropy at T-N and the Curie-Weiss behavior suggest the localized nature of 5f-electrons with the 5f(4) configuration. The detected dHvA frequencies in NpAl4 are small, indicating that the small magnetic Brillouin zone is formed in the antiferromagnetic state. On the other hand, the large Fermi surfaces are observed in a paramagnet UAl4. The detected cyclotron mass is relatively large, ranging from 3.4 to 23 m(0), which is consistent with the electronic specific heat coefficient gamma = 89 mJ/(K-2.mol).