Magnetic and electrical properties in NpAl ₄ and UAl ₄

We grew high-quality single crystals of NpAl ₄, together with UAl ₄, by the Al-flux method and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization and de Haasvan Alphen (dHvA) effect. NpAl ₄ is an antiferromagnet with the Néel temperature T _N =49K. The corresponding magnetic moment is most likely directed along the [010] direction in the orthorhombic 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 NpAL ₄ 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 UA1 ₄. The detected cyclotron mass is relatively large, ranging from 3.4 to 23m ₀ which is consistent with the electronic specific heat coefficient γ = 89mJ/(K ²-mol).