de Haas-van Alphen effect in NpCoGa5

We grew high-quality single crystals of NpCoGa5 and succeeded in observing the de Haas-van Alphen effect. The metamagnetic transition, where the antiferromagnetic state changes into the paramagnetic (field-induced ferromagnetic) state, was observed at Hm=43kOe for H-bar [001]. On increasing the field angle θ, Hm follows the 1/cosθ dependence, indicating an Ising-type antiferromagnet. The main Fermi surface in NpCoGa5 consists of two kinds of cylindrical ones. The detected cyclotron effective mass is considerably large, ranging from 3.2 to 11.5m0 in the paramagnetic state. NpCoGa5 is a heavy-fermion compound and the 5f electrons significantly contribute to the conduction bands