Characteristic metamagnetic transitions in NpRhGa5 and UPtGa5

We measured the high-field magnetization up to 55 T and constructed a magnetic phase diagram for a transuranium antiferromagnet NpRhGa₅ with the tetragonal structure. The magnetization at 4.2 K for an easy-axis H//[100] indicates a metamagnetic transition with a sharp step at H_c = 26 T and saturates above H_s = 38 T, reaching 0.43 μ_B/Np at H_s. On the other hand, the magnetization saturates at H_s = 28 T for a hard-axis H//[001], reaching 0.44 μ_B/Np at H_s. The magnetization indicated that the magnetic moment reduces from 0.96 μ_B/Np at zero field to 0.44 μ_B/Np at the saturation field as a function of magnetic field. These experimental results in NpRhGa₅ are compared with the simple magnetism in UPtGa₅with the similar antiferromagnetic structure.