Magnetic and electrical properties of NpTGa₅ (T = Fe, Rh and Ni)

We grew high-quality single crystals of NpTGa₅ (T = Fe, Rh, and Ni) with a tetragonal structure by the Ga-flux method and measured their electrical resistivity, specific heat, magnetic susceptibility, and magnetization. All the investigated compounds undergo magnetic ordering. NpFeGa₅ orders antiferromagnetically at T_N = 118K and shows another magnetic transition at T* = 78 K. A relatively large electrical resistivity indicates that NpFeGa₅ might be a low-carrier compound. NpRhGa₅ also orders antiferromagnetically at T_N1 = 36 K. Below another magnetic transition at T_N2 = 32 K, the antiferromagnetic easy-axis is most likely changed from [001] to the (001) plane. On the other hand, NpNiGa₅ undergoes ferromagnetic ordering at T_C = 30 K, where the magnetic moment of Np is directed along the [001] direction. Furthermore, below another magnetic transition at T* = 18 K, the ordered moment is discontinuously enlarged and a change of the magnetic structure occurs. The electronic specific heat coefficients are approximately determined as 30, 52, and 100 mJ/K²·mol for NpFeGa ₅, NpRhGa₅, and NpNiGa₅, respectively. The magnetic susceptibility approximately follows the Curie-Weiss law at high temperatures (250 < T < 300 K) with effective magnetic moments ranging from 2.4 to 2.6μ_B/Np for NpRhGa₅ and NpNiGa ₅, indicating that the localized 5f⁴ electronic states are most likely applicable to these compounds at high temperatures.