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 = 118 K 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 100mJ/K(2.)mol for NpFeGa5, NpRhGa5, and NpNiGa5, respectively. The magnetic susceptibility approximately follows the Curie-Weiss law at high temperatures (250 < T < 300K) with effective magnetic moments ranging from 2.4 to 2.6 mu(B)/Np for NpRhGa5 and NpNiGa5, indicating that the localized 5f(4) electronic states are most likely applicable to these compounds at high temperatures.