2005 Volume 74 Issue 8 Pages 2323-2331
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 TN=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 TN1=36 K. Below another magnetic transition at TN2=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 TC=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 NpFeGa5, 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 2.6 μB/Np for NpRhGa5 and NpNiGa5, indicating that the localized 5f4 electronic states are most likely applicable to these compounds at high temperatures.
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