Magnetic anisotropy in NpRhGa5 single crystals

We have investigated oriented single crystals of the antiferromagnet NpRhGa5 (TN∼37 K) by means of magnetization and specific heat measurements. Strong anisotropy effects are observed: when magnetic fields are applied in the basal plane, the Neel temperature is barely affected. In contrast, in fields applied along the c axis, the second-order transition (magnetic ordering) is shifted to lower temperatures and merges into the first-order transition (reorientation of the magnetic moments). A full mapping of this complex magnetic phase diagram as a function of the temperature (down to 1.6 K), applied magnetic field (up to 9 T) and crystal orientation is presented. This study shows that antiferromagnetic order is more fragile when the magnetic field is applied along the c axis, which is similar to the situation observed for the NpCoGa5 homologue compound