Magnetic and electronic properties of NpFeGa₅

NpFeGa₅ single crystals prepared by the Ga-flux method have been investigated by magnetisation, specific heat and ²³⁷Np Mössbauer spectroscopy measurements. An antiferromagnetic (AF) transition has been observed at T_N ∼ 114K. The second AF transition which takes place at T^* ∼ 74K has only been detected in the magnetisation data. A saturated ordered Np moment of 1.00(5) μ_B and the occurrence of an Np³⁺ charge state were inferred from the Mössbauer data. The angle between the Np ordered moment and the basal plane was shown to be ∼ 17°, in good agreement with the value observed by neutron diffraction in the low temperature AF phase. The absence of change of the quadrupole interaction parameters at T^* did not allow us to gain information on the Np moment reorientation at T^*. The trend of the isomer shift along the NpTGa₅ series and the discrepancy between the Np moments determined by neutron diffraction and Mössbauer spectroscopy indicated a partial spatial delocalisation of the 5f electrons in NpFeGa₅.