Magnetization, specific heat, O NMR, and Np Mssbauer study of UNpO

Martel, L.*; Hen, A.*; Tokunaga, Yo  ; Kinnart, F.*; Magnani, N.*; Colineau, E.*; Griveau, J.-C.*; Caciuffo, R.*

We report a study of the magnetic and electronic properties of the UNpO solid solution, based on dc- and ac- magnetisation, Np Mssbauer spectroscopy, O Nuclear Magnetic Resonance (NMR), and specific heat measurements. The compound orders antiferromagnetically at T = 17 K. The different techniques reveal the complexity of this system with:(1) a spatial distribution of ordered moments, (2) a small Np ordered moment, and (3) an additional specific heat anomaly at 7.4 K, with a residual value at very low temperature and a reduced magnetic entropy. The results are compared to the end-members of the series, UO and NpO, as well as to the other solid solutions previously reported in this system. We discuss how the properties of UNpO add new input to the trend previously reported for the series, in view of the models that have been previously proposed.