Ferromagnetic ordering in NpAl2: Magnetic susceptibility and 27Al nuclear magnetic resonance

We report on the magnetic properties of the neptunium based ferromagnetic compound NpAl2. We used magnetization measurements and 27Al NMR spectroscopy to access magnetic features related in the paramagnetic and in the ordered state below TC (56 K). While very precise DC SQUID magnetization measurements confirm the FM state, they show as a reduced reverse field (HC~3000 Oe) and a small hysteresis loop at 5 K, well below the ordered state. The variable offset cumulative spectra (VOCS) acquired in the paramagnetic state shows a high sensitivity of the 27Al nuclei to the ferromagnetic ordering (Knight shifts and line broadening) even at room temperature. The oversimplified picture of NpAl2 as a well localised 5f system used as a reference compound for Mössbauer spectroscopy should be slightly reconsidered with some partly delocalised 5f states interacting at high temperature.JRC.E.4-Nuclear Fuel Safet