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 to the paramagnetic and ordered states (T_C=56 K). While very precise DC SQUID magnetization measurements confirm ferromagnetic ordering, they show a relatively small hysteresis loop at 5 K reduced with a coercive field H_Co~3000 Oe. The variable offset cumulative spectra (VOCS) acquired in the paramagnetic state show a high sensitivity of the ²⁷Al nuclei spectral parameters (Knight shifts and line broadening) to the ferromagnetic ordering, even at room temperature. - Highlights: • We present the magnetic properties of the ferromagnetic NpAl₂ (T_C=56 K). • Magnetization shown reduced small hysteresis loop at 5 K and an H_Co~3000 Oe. • ²⁷Al NMR VOCS were acquired in the paramagnetic state. • Strong variation of Knight shift and line broadening with temperature. • ²⁷Al highly sensitive to ferromagnetic correlations