Magnetic properties of neptunium Laves phases: High-field measurements on NpAl2and NpOs2

The magnetic properties of the cubic $C-15\mathrm{}$-type Laves phases Np${\mathrm{Al}}_2$ and Np${\mathrm{Os}}_2$ have been studied in fields up to 80 kOe by means of bulk magnetization, nuclear $\gamma$-ray resonance (NGR), and neutron-diffraction techniques. Both materials are ferromagnetic. In Np${\mathrm{Al}}_2$, the bulk moment ${\overline{\mu }}_0=1.21\mathrm{}{\mu }_B$, whereas the $5f$ moment, as determined by NGR and neutron-diffraction methods, is $1.50{\mu }_B$. Np${\mathrm{Al}}_2$ is a localized ferromagnet with a small high-field susceptibility and, in common with all other actinide ferromagnets examined so far, exhibits a negative conduction-electron polarization. In contrast, Np${\mathrm{Os}}_2$ behaves like an itinerant ferromagnet with a large high-field susceptibility, and a total moment ${\overline{\mu }}_0=0.44\mathrm{}{\mu }_B$. If the NGR and neutron-diffraction measurements are interpreted in terms of localized $5f$ moments, they give values for the Np moment in Np${\mathrm{Os}}_2$ of $0.40{\mu }_B$ and $0.25{\mu }_B$, respectively. A qualitative discussion is presented to argue that the $5f$ electrons in Np${\mathrm{Os}}_2$ are spatially delocalized and that this will reconcile the discrepancies between the magnetic moments determined by the three techniques. Some additional results for Np${\mathrm{Mn}}_2$ and Np${\mathrm{Ni}}_2$ are also reported.