Magnetic properties of ${\mathrm{NpGa}}_2$ and ${\mathrm{NpSi}}_2$

The magnetic and electronic properties of ${\mathrm{NpGa}}_2$ and ${\mathrm{NpSi}}_2$ were studied by Mössbauer as well as ac and dc magnetization techniques. ${\mathrm{NpGa}}_2$ orders ferromagnetically at ${\mathit{T}}_{\mathit{C}}$=55(5) K with a Np-ion ordered moment of ${\mathrm{\mu }}_{\mathrm{ord}}$=2.39${\mathrm{\mu }}_{\mathit{B}}$, which coincides with the value of the saturation moment ${\mathrm{\mu }}_{\mathrm{sat}}$. The paramagnetic effective moment is ${\mathrm{\mu }}_{\mathrm{eff}}$=2.6${\mathrm{\mu }}_{\mathit{B}}$. The Mössbauer isomer shift (IS) of ${\mathrm{NpGa}}_2$ is +15.2 mm/sec relative to ${\mathrm{NpAl}}_2$, indicating a ${\mathrm{Np}}^{3+}$ charge state. ${\mathrm{NpSi}}_2$ has an IS of +7.9 mm/sec and orders ferromagnetically at ${\mathit{T}}_{\mathit{C}}$=48(5) K with ${\mathrm{\mu }}_{\mathrm{ord}}$=1.1${\mathrm{\mu }}_{\mathit{B}}$, whereas ${\mathrm{\mu }}_{\mathrm{sat}}$≊0.8${\mathrm{\mu }}_{\mathit{B}}$. The paramagnetic effective moment is ${\mathrm{\mu }}_{\mathit{e}\mathit{f}\mathit{f}}$=2.1${\mathrm{\mu }}_{\mathit{B}}$. A comparison of our data with the results for other Np${\mathit{X}}_2$ (X=Al, As, Sb, and Te) compounds indicates that ${\mathrm{NpGa}}_2$ is a localized 5f-electron system, whereas in ${\mathrm{NpSi}}_2$ the 5f electrons are partially delocalized. We show that at present the magnetic properties of the Np${\mathit{X}}_2$ compounds cannot be consistently explained within the conventional crystal-field picture nor by taking into account hybridization dressing or using local-spin-density models.