Magnetic properties of neptunium Laves phases: Np${\mathrm{Al}}_2$, Np${\mathrm{Os}}_2$, Np${\mathrm{Ir}}_2$, and Np${\mathrm{Ru}}_2$

The magnetic properties of a series of neptunium cubic C-15-type Laves phases ($\mathrm{Np}{X}_2$, where $X=\mathrm{Al}, \mathrm{Os}, \mathrm{Ir}, or \mathrm{Ru}$) have been studied between 1.5 and 300 °K by means of magnetization and nuclear $\gamma$-ray resonance measurements. The properties show a systematic variation with neptunium-neptunium distance $d_{\mathrm{Np}}$. Thus, Np${\mathrm{Al}}_2$ ($d_{\mathrm{Np}}=3.37\mathrm{}$ AÅ) is ferromagnetic with a transition temperature $T_c=56\mathrm{}$ °K and an ordered moment of 1.5 ${\mathrm{\mu }}_{\mathrm{B}}$/mole; Np${\mathrm{Os}}_2$ ($d_{\mathrm{Np}}=3.26\mathrm{}$ AÅ) is ferromagnetic with $T_c=7.5\mathrm{}$ °K and an ordered moment of 0.4 ${\mathrm{\mu }}_{\mathrm{B}}$/mole; Np${\mathrm{Ir}}_2$ ($d_{\mathrm{Np}}=3.25\mathrm{}$ AÅ) is antiferromagnetic with $T_c=7.5\mathrm{}$ °K and an ordered moment of 0.6 ${\mathrm{\mu }}_{\mathrm{B}}$/mole; and Np${\mathrm{Ru}}_2$ ($d_{\mathrm{Np}}=3.23\mathrm{}$ AÅ) does not order magnetically down to 1.5 °K. This trend is interpreted in terms of an increase in the delocatlization of the neptunium magnetic moment and the magnetic $5f$ electrons, as $d_{\mathrm{Np}}$ decreases.