Hyperfine Interactions and Relaxation Effects in Laves-Phase Intermetallic Compounds of Neptunium

Abstract

Studies of magnetic properties of cubic Laves-phase compounds of Np have been performed using the M\"ossbauer effect of the 59.5-keV transition in $^{237}\mathrm{Np}$. The compounds studied were Np${\mathrm{Al}}_{2}$, Np${\mathrm{Co}}_{2}$, Np${\mathrm{Ni}}_{2}$, Np${\mathrm{Fe}}_{2}$, and Np${\mathrm{Ir}}_{2}$. Hyperfine-interaction parameters and ordering temperatures were deduced from these measurements. If the values of $\frac{H(T)}{H(0)}$ are plotted as a function of $\frac{T}{{T}_{C}}$, the points (except those of Np${\mathrm{Fe}}_{2}$) lie on the molecular-field-theory curve for $S=\frac{1}{2}$ Quadrupole interactions were found to be very small. This behavior is consistent with the assumption that the ground crystalline state in these compounds is an isolated Kramers doublet. Relaxation phenomena were observed in the spectra. The spectra could be fitted by assuming relaxation effects within the Kramers doublet with a relaxation time of \ensuremath{\sim} 4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}11}$ sec. Isomer shifts were measured and found to change regularly with the size of the unit cell.