Crystal-field effects and magnetic behavior inRNi5andRCo5+xrare-earth compounds

Several R${\mathrm{Ni}}_5$ and R${\mathrm{Co}}_{5+\mathrm{x}}$ compounds were studied by rare-earth Mössbauer spectroscopy ${(}^{161}$Dy, ${}_{}{}^{166}{}_{}{}^{}\mathrm{Er}$, ${}_{}{}^{169}{}_{}{}^{}\mathrm{Tm}$). We solved the discrepancy existing in the literature concerning the magnetic ground state in ${\mathrm{ErNi}}_5$. Two different crystal-field-split level schemes were proposed. Our data showed that the scheme with the ‖±(15/2〉 as ground-state doublet is the correct one. We also found that ${\mathrm{ErNi}}_5$ and ${\mathrm{TmNi}}_5$ show slow paramagnetic relaxation above $T_c$ in contrast with ${\mathrm{DyNi}}_5$, which shows normal relaxation behavior. This difference in relaxation behavior is attributed to differences in the magnetic ground state, being the ‖±1/2〉 doublet in ${\mathrm{DyNi}}_5$, while it is the ‖±(15/2〉 doublet in ${\mathrm{ErNi}}_5$ and the ‖±6〉 doublet in ${\mathrm{TmNi}}_5$. From the lattice contribution of the electric quadrupole splitting in R${\mathrm{Co}}_{5+\mathrm{x}}$, we determined the concentration dependence of the second-order crystal-field potential $A_2^0$. Initially it is nearly concentration independent, but decreases for higher x values. For the analysis of the data obtained on ${\mathrm{DyCo}}_{5.2}$, two second-order terms ($A_2^0$ and $A_2^2$) were taken into consideration.