Exchange and crystal field in Sm-based magnets. II. Phenomenological analysis and density functional calculations

Abstract

A technique of determining the exchange field ${B}_{\mathrm{ex}}$ on the $4f$ shell of Sm atoms in Sm-based magnets is proposed. It makes use of the $4f$ intermultiplet transition in Sm, observed in inelastic neutron scattering (INS) experiments. The method is used to analyze previously published data for a number of Sm-Fe and Sm-Co intermetallics, for all of which ${B}_{\mathrm{ex}}$ is determined. Additional information on intramultiplet transitions in ${\mathrm{SmCo}}_{5}$ and ${\mathrm{Sm}}_{2}{\mathrm{Co}}_{17}$ makes it possible to obtain more accurate ${B}_{\mathrm{ex}}$ values as well as to estimate the leading crystal field parameter (CFP) ${A}_{2}^{0}$ for these compounds. For the same systems an independent determination of ${A}_{2}^{0}$ is carried out using published magnetization curves and the ${B}_{\mathrm{ex}}$ values found from the INS spectra. The two ``experimental'' values of ${A}_{2}^{0}$ (INS and magnetization) agree well. For comparison, theoretical Sm-Co exchange fields and CFP for ${\mathrm{SmCo}}_{5}$ and ${\mathrm{Sm}}_{2}{\mathrm{Co}}_{17}$ are obtained from full-potential density-functional calculations. The theoretical ${A}_{2}^{0}〈{r}^{2}〉$ are shifted toward more negative values with respect to their experimental counterparts by a few millielectronvolts. The calculated Sm-Co exchange fields are in fair agreement with the experimentally determined values of the total exchange field on Sm, ${B}_{\mathrm{ex}},$ the weak Sm-Sm exchange interaction being accountable for the remaining small discrepancies.