Exchange and crystal field effects in the ESR spectra ofEu2+inLaB6

Abstract

Electron spin resonance of ${\mathrm{Eu}}^{2+}$ ($4{f}^{7}$, $S=7∕2$) in a La hexaboride $(\mathrm{La}{\mathrm{B}}_{6})$ single crystal shows a single anisotropic Dysonian resonance. From the observed negative $g$ shift of the resonance, it is inferred that the ${\mathrm{Eu}}^{2+}$ ions are covalent exchange coupled to the $\mathrm{B}\phantom{\rule{0.2em}{0ex}}2p$-like host conduction electrons. From the anisotropy of the spectra (linewidth and field for resonance), we found that the $S$ ground state of ${\mathrm{Eu}}^{2+}$ ions experience a cubic crystal field of a negative fourth order crystal field parameter (CFP), ${b}_{4}=\ensuremath{-}11.5(2.0)\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$, in agreement with the negative fourth order CFP, ${A}_{4}$, found for the non-$S$ ground state $R$ hexaborides. These results support covalency as the dominant contribution to the fourth order CFP for the whole $R$ hexaboride family.