Gradual transition from insulator to semimetal ofCa1−xEuxB6with increasing Eu concentration

Abstract

The local environment of ${\mathrm{Eu}}^{2+}$ $(4{f}^{7},S=7∕2)$ in ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}{\mathrm{B}}_{6}$ $(0.003\ensuremath{\leqslant}x\ensuremath{\leqslant}1.00)$ is investigated by means of electron spin resonance (ESR). For $x\ensuremath{\lesssim}0.003$ the spectra show resolved fine and hyperfine structures due to the cubic crystal electric field and nuclear hyperfine field, respectively. The resonances have Lorentzian line shape, indicating an insulating environment for the ${\mathrm{Eu}}^{2+}$ ions. For $0.003\ensuremath{\lesssim}x\ensuremath{\lesssim}0.07$, as $x$ increases, the ESR lines broaden due to local distortions caused by the $\mathrm{Eu}∕\mathrm{Ca}$ ions substitution. For $0.07\ensuremath{\lesssim}x\ensuremath{\lesssim}0.30$, the lines broaden further and the spectra gradually change from Lorentzian to Dysonian resonances, suggesting a coexistence of both insulating and metallic environments for the ${\mathrm{Eu}}^{2+}$ ions. In contrast to ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}{\mathrm{B}}_{6}$, the fine structure is still observable up to $x\ensuremath{\approx}0.15$. For $x\ensuremath{\gtrsim}0.30$ the fine and hyperfine structures are no longer observed, the line width increases, and the line shape is purely Dysonian, anticipating the semimetallic character of ${\mathrm{EuB}}_{6}$. This broadening is attributed to a spin-flip scattering relaxation process due to the exchange interaction between conduction and ${\mathrm{Eu}}^{2+}$ $4f$ electrons. High-field ESR measurements for $x\ensuremath{\gtrsim}0.15$ reveal smaller and anisotropic linewidths, which are attributed to magnetic polarons and Fermi surface effects, respectively.