Gradual transition from insulator to semimetal of Ca${}_{1-x}$Eu${}_x$B${}_6$ with increasing Eu concentration

Abstract

The local environment of Eu${}^{2+}$ ($4f^7$, $S=7/2$) in Ca${}_{1-x}$Eu${}_x$B${}_6$ ($0.003⩽x⩽1.00$) is investigated by means of electron spin resonance (ESR). For $x\lesssim 0.003$ the spectra show resolved \textit{fine} and \textit{hyperfine} structures due to the cubic crystal \textit{electric} field and nuclear \textit{hyperfine} field, respectively. The resonances have Lorentzian line shape, indicating an \textit{insulating} environment for the Eu${}^{2+}$ ions. For $0.003\lesssim x\lesssim 0.07$, as $x$ increases, the ESR lines broaden due to local distortions caused by the Eu/Ca ions substitution. For $0.07\lesssim x\lesssim 0.30$, the lines broaden further and the spectra gradually change from Lorentzian to Dysonian resonances, suggesting a coexistence of both \textit{insulating} and \textit{metallic} environments for the Eu${}^{2+}$ ions. In contrast to Ca${}_{1-x}$Gd${}_x$B${}_6$, the \textit{fine} structure is still observable up to $x\approx 0.15$. For $x\gtrsim 0.30$ the \textit{fine} and \textit{hyperfine} structures are no longer observed, the line width increases, and the line shape is purely Dysonian anticipating the \textit{semimetallic} character of EuB${}_6$. This broadening is attributed to a spin-flip scattering relaxation process due to the exchange interaction between conduction and Eu${}^{2+}$ $4f$ electrons. High field ESR measurements for $x\gtrsim 0.15$ reveal smaller and anisotropic line widths, which are attributed to magnetic polarons and Fermi surface effects, respectively.

Submitted to PRB