DifferentGd3+sites in dopedCaB6:An electron spin resonance study

The local environment of ${\mathrm{Gd}}^{3+}$ ${(4f}^7,\hspace{0.5em}S=7/2)$ ions in single crystals of ${\mathrm{Ca}}_{1-x}{\mathrm{Gd}}_x{\mathrm{B}}_6$ $(0.0001\mathrm{}\lesssim x\lesssim 0.01)$ is studied by means of electron spin resonance (ESR). The spectra for low concentration samples $(x\lesssim 0.001)$ show a split spectrum due to cubic crystal field effects (CFE). The line shape of each fine structure line is Lorentzian, indicating an insulating environment for the ${\mathrm{Gd}}^{3+}$ ions. For higher concentrations $(0.003\mathrm{}\lesssim x\lesssim 0.01),$ the spectra show a single resonance $[g=1.992\mathrm{}(4),\Delta H_{1/2}\approx 20–60\hspace{0.5em}\mathrm{Oe}]$ with no CFE and Dysonian line shape indicating metallic environment for the ${\mathrm{Gd}}^{3+}$ ions. For intermediate concentrations, a coexistence of spectra corresponding to insulating and metallic regions is observed. Most of the measured samples show the weak ferromagnetism (WF) as reported for ${\mathrm{Ca}}_{0.995}{\mathrm{La}}_{0.005}{\mathrm{B}}_6,$ but, surprisingly, this WF has no effect in our ESR spectra either for metallic or insulating environments. This result suggests that the WF in these systems might be isolated in clusters (defect-rich regions) and its relationship with metallicity is nontrivial.