ESR of different Gd 3+ sites in CaB 6

Abstract

The environment of Gd 3+ (4f 7, S= 7 2 ) ions in single crystals of Ca l− x Gd x B 6 (0.0001⩽ x⩽0.01) is studied by electron spin resonance (ESR). The spectra for x⩽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 Gd 3+ ions. For 0.003⩽ x⩽0.01, the spectra show a single resonance ( g≈1.992(4), Δ H l/2 ≈60 Oe) with no CFE and Dysonian line shape indicating metallic environment for the 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 Ca 0.995La 0.005B 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.