R. R. Urbano

Gradual transition from insulator to semimetal of Ca1-x Eux B6 with increasing Eu concentration

The local environment of Eu2+ (4 f7, S=7 2) in Ca1-x Eux B6 (0.003≤x≤1.00) is investigated by means of electron spin resonance (ESR). For x 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 Eu2+ ions. For 0.003 x 0.07, as x increases, the ESR lines broaden due to local distortions caused by the Eu Ca ions substitution. For 0.07 x 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 Eu2+ ions. In contrast to Ca1-x Gdx B6, the fine structure is still observable up to x≈0.15. For x 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 EuB6. This broadening is attributed to a spin-flip scattering relaxation process due to the exchange interaction between conduction and Eu2+ 4f electrons. High-field ESR measurements for x 0.15 reveal smaller and anisotropic linewidths, which are attributed to magnetic polarons and Fermi surface effects, respectively.

Electron spin resonance of Gd3+ in GdmMnIn3m+2n (M=Rh,Ir; n=0,1; m=1,2) antiferromagnets

We report electron spin resonance experiments of Gd3+ in the GdmMnIn3m+2n (M=Rh,Ir; n=0,1; m=1,2) intermetallic compounds. For T>TN∼45K, all compounds present a single Dysonian resonance and show a Korringa-like temperature dependence of the linewidth, ΔH=a+bT. The residual linewidth a is strongly affected by the transition metal M=Rh or Ir and/or by the layering (m=1 or 2) or change in structure (n=0,1). The residual linewidth is associated with an unresolved crystalline electrical field (CEF) fine structure. Consequently, a systematic evolution of the CEF in the GdmMnIn3m+2n compounds is inferred. A discussion to what extent our results can explain to the CEF effects observed in isostructural R-based compounds will be given.