Abstract
Local environment of Gd3+ and Eu2+ 4f7 ions, S = 7/2, in Ca1–x Eux B6 (0.0001 ≤ x ≤ 0.30) and Cax Gdx B6 (0.0001 ≤ x ≤ 0.01) is investigated by means of electron spin resonance (ESR). For x ≤ 0.001 the spectra show resolved fine structures due to the cubic crystal electric field and, in the case of Eu, the hyperfine structure due to the nuclear hyperfine field is also observed. The resonances have Lorentzian line shape, indicating insulating host for the Gd3+ and Eu2+ ions. As x increases, the ESR lines broaden due to local distortions caused by the Ca/Gd,Eu ions substitution. For Gd (x ≈ 0.001) and Eu (x ≈ 0.02), the spectra present superposition of Lorentzian and Dysonian resonances, suggesting a coexistence of insulating and metallic hosts for the Gd3+ and Eu2+ ions. The Gd3+ and Eu2+ fine structures are still observable up to x ≈ 0.003 for Gd and x ≈ 0.15 for Eu. For larger values of x the fine and hyperfine structures are no longer observed, the line width increases, and the line shape becomes pure Dysonian anticipating the metallic and semimetallic character of GdB6 and EuB6, respectively. These results clearly show that in the low concentration regime the Ca1–x Rx B6 (R = Gd, Eu) systems are intrinsically inhomogeneous. No evidence of weak ferromagnetism (WF) was found in the ESR spectra of either metallic or insulating phases of these compounds, suggesting that, if WF is present in these materials, the Gd3+ and Eu2+ 4f7-electrons are shielded from the WF field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Highlights
IntroductionHigh-resolution angle-resolved photoemission (ARPES) by Souma et al [18] revealed an energy gap of about 1 eV between the valence and conduction bands and a carrier density of the order of 5 × 1019 cm–3 for their CaB6 single crystals
The simple cubic system of Ca1–xRxB6 (R = rare-earths, specially La) has become the focus of extensive scientific investigations since the reported weak-ferromagnetism (WF) at high-temperature Tc ~ 600–800 K in these materials by Young et al [1]
For x = 0.003 the spectrum shows the fine and hyperfine structures corresponding to seven groups (4f 7, S = 7/2) of twelve hyperfine resonances due to the 151Eu2+ (47.8%; I = 5/2) and 153Eu2+ (52.2%; I = 5/2) isotopes [25]
Summary
High-resolution ARPES by Souma et al [18] revealed an energy gap of about 1 eV between the valence and conduction bands and a carrier density of the order of 5 × 1019 cm–3 for their CaB6 single crystals. Vonlanthen et al [12] reported that, depending on the crystal growth method, undoped CaB6 could show WF. They argue that self-doping attributed to defects might occur. It has been argued that CaB6 is a ~1 eV-gap semiconductor and that the intrinsic WF could be hidden by the FM of Fe and Ni impurities at the surface of the crystals [21]
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