The Eu2+ (4 f7, S=72) g value in Ca1-x Eux B6 (0.15≲x≤1.00) was measured by means of electron spin resonance at two frequencies (fields), 9.4 (≈3.4 kOe) and 34.4 GHz (≈12.1 kOe). The g value was found to be anisotropic and magnetic-field dependent. The amplitude of the anisotropy increases at low temperatures. The observed angular and temperature dependences of the g value suggest tetragonal symmetry caused, presumably, by a distortion along a direction perpendicular to the largest crystal face, the [001] direction. Due to the platelet shape of the samples, part of the anisotropy of the g value can also be attributed to demagnetization effects. The g values decrease at higher fields, which is interpreted in terms of a two-band model involving an exchange interaction between the localized Eu2+ 4 f7 electrons with conduction Eu2+ 5d -like electrons and B 2p -like holes. © 2006 American Institute of Physics.

Local environment of Gd 3+ and Eu 2+ 4f 7 ions, S =7/2, in Ca 1-xEu xB 6 (0.0001 ≤ x ≤ 0.30) and Ca xGd xB 6 (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 Gd 3+ and Eu 2+ 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 Gd 3+ and Eu 2+ ions. The Gd 3+ and Eu 2+ 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 GdB 6 and EuB 6, respectively. These results clearly show that in the low concentration regime the Ca 1-xR xB 6 = 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 Gd 3+ and Eu 2+ 4f 7 -electrons are shielded from the WF field. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

We report electron spin resonance (ESR) measurements in the Gd3+ doped semiconducting filled skutterudite compound Ce1-xGd xFe4P12 (x-0.001). As the temperature T varies from T- 150 K to T- 165 K, the Gd3+ ESR fine and hyperfine structures coalesce into a broad inhomogeneous single resonance. At T- 200 K the line narrows and as T increases further, the resonance becomes homogeneous with a thermal broadening of 1.1(2) Oe/K. These results suggest that the origin of these features may be associated with a subtle interdependence of thermally activated mechanisms that combine: (i) an increase with T of the density of activated conduction carriers across the T-dependent semiconducting pseudogap; (ii) the Gd3+ Korringa relaxation process due to an exchange interaction JfdS.s between the Gd3+ localized magnetic moments and the thermally activated conduction carriers; and (iii) a relatively weak confining potential of the rare earth ions inside the oversized (Fe 2P3)4 cage, which allows the rare earths to become rattler Einstein oscillators above T- 148 K. We argue that the rattling of the Gd3+ ions, via a motional narrowing mechanism, also contributes to the coalescence of the ESR fine and hyperfine structure. © 2011 American Physical Society.

The environment of Gd3 + / Eu2 + (4 f7, S = frac(7, 2)) in Ca1 - x Rx B6 (R = Gd, Eu ; 0.0001 ≤ x ≤ 0.30) is studied by electron spin resonance (ESR). For x ≲ 0.001 the spectra show Lorentzian shape (insulating phase). As x increases, the spectra present a superposition of Lorentzian and Dysonian resonances (coexistence of insulating and metallic phases). For x ≳ 0.01, the line shape becomes pure Dysonian (metallic phase). Thus, the intermediate concentration regime of Ca1 - x Rx B6 is intrinsically inhomogeneous. These compounds show no weak ferromagnetism. © 2006 Elsevier B.V. All rights reserved.

The controversial ground-state properties of the Kondo insulator Sm B6 have been investigated using B11 NMR in very high magnetic fields up to 37 T. We find evidence that, following the development of a gap in the conduction-band density of states below 100 K, the in-gap states dominate the nuclear relaxation at temperatures less than 10 K. The Korringa product 1/ K2 T1 T exhibits anomalous behavior in this range and the application of high magnetic fields leads to suppression of nuclear relaxation. The hybridization gap, however, remains open up to 37 T. The behavior of the relaxation at low temperatures suggests a strong field dependence of the in-gap states and rules out the possibility that bound states arise from B6 vacancies. A simple density-of-states model and a band scheme are introduced to account for these observations. © 2007 The American Physical Society.

Transport, magnetic, and thermal properties at high magnetic fields (H) and low temperatures (T) of the heavy-fermion compound CeAuSb2 are reported. At H=0 this layered system exhibits antiferromagnetic order below TN=6K. Applying B along the interplane direction, leads to a continuous suppression of TN and a quantum critical point at Hc 5.4T. Although it exhibits Fermi-liquid behavior within the Néel phase, in the paramagnetic state the fluctuations associated with Hc give rise to unconventional behavior in the resistivity (sublinear in T) and to a TlnT dependence in the magnetic contribution to the specific heat. For H>Hc and low T the electrical resistivity exhibits an unusual T3 dependence. © 2005 The American Physical Society.

Transport, magnetic and thermal properties at high magnetic fields (H) and low temperatures (T) of the heavy fermion compound CeAuSb2 are reported. At H=0 this layered system exhibits antiferromagnetic order below TN = 6K. Applying B along the inter-plane direction, leads to a continuous suppression of TN and a quantum critical point at H c ≅ 5.4 T. Although it exhibits Fermi liquid behavior within the Neel phase, in the paramagnetic state the fluctuations associated with H c give rise to unconventional behavior in the resistivity (sub-linear in T) and to a TlnT dependence in the magnetic contribution to the specific heat. For H > Hc, and low T the electrical resistivity exhibits an unusual T3-dependence. © 2006 American Institute of Physics.

The angular, magnetic field, and temperature dependence of the Eu2+ (4 f7, S=7/2) g -value in Ca1-x Eux B6 (0.15 x≤1.00) is measured by means of electron spin resonance at two microwave frequencies, 9.4 and 34.4 GHz. The g -value is found to be anisotropic and magnetic-field-dependent. The reduction with field of the positive g -shift is interpreted in terms of the exchange interaction between the localized Eu2+ 4 f7 electrons with the conduction Eu2+ 5d -like electrons and B 2p -like holes. The angular, temperature, and x dependence of the anisotropy of the g -value can be attributed to demagnetization effects due to the platelet-like shape of the samples. High-resolution synchrotron x-ray diffraction experiments show the absence of distortions larger than δa/a∼ 10-4. However, a mechanism for the anisotropy based on a weak tetragonal lattice distortion arising from the crystal surface cannot be excluded. © 2006 The American Physical Society.

The temperature dependence of the specific heat and of the resistivity under pressure has been measured for single crystals of the semiconductor Ce3 Au3 Sb4. The transport data follow an exponential activation and variable range hopping at low T, consistent with weak disorder and localization, while C / T has a - ln T dependence with large entropy. Thus the properties of Ce3 Au3 Sb4 are very different from those of ordinary Kondo insulators. © 2007.