Electron spin resonance of Gd3+ in the normal state of RNi2B2C (R=Y,Lu)

Electron spin resonance (ESR) of Gd3+ in the normal state (T>T-c) of R1-xGdxNi2B2C (R=Y,Lu) is reported. The results show that the exchange coupling between the rare-earth localized magnetic moment and the conduction electrons depends on the conduction electrons momentum transfer (\k(F)(in)-k(F)(out)\ = q), i.e., J(fs)(q). The temperature dependence of the ESR linewidth yields a value for one of the exchange parameters, [J(fs)(2)(q)](EF)(1/2), which is in agreement with that estimated from the slope of the initial linear decrease of T-c by the Gd3+ impurities. These results indicate that the R1-xGdxNi2B2C (R=Y,Lu) compounds behave as conventional BCS superconductors, in agreement with previous reports. [S0163-1829(98)02806-9].5763668367

Magnetic polaron and Fermi surface effects in the spin-flip scattering of EuB6

The spin-flip scattering (SFS) between conduction and 4f(7) Eu2+ (S-8(7/2)) electrons in the paramagnetic phase of EuB6 (Tgreater than or equal to2T(c)similar or equal to30 K) is studied by means of electron spin resonance (ESR) at three frequencies. The single Dysonian resonance observed in all cases suggests a metallic environment for the Eu2+ ions. The ESR at high field, Hsimilar or equal to12.05 kG (nusimilar or equal to33.9 GHz), has an anisotropic linewidth with cubic symmetry. The low-field, 1.46 kG (4.1 Ghz) and 3.35 kG (9.5 GHz), ESR linewidths are unexpectedly broader and have a smaller anisotropy than at the higher field. The unconventional narrowing and anisotropy of the linewidth at higher fields are indicative of a homogeneous resonance and microscopic evidence for a strong reduction in spin-flip scattering between the spins of Eu2+ and the states in the electron and hole pockets at the X points of the Brillouin zone by magnetic polarons.701

Exchange and crystal field effects in the ESR spectra of Eu(2+) in LaB(6)

Electron spin resonance of Eu(2+) (4f(7), S=7/2) in a La hexaboride (LaB(6)) single crystal shows a single anisotropic Dysonian resonance. From the observed negative g shift of the resonance, it is inferred that the Eu(2+) ions are covalent exchange coupled to the B 2p-like host conduction electrons. From the anisotropy of the spectra (linewidth and field for resonance), we found that the S ground state of Eu(2+) ions experience a cubic crystal field of a negative fourth order crystal field parameter (CFP), b(4)=-11.5(2.0) Oe, in agreement with the negative fourth order CFP, A(4), found for the non-S ground state R hexaborides. These results support covalency as the dominant contribution to the fourth order CFP for the whole R hexaboride family.761

ESR of Gd3+ in the Kondo-lattice compound YbAgCu4 and its reference compounds RAgCu4 (R = Y,Lu)

Low-temperature (T<30 K) electron-spin-resonance (ESR) experiments of Gd3+ diluted in the Kondo-lattice compound YbAgCu4, and its reference compounds YAgCu4 and LuAgCu4, are interpreted in terms of an enhanced density of states at the Fermi level for the Yb-based compound. The results of susceptibility and ESR (Korringa rate and g-shift) measurements show negligible electron-electron exchange enhancement for all the studied compounds. The exchange interaction between the Gd3+ local moment and the conduction electrons(c-e) is c-e wave vector dependent in all three compounds.56148933893

Antiferromagnetic ordering of divalent Eu in EuCu2Si2 single crystals

We report the synthesis, from an indium flux, of single crystals of EuCu2Si2. In contrast to previous studies of polycrystalline samples in which intermediate-valent behavior for Eu is reported, we find that in single crystals of EuCu2Si2 the behavior of Eu is divalent, including the presence of antiferromagnetic order at 10 K. The origins of these variations in ground-state properties are discussed in terms of effective chemical pressure and local changes in chemical environment.63

Anomalous phonon shifts in the paramagnetic phase of multiferroic RMn2O5 (R=Bi, Eu, Dy): Possible manifestations of unconventional magnetic correlations

A Raman spectroscopic study of the high-frequency optical phonons in single crystals of the multiferroic system RMn2O5 (R=Bi, Eu, Dy) was performed. All studied materials show anomalous phonon shifts, below a new characteristic temperature for these materials, T(*)similar to 60-65 K. The sign and magnitude of such shifts appear to be correlated with the ionic radius of R, envolving from softenings for R=Bi to hardenings for R=Dy and showing an intermediary behavior for R=Eu. Additional phonon anomalies were identified below similar to T(N)similar to 40-43 K, reflecting the onset of long-range ferroelectric and/or magnetic order of the Mn sublattice. Complementary dc-magnetic susceptibility [chi(T)] measurements for BiMn2O5 up to 800 K yield a Curie-Weiss temperature theta(CW)=-253(3) K, revealing a fairly large frustration ratio (parallel to theta(CW)parallel to/T-N=6.3). Deviations of chi(T) from a Curie-Weiss paramagnetic behavior due to magnetic correlations were observed below temperatures of the order of parallel to theta(CW)parallel to, with the inverse susceptibility showing inflection points at similar to 160 K and similar to T-*. Supported by chi(T) data, the anomalous Raman phonon shifts below T-* are interpreted in terms of the spin-phonon coupling, in a scenario of strong magnetic correlations. Overall, these results support significant magnetic frustration, introduce a new characteristic temperature (T-*), and suggest a surprisingly rich behavior for the magnetic correlations in the paramagnetic phase of this system.731

Coexisting on-center and off-center Yb3+ sites in Ce1-xYbxFe4P12 skutterudites

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Electron-spin-resonance (ESR) measurements performed on the filled skutterudite system Ce1-xYbxFe4P12 (x less than or similar to 0.003) unequivocally reveal the coexistence of two Yb3+ resonances associated with sites of considerably different occupations and temperature behaviors. Detailed analysis of the ESR data suggests a scenario where the fraction of oversized (Fe2P3)(4) cages that host Yb ions are filled with a low occupation of on-center Yb3+ sites and a highly occupied T-dependent distribution of off-center Yb3+ sites. Analysis of the Yb-171(3+)(I=1/2) isotope hyperfine splittings reveal that these two sites are associated with a low (similar to 1 GHz) and a high (greater than or similar to 15 GHz) rattling frequency, respectively. Our findings introduce Yb3+ in T-h symmetry systems and use the Yb3+ ESR as a sensitive microscopic probe to investigate the Yb3+ ions dynamics.805Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)DOE [DE-FG02-98ER45707]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)DOE [DE-FG02-98ER45707

Magnetic field dependence and bottlenecklike behavior of the ESR spectra in YbRh2Si2

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Electron spin resonance (ESR) experiments at different fields or frequencies (4.1 <=nu <= 34.4 GHz) in the Kondo lattice (T-K similar or equal to 25 K) YbRh2Si2 single-crystal compounds confirmed the observation of a single anisotropic Dysonian resonance with g(perpendicular to c)congruent to 3.55 and no hyperfine components for 4.2 less than or similar to T less than or similar to 20 K. However, our studies differently reveal that (i) the ESR spectra for H-perpendicular to c show strong-field-dependent spin-lattice relaxation, (ii) a weak-field and temperature-dependent effective g value, (iii) a dramatic suppression of the ESR intensity beyond 15% of Lu doping, and (iv) a strong sample and Lu-doping (<= 15%) dependence of the ESR data. These results suggest a different scenario where the ESR signal may be associated to a coupled Yb3+-conduction electron resonant collective mode with a strong bottleneck and dynamiclike behavior.793Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)NSF (USA)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq