Quantum Oscillations in EuFe2As2 single crystals

Quantum oscillation measurements can provide important information about the Fermi surface (FS) properties of strongly correlated metals. Here, we report a Shubnikov-de Haas (SdH) effect study on the pnictide parent compounds EuFe$_{2}$As$_{2}$ (Eu122) and BaFe$_{2}$As$_{2}$ (Ba122) grown by In-flux. Although both members are isovalent compounds with approximately the same density of states at the Fermi level, our results reveal subtle changes in their fermiology. Eu122 displays a complex pattern in the Fourier spectrum, with band splitting, magnetic breakdown orbits, and effective masses sistematically larger when compared to Ba122, indicating that the former is a more correlated metal. Moreover, the observed pockets in Eu122 are more isotropic and 3D-like, suggesting an equal contribution from the Fe $3d$ orbitals to the FS. We speculate that these FS changes may be responsible for the higher spin-density wave ordering temperature in Eu122.Comment: 5 pages, 4 figure

Angular dependence of giant magnetoimpedance in an amorphous Co-Fe-Si-B ribbon

The field response of impedance is studied in a stress-annealed amorphous ribbon as a function of the angle of application of the external magnetic field in order to verify the role of induced anisotropies (and their distribution) and demagnetizing factors in the giant magnetoimpedance (GMI) phenomenon which occurs in soft magnetic materials. The experimental results are well explained by a theoretical model, based on the simultaneous solution of Maxwell equations and the Landau-Lifshitz equation of motion. Demagnetizing effects are properly taken into account in the case of ribbons or thin films. The physical parameters necessary to test the theory were obtained through complementary measurements of the ferromagnetic resonance and temperature dependence of magnetization. The results clearly indicate the enormous influence of the distribution of anisotropies on the GMI effect. Also, an experimental procedure for determining the easy-axis distribution function is proposed. [S0163-1829(99)15433-X].6096685669

Spin structure and first-order transition of GdIn(3): Near-surface magnetism, buried amplitude-modulated phase, and interface delocalization

A resonant x-ray magnetic diffraction study was performed for pure and (Cd,Ga)-doped GdIn(3) single crystals with cubic structure. All studied samples show an equal-magnitude antiferromagnetic spin structure with propagation vector (tau) over right arrow=[1/2, 1/2,0] at low temperatures, corresponding to a parallel spin propagation along the (c) over right arrow direction (normal to the studied surfaces) and antiparallel propagation along (a) over right arrow and (b) over right arrow. A complex magnetic behavior in the submicrometric near-surface region (NSR) was found close to T(N)(bulk) similar to 44 K. For both pure and substituted samples, a fairly strong signal from the equal-magnitude magnetic phase was found to survive above T(N)(bulk) and abruptly disappears at T(N)(NSR) similar to T(N)(bulk)+ 0.7 K, indicating that the NSR may show a larger T(N) than the bulk for all studied samples. For the pure compound only, satellite peaks consistent with an amplitude-modulated magnetic phase with a wavelength of 380 angstrom were found between T(N)(bulk) and T(N)(NSR). A successful fit of the scattering profile around several magnetic Bragg positions and photon energies, using a simple phase coexistence model, confirmed that the amplitude-modulated phase develops underneath the most superficial region showing the equal magnitude structure. The evolution of the magnetic scattering profile on cooling indicates that the interface between equal-magnitude and amplitude-modulated phases diverges towards the bulk as T -> T(N)(bulk) from above. A detailed analysis of the magnetic scattering, as well as the existence of a single bulk transition within the experimental sensitivity of our specific heat and magnetic susceptibility measurements, in contrast to the rich behavior shown by the near-surface region, indicates that the amplitude-modulated phase is not bulk representative, being actually sandwiched between the bulk paramagnetic and the equal-magnitude phases. Depth-temperature phase diagrams for pure and (Ga,Cd)-doped GdIn(3) are drawn on the basis of our results, which are discussed in terms of a three-phase coexistence scenario theoretically proposed for first-order transitions in the NSR.77

Vibrational and electronic excitations in the (Ce,La)MIn5 (M = Co,Rh) heavy-fermion family

We present a systematic study at ambient pressure of the phononic and electronic Raman-active excitations in the ab plane of the (Ce,La)MIn5 (M=Co,Rh) heavy-fermion family. We found that the characteristic Raman spectra of this family of compounds display two phonon modes at similar to 38 and similar to 165 cm(-1) and a broad electronic background centered at similar to 40 cm(-1). For CeCoIn5, the temperature dependence of these excitations shows anomalous behavior near T-*=45 K that may indicate a nontrivial renormalization of the electronic structure driven by strong correlations between hybridized 4f electrons.75

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

Perturbing the superconducting planes in CeCoIn5 by Sn substitution

In contrast to substitution on the Co or Ce site, Sn substitution has a remarkably strong effect on superconductivity in CeCoIn5-xSnx, with T-c -> 0 beyond only similar to 3.6% Sn. Instead of being randomly distributed on in-plane and out-of-plane In sites, extended x-ray absorption fine structure measurements show the Sn atoms preferentially substitute within the Ce-In plane. This result highlights the importance of the In(1) site to impurity scattering and clearly demonstrates the two-dimensional nature of superconductivity in CeCoIn5.95

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

Evolution of Eu2+ spin dynamics in Ba1-xEuxFe2As2

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Single crystals of Ba1-xEuxFe2As2 were studied by magnetic susceptibility, heat capacity, resistivity, and electron spin resonance (ESR) measurements. Spin-density wave (at T-SDW) and antiferromagnetic (at T-N) phase transitions were mapped as a function of x. For x >= 0.2, we found a single Eu2+ ESR Dysonian line that presents an isotropic linear increase (Korringa) of its linewidth (Delta H) above T-SDW which systematically decreases with decreasing x. In contrast, for a critical concentration x(c) (0.10 < x(c) < 0.20), Delta H decreases with increasing T, suggesting a distinct relaxation process that we associate with a Eu2+ Kondo single impurity regime. The Korringa rate suppression towards the Ba-rich compounds is claimed to be due to the reduction of the q-dependent exchange interaction between the Eu2+ f electrons and the conduction electrons, which is likely associated with an increasing of localization of Fe d electrons. This result may help the understanding of the SDW phase suppression (that can lead to superconductivity) in this class of materials.8616Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FINEP-BrazilFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [2006/60440-0, 2009/09247-3, 2010/11949-3, 2011/01564-0, 2011/23650-5

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