[en] Using the magnetic circular dichroism in X-ray absorption at the L_2,3-edges of the 3d-transition metals, we determined the element specific ratio between orbital and spin magnetic moment of the Heusler compounds Co₂CrAl, Co₂Cr_0.6Fe_0.4Al and Co₂FeAl. The orbital magnetic moment per spin is large (0.1-0.2) compared to bulk values of Fe and Co metals

No abstract available

[en] In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba_0.65Na_0.35Fe₂As₂. The isothermal magnetic field (H ∥ c axis) dependent magnetization (M) loops exhibit a second peak (SP) or ‘fishtail effect’, as well as remarkable flux jumps at low temperatures. The critical current density J_c obtained from the M(H) loops is rather high, of the order of 10⁶ A cm⁻². The analysis of the temperature- and field-dependent J_c implies that high J_c is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well as on the field sweep rate. The field for the first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field-dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggest that SP in M(H) is likely related to the crossover in creep dynamics from an elastic to a plastic mechanism. We have constructed the vortex phase diagram on the field–temperature plane. (paper)

[en] ⁷⁵As NMR investigation of a single crystal of superconducting LiFeAs is presented. The Knight shift and the in situ ac susceptibility measurements as a function of temperature and external field are indicative of two superconducting (SC) transition temperatures, each of which is associated with its own upper critical field. Strikingly, the Knight shift maintains its normal state value over a temperature range in the SC state before it drops abruptly, being consistent with spin-singlet pairing. Together with our previous NMR study, the anomalous SC state featuring the constant Knight shift is attributed to the extremely sensitive SC properties of LiFeAs, probably stemming from its proximity to a critical instability. (fast track communication)

[en] We employed a modified Bridgman and self-flux methods to grow single crystals of A(Fe, Co, Ni)₂As₂ (A=Ba,Sr) and (A_1-xK_x)Fe₂As₂(A=Ba,Eu) series, respectively. Using these techniques large single-crystals with dimensions up to 25 x 10 mm² and thickness up to 1 mm were grown. Susceptibility and resistivity of these crystals were studied. Narrow superconducting transition widths (∼0.5 K) and large residual resistivity ratios (∼ 7) indicate the high-quality of our single crystals. Here we present details of our crystal growth methods, structural characterizations and physical properties.

[en] In the present work we have studied polycrystalline samples of the (Gd,La)O_1-xF_xFeAs superconductor with different levels of fluorine and gadolinium doping by means of high field/high frequency electron spin resonance (HF-ESR) spectroscopy. The Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state occurring in the parent (Gd,La)OFeAs compounds at temperatures T_SDW∝130-150 K. In addition, in case of LaO_1-xF_xFeAs with 5% Gd doping there is a clear indication of the suppression of the magnetic order in the samples with the superconducting ground state. The GdOFeAs samples reveal an antiferromagnetic coupling between Gd and Fe planes which reflects in the splitting and shift of Gd ESR line below T_SDW∝130 K. Surprisingly, the 15% and 17% fluorine doped GdOFeAs samples, with T_c=20 K and T_c=45 K respectively, show the low-T ESR response similar to the undoped sample which indicates that the SDW is not completely suppressed in the SC samples. We compare HF-ESR data with results of transport measurements on these samples and discuss a possible contribution of magnetic rare-earths to the interplay between magnetism of the FeAs planes and superconductivity which evolves upon the fluorine doping.

[en] We investigated the magnetic spin-1 perovskite Ba${}_3$NiNb${}_2$O${}_9$ by means of complex ac susceptibility measurements at extreme sample conditions. Ba${}_3$NiNb${}_2$O${}_9$ with cubic perovskite structure (Pm-3m) has a random occupation of Nb(66 %)/Ni(33 %) at the center of the cubic perovskite unit cell. Different from the isostoichiometric sister compound, Ba${}_3$NiNb${}_2$O${}_9$ with P-3m1 structure which shows both uud-spin configuration and multiferroicity, the magnetic properties of the investigated system have not been studied below 2 K yet. For our single crystals, we observe a spin freezing transition at around 0.7 K. Furthermore, the peak of ${\mathrm{\chi <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ is suppressed by applying an external dc field of 200 mT and ${\mathrm{\chi <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}\mathrm{\prime <!--\; ???\; -->}}$ shows a sudden onset near the freezing temperature.

[en] In this work we have studied by means of electron spin resonance (ESR) spectroscopy polycrystalline samples of LaO_1-xF_xFeAs (x = 0, 0.1) with small levels of Gd doping (2% and 5% with respect to La). The Gd³⁺ ion is an established spin probe to study local magnetic and charge environments in the crystal lattice. In the parent compound the Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state taking place at a temperature T_SDW∝130 K. Here, the analysis of the low-T ESR spectra gives evidence for the occurrence of magnetically non-equivalent Gd sites and possibly sites with different local charge environments. In the case of the superconducting samples with 10% of the F doping (T_c = 25 K) the Gd ESR gives no indication of the SDW order.

[en] We report a low-temperature specific heat study of single crystals of the heavily hole doped superconductor Ca_0_._3_2Na_0_._6_8Fe_2As_2. This compound exhibits bulk superconductivity with a transition temperature T_c ∼ 34 K, which is evident from the magnetization, transport, and specific heat measurements. The zero field data manifests a significant electronic specific heat in the normal state with a Sommerfeld coefficient γ ∼ 53 mJ/mol K"2. Using a multi-band Eliashberg analysis, we demonstrate that the dependence of the zero field specific heat in the superconducting state is well described by a three-band model with an unconventional s_± pairing symmetry and gap magnitudes Δ_i of about 2.35, 7.48, and -7.50 meV. Our analysis indicates a non-negligible attractive intraband coupling,which contributes significantly to the relatively high value of T_c. The Fermi surface averaged repulsive and attractive coupling strengths are of comparable size and outside the strong coupling limit frequently adopted for describing high-T_c iron pnictide superconductors. We further infer a total mass renormalization of the order of five, including the effects of correlations and electron-boson interactions.

[en] We have performed inelastic neutron scattering (INS) studies on two undoped compounds: superconducting paramagnetic LiFeAs and non-superconducting antiferromagnetic (AF) BaFe_2As_2. INS using LiFeAs single crystals revealed clear evidence for AF fluctuations at an incommensurate Q-vector, which vanish at low energies. The excitations clearly responds to the opening of the superconducting gap by a redistribution of spectral weight. Applying more sophisticated methods like polarized neutron scattering with longitudinal polarization analysis to BaFe_2As_2 single crystals revealed anisotropic fluctuations, where the out-of-plane mode lies lower than the in-plane mode. This finding is astonishing as the static moment aligns along the FeAs layers. The fit of a J_1-J_2 model using the coupling constants of SrFe_2As_2 and taking into account the instrumental resolution yielded the single-ion anisotropies responsible for both gaps. In comparison to theoretical values we find that the order of the gaps is reversed and that the spin-orbit coupling is underestimated in the calculations.