G. Wortmann

151Eu-Mössbauer study of the high-Tc superconductor EuBa2Cu3O7−x

Abstract The temperature dependence of the isomer shift S and recoil-free fraction ƒ of the 21.6 keV Mossbauer resonance of 151Eu is studied in the high-Tc superconductor EuBa2Cu3O7−x between 4.2 and 300 K. Eu is found to be present as stable Eu3+ ion with an isomer shift and a Mossbauer Debye temperature typical for ionic oxygen surroundings. A possible change in S at Tc is smaller than 0.02 mm/s. The Eu3+ nuclei experience an electric-quadrupole interaction with a main axis of the electric-field gradient tensor V = −3.1(5) × 10 17 V cm 2 and an asymmetry parameter η ≠ 0.

Mössbauer study of the magnetic high-Tc superconductor GdBa2Cu3O7−δ

Abstract The magnetic ordering of the Gd sublattice in superconducting GdBa2Cu3O7-δ is studied by Mossbauer spectroscopy using the 86.5-keV gamma resonance of 155Gd. Below the Neel temperature of TN ≌ 2.4 K, the magnetic hyperfine field at the Gd nucleus reflects the increasing local sublattice magnetization extrapolating to a saturation value of Beff(T=0 K) ≌ 31.5 T. The effective magnetic hyperfine field is found to be parallel to the main axis of the electric-field-gradient tensor, which is characterized by an asymmetry parameter of n = 0.40 ± 0.05. The observed isomer shift and the value of Beff are typical for trivalent Gd compounds with negligible conduction-electron contributions.

Magnetic order of the Pr sublattice in tetragonal and orthorhombic Pr1−xGdxBa2Cu3O7−δ observed by 155Gd-Mössbauer spectroscopy

Abstract 155 Gd-Mossbauer studies of (Pr 0.925 Gd 0.075 )Ba 2 Cu 3 O 7−δ in the tetragonal ( δ = 0.9) and orthorhombic ( δ = 0.1) phase reveal transitions to magnetic order at T N = 7.5 K and 16.5 K, respectively, of the Pr sublattice. The hyperfine interaction parameters of 155 Gd in the orthorhombic sample above T N compare well with those data obtained in orthorhombic GdBa 2 Cu 3 O 7−δ and prove that Gd substitutes for Pr. The marked difference in ordering temperatures of the Pr sublattice for the two phases is discussed in the terms of the Pr-4f electron hybridization depending sensitively on the oxygen content δ.

Mössbauer studies of YBa2Cu3O7−δ-type high-Tc superconductors

The local magnetic, electronic, and structural properties of (RE)Ba2Cu3O7−δ supercondcutors (RE=Gd, Dy, and Eu) were studied by Mössbauer spectroscopy using the resonances of155Gd,161Dy,151Eu, and57Eu. In GdBa2Cu3O7−δ, different magnetic ordering behaviors of the Gd sublattice are found for the orthorhombic (superconducting) and tetragonal (non-superconducting) phases. In DyBa2Cu3O7−δ, the magnetic moments of the respective CEF ground states in the orthorhombic and tetragonal phases are derived from paramagnetic hyperfine splittings at 1.4 K. In both DyBa2Cu3O7−δ and EuBa2Cu3O7−δ, anomalies connected with the superconducting transitions in isomer shift, recoil-free fraction, and relaxation behavior were looked for, but not found. The electric-quadrupole splittings observed for both systems are discussed in connection with the lattice EFGs derived for the Gd system. In GdBa2 (Cu0.995Fe0.005)3O7−δ, the local properties of the various Fe sites are investigated over a wide temperature range in both the orthorhombic and tetragonal phase. The magnetic ordering of the Gd sublattice in the orthorhombic phase and of the Cu(2) sublattice in the tetragonal phase, respectively, is monitored via the magnetic splittings at the various Fe sites. Possible assignments of Cu(1) and Cu(2) sites as well as different oxygen configurations around the substituted Fe ions are discussed.

Gd–LIII EXAFS study of structural and dynamic properties of Gd@C82 between 10 and 300 K

The location of endohedral Gd ions in Gd@C 82 is studied by Gd-L 111 X-ray absorption spectroscopy in the temperature range 10-300 K. The near-edge data point to a covalent bond of trivalent Gd ions with the carbon cage. The EXAFS data can be well fitted with a Gd position above a carbon hexagon with two neighbour shells of carbon at distances R 1,2 =2.49(3) and 2.95(5) A, corresponding to a large off-center position of about 1.8 A from the center of the C 82 cage. The second cumulants of these distances are relatively large and weakly depending on temperature. This points to a considerable distribution in Gd-C and Gd-C 2 distances and confirms the relatively strong binding of the Gd ion to the C 82 cage. These results will be discussed together with previous EXAFS studies and theoretical calculations of Y@C 82 and La@C 82 .

Ionic diffusion and local hopping in copper chalcohalide glasses measured using 64Cu tracer and 129I-Mössbauer spectroscopy

For the first time, 64Cu tracer measurements of ionic diffusion were performed for several copper-rich glass compositions in the CuIue5f8As2Se3, CuIue5f8SbI3ue5f8As2Se3, CuIue5f8PbI2ue5f8As2Se3, CuIue5f8PbI2ue5f8SbI3ue5f8As2Se3 and Cu2Seue5f8As2Se3 systems. In accordance with previous a.c. impedance results and Wagner d.c. polarization measurements, it was found that pure Cu+ ion-conducting glasses (50CuIue5f817PbI2ue5f833As2Se3 and 50CuIue5f820PbI2ue5f810SbI3ue5f820As2Se3) exhibit the highest copper tracer diffusion coefficients, DCu, and the lowest diffusion activation energies. The values of DCu at room temperature are higher by 4.5–5.5 orders of magnitude than those in an As2Se3 glass. The Haven ratio, HR, is found to be 0.52–0.61 (ternary glass) and 0.93–1.00 (quaternary glass). Short-range diffusional displacements of the iodide ions induced by the hopping Cu+ ions are also detected in the CuIue5f8PbI2ue5f8SbI3ue5f8As2Se3 glassy system using 129I-Mossbauer spectroscopy in the temperature range of 4.2 to 305 K. The activation energy of local hopping, Eh ≈ 0.31 eV, is very similar to that of bulk ionic conductivity (0.37 eV) and copper diffusion (≈ 0.33 eV). In contrast to CuI-based vitreous alloys, 50Cu2Seue5f850As2Se3 glass exhibits DCu that are two to five orders of magnitude lower, and the copper ion transport number,tCu+, is between 10−3 and 10−4 in the temperature range 140–170 °C.

Synthetic metals: Applications of the Mössbauer effect and other methods

A review is given about the current activities in the field of the “synthetic metals”. This nomenclature is used for materials which consist basically of non-metallic components, but reach metallic conductivities by doping or intercalation reactions with additional components acting as donors or acceptors. Typical examples are conducting polymers like doped polyacetylene or graphite intercalation compounds. The contributions of Mössbauer studies in understanding the electronic, magnetic, and structural properties of such systems are outlined. Comparison with other spectroscopic methods is given in selected cases.

Magnetic ordering and spin fluctuation behavior in compounds of the type, Ce2(Pd,Rh)2In

The results of heat capacity (C), electrical resistivity and Ce L III -edge measurements on the alloys, Ce 2 Pd 2 In, Ce 2 Rh 2 In and some of their derivatives, are reported. The temperature dependence of C data of Ce 2 Pd 2 In exhibits features attributable to short range magnetic correlations preceding a sharp upturn at 4 K arising from long range magnetic ordering. An increase in Pd/In ratio apparently diminishes the magnetic transition temperature and the features due to spin fluctuation effects. The low temperature values of C/T of Y 1.6 Ce 0.4 Pd 2 In and Lu 1.6 Ce 0.4 Pd 2 In are large, attributable to heavy fermion behavior. In the compound Ce 2 Rh 2 In, Ce is found to be strongly mixed valent, with a large spin fluctuation temperature.

Magnetic ordering of the Gd sublattice in superconducting GdBa2Cu3O7−δ

Abstract The magnetic ordering behavior of the Gd sublattice in superconducting GdBa2Cu3O7−δ is studied by1 5 5 Gd-Mo¨ssbauer spectroscopy from 1.2 K to 20 K. From the observed temperature dependence of the magnetic hyperfine field, the Gd sublattice magnetization is derived up to the magnetic ordering temperature,TN = 2.27(2) K, where a first-order-like transition into a regime with strong magnetic fluctuations is observed. The results are discussed in the framework of theories developed for the interplay between magnetic order and superconductivity.

Spin reorientation transition in Nd2Fe14B studied by 145Nd-Mössbauer spectroscopy

Abstract Detailed information on magnitude and orientation of the Nd sublattice magnetization is obtained from Mossbauer studies of a Nd2Fe14B single crystal using the 72.5 keV Mossbauer transition of 145Nd, At 4.2 K, the average Nd moment (3.20 μB) is tilted by an angle of 36° relative to the c-axis, non-collinear with the Fe moment. The temperature dependence of the Nd hyperfine field exhibits a sharp drop at the spin reorientation temperature (≅140 K) accounting for the kink observed in the total magnetization curve. Most elaborate molecular and crystalline field calculations do not reproduce this kink nor the opening angle between the Fe and Nd moments.