A.I. Mansour

Dramatic strain induced modification of the low field anisotropic magnetoresistance in ultrathin manganite films

The authors investigated the anisotropic magnetoresistance (AMR) in ultrathin La-based manganite films grown on various substrates. It was found that depending on the strain state, the AMR in some of these systems exceeds 100% and can even change sign. These changes are very dramatic when compared to the few percent change in AMR in conventional ferromagnets. The mechanism behind these large changes in the AMR is discussed.

Metal-insulator transition, specific heat, and grain-boundary-induced disorder in Sm0.55Sr0.45MnO3

The effects of the grain-boundary-induced lattice disorder on the resistivity, the magnetization, and the specific heat of a prototypical manganite Sm0.55Sr0.45MnO3 near half doping were investigated at temperatures near the metal-insulator transition. An increasing lattice disorder softens the magnetic phase transition from a first order phase transition into a second order transition. Furthermore, the peaks in the resistivity and specific heat are broaden and there is an increase in the charge-carrier scattering rates in the metallic and insulating states. The origin of these phenomena is discussed.

Lattice-strain control of magnetoresistivity and magnetoresistive anisotropy induced by ion-beam milling in La0.65Ca0.35MnO3 films near the metal-insulator transition

The lattice-disorder-induced changes in the magnetoresistivity and the magnetoresistive anisotropy of epitaxial ion-beam-milled La0.65Ca0.35MnO3 films were investigated. The results suggest that, for films less than 20nm thick, an increase of the magnetoresistivity and a reduction in the magnetoresistive anisotropy of the ion-beam-milled films are governed mostly by the point-defect-induced deformation, in contrast to the as-grown films where an increase of both the magnetoresistivity and the magnetoresistive anisotropy is determined by the epitaxial strain.

Temperature dependence of the persistent critical current and instabilities in MgB2 thin films

Temperature dependences of the persistent critical current density Jc(T) and its relaxation rate S=dlnJ∕dlnt were measured from 8.0K to Tc in MgB2 ring-shaped thin films. Jc(T) was observed to approach a Ginzburg-Landau-like temperature dependence with a decreasing Tc in magnesium deficient films. No correlation was found between the current density and the magnitude of the relaxation rate, suggesting percolative (filamentary) flow of the supercurrent. A suppression of the supercurrent density (instability) was observed between 8.0 and 10.5K in MgB2 thin films of Tc above 30K, but not in films of lower transition temperatures. Vacuum annealing shifts the instability to lower temperatures.

Softening of the first-order magnetic phase transition and magnetotransport properties of Sm0.55Sr0.45MnO3 manganite

We have investigated the changes in the magnetotransport properties of Sm0.55Sr0.45MnO3 manganites caused by the disorder-induced softening of the first-order magnetic phase transition into a second-order one. We analyzed the low field magnetization, magnetotransport, and specific heat in the presence of the first-order and the second-order magnetic phase transitions.

Superconducting properties of Tl-2223 phase substituted by iron

Bulk superconducting samples of type Tl2Ba2Ca2Cu3-xFexO10-δ; with 0 ≤ x ≤ 0.4, have been prepared using a single step of solid-state reaction. The prepared samples have been characterized using X-ray powder diffraction (XRD), scanning electron microscope (SEM) and microprobe analysis (MPA). The tetragonal structure of Tl-2223 did not change with the partial replacement of Cu2+ by Fe3+ ions, whereas the lattice parameters were found to vary as function of Fe-content. The superconducting transition temperature Tc determined from electrical resistivity and ac magnetic susceptibility measurements shows suppression in its value as Fecontent increases. The suppression in Tc was attributed to the magnetic disorder and Cooperpairs breaking. The critical current density Jc and field irreversibility Bir were calculated as function of Fe-content.

Persistent supercurrents in ring-shaped Bi2Sr2CaCu2Ox single crystal

A superconducting ring could be used to simulate the supercurrent conduction, and its interaction with magnetic vortices in a superconducting solenoid, allowing one to investigate the nature of the vortex structure and its pinning in the presence of the persistent supercurrent. The dissipation of the persistent supercurrent has been studied in a ring-shaped high purity single crystal of Bi2Sr2CaCu2Ox in order to obtain the information about the exponent μ, a parameter in the scaling relation between the effective energy barrier against vortex motion Ueff and the persistent current density J. The measurements of the persistent supercurrent decay show a transition from a strongly nonlogarithmic to a logarithmic decay regime with an increasing temperature. In response to a small increase in the concentration of oxygen vacancies μ decreases in the logarithmic decay regime but remains almost constant in the nonlogarithmic one.

Local magnetic properties of Y12Co5Bi and Gd12Co5Bi studied by muon spin relaxation

The recently discovered (RE)12Co5Bi (where RE is a rare earth element) system has rich magnetic phase diagrams. We applied muon spin relaxation to study the local magnetism in polycrystalline samples of Y12Co5Bi and Gd12Co5Bi. Our results indicate a magnetic transition at around 100K for Gd12Co5Bi. By contrast, Y12Co5Bi does not show a magnetic transition as expected since Y3+ does not contain any f electrons. These results are consistent with the dc susceptibility measurements. We also estimate and discuss the fluctuation rates in the high temperature paramagnetic regime for Gd12Co5Bi.

C-axis persistent current and Cooper-pair tunneling through intrinsic Josephson junctions in a ring-shaped YBa2Cu3O7−δ film

We report the direct experimental observation of the temperature dependence of the persistent current Icj due to tunneling Cooper pairs along the c-axis intrinsic Josephson junctions integrated into YBa2Cu3O7−δ ring-shaped thin films. The measured Icj exhibits a linear temperature dependence over a wide range of temperatures well below Tc. Similar behavior was observed in different samples, confirming the reproducibility of the fabrication technique and the reliability of the results.

INTRINSIC NANOSCALE DISORDER IN HTSC: EFFECT ON THE PHYSICAL PROPERTIES

Recent STM studies revealed nanoscale electronic disorder on the crystal surface in many cuprates. In BSCCO strong correlations between oxygen defect distributions on its surface and both the gap map and the coherence peak amplitude showed that the off-center distortions in the positions of oxygen atoms are responsible for most of the electronic disorder. Additional information on nanoscale inhomogeneities and its relationship to the lattice strain and oxygen redistribution can be obtained from the electrical transport that uses Percolating Persistent Supercurrents which are known to bypass regions of a reduced order parameter (macroscopic crystal defects). Our investigations identified universal (sample independent) features in the superconducting properties which can be attributed to the presence of a nanoscale inhomogeneity, such as nanogranular Josephson effects, strain-induced filamentary and percolative flow of the transport current, etc. Local oxygen redistribution, induced either by careful low temperature annealing or by room temperature aging, modifies both the superconducting and the normal state properties.