Field Of YBCO Research Articles

Effect of Interface-Induced Exchange Fields on Cuprate-Manganite Spin Switches

We examine the anomalous inverse spin switch behavior in La0.7Ca0.3MnO3(LCMO)/YBa2Cu3O7-δ (YBCO)/LCMO trilayers by combined transport studies and polarized neutron reflectometry. Measuring magnetization profiles and magnetoresistance in an in-plane rotating magnetic field, we prove that, contrary to many accepted theoretical scenarios, the relative orientation between the two LCMO's magnetizations is not sufficient to determine the magnetoresistance. Rather the field dependence of magnetoresistance is explained by the interplay between the applied magnetic field and the (exponential tail of the) induced exchange field in YBCO, the latter originating from the electronic reconstruction at the LCMO/YBCO interfaces.

The Effect of a Ferromagnetic Medium on the Field of YBCO Trapped Field Magnets

Trapped field magnets (TFMs) are characterized by the maximum field they can retain when they are activated in air (~vacuum). In some applications however, TFMs are partially embedded in a ferromagnetic medium. We consider, experimentally and theoretically, some simple geometries involving contact between TFMs and ferromagnetic media. We find: an increase in maximum TFM field exceeding a factor of 2.53; a change in field profile from the well-known conical TFM field to a uniform field in the ferromagnetic material; and fields produced by present day TFMs capable of saturating Hyperco-50 alloy. The observed flux creep rate of the TFM field was found to be the same in air and in contact with ferromagnetic material.

Anisotropic behaviour of the melting line and the low critical field in YBCO

The anisotropy of the melting line and the low field critical point H lcp of the vortex lattice has been determined in a twinned YBa 2Cu 3O 7− δ single crystal. We observe that the melting line follows the intrinsic angular behaviour related to the sample anisotropy and terminates in a low critical point, H lcp. This low critical field follows the same anisotropic behaviour as the melting line when the disordered phase is originated by point defects, while an upward shift of H lcp is observed when the vortices are pinned at twin boundaries.

Analysis of the excess resistivity induced by a low magnetic field in YBCO and BSCCO compounds

The results of a (weak) DC magnetic field induced excess resistivity in granular high-temperature superconductors are analyzed. Assuming a random mixture of superconducting and normal regions, the two characteristic excess resistivity peaks are interpreted as originating from two different mechanisms: the higher temperature peak in excess resistivity is attributed to a distribution of the critical temperature in individual grains affected by the applied magnetic field, this results in a quasi-percolative normal current energy dissipation. The lower temperature wide peak is attributed to an interplay between strong pinning due to the spatial inhomogeneity of the condensation energy on one hand and to the onset of proximity effect with decreasing temperature, on the other. The results obtained with various concentrations of Ag admixture and under varying magnetic field are consistent with the predictions of this model.

A 94 GHz interferometer for use in establishing the upper critical field in YBCO

This paper describes a 94 GHz interferometer that was used to measure the change in the complex impedance in a thin-film, high-temperature superconductor (YBCO) when under the influence of an extremely high-pulsed magnetic field. It will review the design features necessary to meet the experimental requirements, describe the circuit used, outline the experimental setup, and present a set of data obtained at 4 K.

Magnetic flux penetration and current density in superconducting thin films

The magnetic behaviour of type-II superconducting thin films subjected to a transverse external field can be explained taking demagnetising effects into account. We studied this problem following two different approaches: on the one hand, we computed flux diffusion and circulating currents, on the basis of a recent model presented in the literature; on the other hand, we carried out measurements of local magnetic field on the film surface by means of a self-made Hall probe magnetometer. We compared the measured and the computed hysteresis loops and magnetisation decays and we found good agreement. By the fit of the experimental hysteresis loops, we also obtained the dependence of the critical current density on temperature and local magnetic field in YBCO thin films.

Josephson behaviour and flux penetration effects in YBCO double tilt-angle step-edge junctions

YBCO thin-film step-edge junctions have been prepared whose characteristics show features corresponding to the two tilt-angle grain-boundary junctions formed at the step. The I-V characteristics of these junctions are fitted to high accuracy by a model consisting of two noisy resistively-shunted Josephson junctions. In applied magnetic fields up to a few hundred microtesla, both junction critical currents show reversible flux modulation, in one case indicating excellent uniformity of the critical current distribution along the step. In stronger magnetic fields, the junction critical current modulation shows evidence of flux penetration into the film, i.e. the mixed state. Taking demagnetising effects into account the lower critical field of YBCO at 77 K, /spl mu//sub 0/H/sub e1,e/, is found to be 12 mT. >