V.V. Chabanenko

Flux Jumps and H-T Diagram of Instability for MgB2

We present a study of magneto-thermal instabilities in polycrystalline MgB2 superconductor, by magnetic hysteresis loop measurements and by investigations of magnetic flux dynamics with a miniature Hall probe. Temperature and magnetic field ranges where the flux jumps may be observed have been determined. On the basis of measurements of the magnetic flux dynamics, an average magnetic diffusivity describing the process of the flux jump is estimated. This parameter is compared with the thermal and magnetic diffusivities calculated on the basis of available data for thermal conductivity, heat capacity and resistivity. It is shown that the estimated value of the field of the first flux jump is influenced significantly by the field dependence of specific heat. In order to explain the observed phenomenon, the temperature reached by the sample during the flux jump at different magnetic fields is calculated.

Magnetothermal instabilities in type II superconductors: The influence of magnetic irreversibility

Complete magnetization and magnetostriction loops for type II superconductors were calculated in terms of the following pinned critical state models which incorporate the flux jump instability criterion: the Kim–Anderson model, the exponential model, the linear model, and the model with peak effect. These results were used for constructing magnetic field–temperature (H–T) diagrams of instability of these critical state models. Magnetothermal instabilities of superconductors with peak effect were investigated experimentally and theoretically. Regions in the H–T diagram where flux jumps occur were built and compared with experimental M(H) hysteresis loops. We have explained the phenomenon of “island” jumps. The influence of flux trapping on the flux jump regions of magnetic fields was studied.

Observation of an odd longitudinal magnetoresistance in YBa2Cu3O7−δ single crystals with unidirected twins

Abstract Unidirected twin planes are the cause of anisotropic pinning of vortices in YBa 2 Cu 3 O 7− δ (below YBaCuO) single crystals. In contrast to the case of only isotropic pinning, two new effects may appear due to the pinning anisotropy: guiding of vortices along twin planes and anisotropy of the Hall resistivity. The former leads to the appearance of the even (with respect to the reversal of a magnetic field direction) transverse resistivity and influences the Hall angle, while the last causes new additional odd contribution to the longitudinal resistivity. The paper presents magnetoresistivity measurements in the vortex liquid phase of YBaCuO single crystals with different angles between unidirected twins and current direction in the magnetic fields up to 12 T. Field inversion was used for separation of the even and odd components of the measured transverse and longitudinal voltages in order to select new contributions to the resistivity in the case of anisotropic pinning. The results are consistent with the model of anisotropic pinning where the anisotropy of the Hall drag coefficient is caused by twins.

Pinning induced magnetostriction in ceramic high temperature superconductors

Abstract Transverse and longitudinal magnetostriction was measured in ceramic YBa2Cu3O7−δ and Hg0.8Pb0.2Ba2Ca2Cu3O8+δ superconductors by the strain gauge technique. The measurements were performed in an external magnetic field up to 12 T. The observed magnetostriction was shown to consist of two components. The first one is connected with the intragrain — and the second one with the intergrain screening currents. Both components of the magnetostriction were analyzed independently in frames of an isotropic theory of the pinning induced magnetostriction.

Guiding of vortices in YBa2Cu3O7−δ single crystals with unidirected twins

Abstract Guided motion (GM) of vortices in vortex liquid phase of YBa2Cu3O7−δ (YBaCuO) single crystal with unidirected twins is studied by measuring transverse voltages in geometry where the current is flowing in the a–b plane and the external magnetic field H is oriented along the c-axis. Field inversion was used for separation of the even component of the measured transverse and longitudinal voltages in order to select new even contribution to magnetoresistivity caused by GM of vortices. The results are discussed in terms of the phenomenological approach to the anisotropic pinning caused by unidirected twins.

H-T PHASE DIAGRAM FOR THE GIANT MAGNETIC FLUX JUMPS IN LOW TEMPERATURE SUPERCONDUCTORS AND HIGH TEMPERATURE SUPERCONDUCTORS

We have studied magnetothermal instabilities both theoretically and experimentally. Magnetostriction and magnetization hysteresis loops with flux jumps were calculated over a wide range of experimental parameters employing three critical state models: the original Bean model, the Kim–Anderson model, and the exponential model. The influence of the magnetic prehistory on the flux jumps, magnetostriction, and the magnetization was investigated for the LaSrCuO crystal. H–T diagrams of flux instabilities were constructed from calculation and from experimental results. One can see good qualitative agreement between these two diagrams.

The Peculiarities of Magnetic Flux Dynamics at Magnetothermal Instability in Textured Bi2Sr2CaCu2O8+δ

The magnetic flux dynamics was investigated in a textured Bi2Sr2CaCu2O8+δ sample using a miniature Hall probe placed on the surface of the superconductor. The process of magnetic flux penetration into the superconductor was found to have a complex structure and consist of two processes with different time scales. The first one has a length of tens of microseconds and is considered as locally adiabatic. The second one has a length comparable with the time of thermal diffusion. The importance of the above two processes in the development of magnetic instability is analyzed.

Failure of textured YBaCuO samples in the strong magnetic field

Abstract It has been shown that the effect of strong magnetic field on the textured YBaCuO superconductors leads to a new phenomenon — magnetolamination. This phenomenon consists in mechanical lamination parallel to the ab -plane into the thin platelets under the high magnetic field cycling. A mechanism responsible for the textured sample cracking due to the high magnetic field is proposed and discussed in detail.

Reversible mechanism of magnetothermal instabilities in melt-textured YBaCuO

Abstract The anisotropy of the development of instabilities in the metl-textured YBaCuO samples with remanent magnetization of 600 emu/g has been studied in a magnetic field up to 12 T. Large 500 emu/g jumps of the magnetic moment of the sample due to the development of the magnetothermal instability have been observed. A giant (10 −4 ) magnetostriction caused by the magnetic flux pinning has been revealed. Stabilization of the critical state of the sample by reversal of magnetization has been found. However, when the sample is heated to temperatures higher than 100 K and subsequently cooled the magnetic jumps restore completely. A new reversible mechanism of instability generation based on the phenomenon reversibility of plastic deformations of high-temperature superconductors (HTS) is proposed to explain the regeneration of critical state instability.

Giant magnetostriction and flux instabilities in textured YBaCuO plates

The anisotropy of unusually large magnetostriction arising from flux pinning for melt processing YBaCuO was investigated. The sample length changes ΔL in the ab-plane when the magnetic field H is parallel and perpendicular to the c-axis were measured. Large jumps of the magnetostriction coefficient associated with flux jumps were found. Complicated deformation of the sample under remagnetization was observed.