Magnetic field penetration in MgB₂ single crystals: pinning and Meissner holes

The evolution of flux distribution in MgB₂ single crystals during their remagnetization was imaged with magneto-optical technique. Meissner holes, formed as the areas where the annihilation of vortices and antivortices takes place, were found at the boundary between oppositely magnetized parts of the crystal. Gradient of magnetic induction in the vicinity of Meissner holes was found to be enhanced. Finger-like structures of convex shape, formed during the penetration of magnetic flux inside the crystal, were observed and explained as an effect of inhomogeneous pinning and demagnetizing field redistribution in the sample

Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d)

Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d) have been studied by means of magnetization measurements in the temperature range between 1.95 K and Tc, in an external magnetic field up to 9 T. Flux jumps were found in the temperature range 1.95 K - 6 K, with the external magnetic field parallel to the c axis of the investigated sample. The effect of sample history on magnetic flux jumping was studied and it was found to be well accounted for by the available theoretical models. The magnetic field sweep rate strongly influences the flux jumping and this effect was interpreted in terms of the influence of both flux creep and the thermal environment of the sample. Strong flux creep was found in the temperature and magnetic field range where flux jumps occur suggesting a relationship between the two. The heat exchange conditions between the sample and the experimental environment also influence the flux jumping behavior. Both these effects stabilize the sample against flux instabilities, and this stabilizing effect increases with decreasing magnetic field sweep rate. Demagnetizing effects are also shown to have a significant influence on flux jumping.Comment: 10 pages, 6 figures, RevTeX4, submitted to Phys. Rev.

Guided vortex motion in superconductors with a square antidot lattice

We have measured the in-plane anisotropy of the vortex mobility in a thin Pb film with a square array of antidots. The Lorentz force, acting on the vortices, was rotated by adding two perpendicular currents and keeping the amplitude of the net current constant. One set of voltage probes was used to detect the vortex motion. We show that the pinning landscape provided by the square antidot lattice influences the vortex motion in two different ways. First, the modulus of the vortex velocity becomes angular dependent with a lower mobility along the diagonals of the pinning array. Second, the vortex displacement is preferentially parallel to the principal axes of the underlying pinning lattice, giving rise to a misalignment between the vortex velocity and the applied Lorentz force. We show that this anisotropic vortex motion is temperature dependent and progressively fades out when approaching the normal state.Comment: 5 pages, 4 figure

The influence of deformation by equal-channel multiple-angle pressing and durable thermal treatment on phase composition and physical properties of NbTi alloy

The influence of deformation by equal-channel multiple-angle pressing and of thermal treatment on phase composition, state of structure and current-carrying capacity of NbTi alloy superconductor have been studied. Deformation is shown to result in the decreasing of subgrain size (100.88200 nm). In the temperature range to 450 ℃, the used structure was found thermally stable. This is explained by precipitation of disperse particles of the second α-phase at the subgrain boundaries. After the deformation there was a distinct increase in magnetization of the alloy due to magnetic flux fixing at boundaries of crystallites and at α-phase precipitations.Дослiджено вплив деформацiї рiвноканальним багатокутовим пресуванням та термообробки на фазовий склад, структурний стан i струмонесучу здатнiсть надпровiдника зi сплаву NbTi. Виявлено, що деформацiя спричиняе зменшення розмiрiв субзерен (100...200 нм). Встановлено термостабiльнiсть структури, сформованоi пресуванням у дiапазонi температур До 450 ℃, що пояснюється видiленням дисперсних частинок другої α-фази на межах субзерен. Пiсля деформацiї спостерiгається виразне пiдвищення намагнiчування сплаву внаслiдок закрiплення магнiтного потоку на межах кристалiтiв та видiленнях α-фази.Исследовано влияние деформации равноканальным многоугловым прессованием и термообработки на фазовый состав, структурное состояние и токонесущую способность сверхпроводника из сплава NbTi. Установлено, что деформация приводит к уменьшению размеров субзерен (100.88200 нм). Установлена термостабильность структуры, сформированной прессованием в диапазоне температур до 450 ℃, что объясняется выделением дисперсных частиц второй α-фазы по границам субзерен. После деформации наблюдается отчетливое повышение намагничивания сплава вследствие закрепления магнитного потока на границах кристаллитов и выделениях α-фазы