D. Majer

Plastic Vortex Creep in YBa2Cu3O7-x Crystals.

Local magnetic relaxation measurements in YBa2Cu3O72x crystals show evidence for plastic vortex creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the “fishtail” magnetization. In this range the activation energy Upl, which decreases with field, drops below the elastic (collective) creep activation energy, Uel, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa 2Cu3O72x. [S0031-9007(96)00878-2]

Plastic Vortex Creep in YBa{sub {bold 2}}Cu{sub {bold 3}}O{sub {bold 7{minus}}}{ital x} Crystals

Local magnetic relaxation measurements in YBa2Cu3O72x crystals show evidence for plastic vortex creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the “fishtail” magnetization. In this range the activation energy Upl, which decreases with field, drops below the elastic (collective) creep activation energy, Uel, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa 2Cu3O72x. [S0031-9007(96)00878-2]

Nature of the Irreversibility Line in Bi2Sr2CaCu2O8

Local vortex dynamics in Bi2Sr2CaCu2O8 single crystals was studied using novel microscopic GaAs/AlGaAs Hall-sensor arrays. The irreversibility line (IL) is found to exist in the absence of bulk pinning. At high temperatures the IL is due to geometrical barriers whereas at intermediate temperatures the irreversible behavior is determined by surface barriers. Bulk pinning governs the IL only at T < 22 K.

Measurement of the stray field emanating from magnetic force microscope tips by Hall effect microsensors

We describe the use of micronic Hall sensors as magnetic-field profilometers with submicron resolution. The procedure involves the deconvolution of Hall voltage maps produced by scanning the field source over the sensor, with a scanning probe microscope. The response function of an infinite Hall cross is calculated analytically in the two-dimensional case, using conformal mapping techniques. Various methods of deconvolution of the Hall voltage maps are presented and compared. The calculated response function is used for the deconvolutions, and different effective sensor sizes are tried. It is shown that the remaining main uncertainties come from the ignorance of the true response function of the sensor, ascribed to the charge depletion phenomenon that is known to occur at the sensor edges. The method is applied to thin-film magnetic force microscope tips for which a precise knowledge of the tips field at sample location proves crucial to image interpretation. Maximum fields in the range 10–100 Oe are found at a distance known to be about 100 nm from the tip contact surface, depending on the tip coating thickness and magnetization direction.

Measurement of the magnetic induction vector in superconductors using a double-layer Hall sensor array

We describe an experimental technique for simultaneous measurement of both the normal (Bz) and the in-plane (Bx) components of the magnetic induction field near the surface of a superconducting sample. This technique utilizes a novel design of a double-layered Hall sensor array fabricated from a GaAs/AlGaAs heterostructure containing two parallel layers of a two-dimensional electron gas. The effectiveness of this technique is demonstrated in measurements of Bx and Bz and the current distribution at the surface of a thin YBa2Cu3O7 crystal.

Investigation of flux creep in high-Tc superconductors using Hall-sensor array

The details of a new method for studying thermally activated flux creep in thin superconducting samples is described. The method employs a linear array of microscopic Hall sensors to measure the time and spatial dependence of the magnetic induction across the sample. These data are analyzed on the basis of the local rate equation for thermally activated flux motion, taking into account both the in-plane and out-of-plane components of the induction field. Following this analysis, flux creep parameters, such as the flux-line current density, flux-line average velocity, and the activation energy for flux creep, can be directly determined as a function of position and time. New experimental data in a superconducting Nd1.85Ce0.15CuO4−δ crystal demonstrate this method.

Geometrical barriers in type II superconductors

Abstract Vortex penetration in thin flat samples in perpendicular fields is governed by geometrical barrier effect. The main features of this novel barrier is vortex accumulation in the center of the sample and irreversible magnetization in the absence of bulk pinning. The developed theoretical model is confirmed by experimental results.

Surface currents and bulk pinning in Bi2Sr2CaCu2O8

Abstract The mechanisms giving rise to the irreversibility line (IL) in the phase diagram of Bi2Sr2CaCu2O8 and the mechanism behind the second maximum in the magnetization loops were determined using a novel Hall-sensor array. The IL is caused by bulk pinning at lower temperatures and by irreversible shielding currents above 22K. The second maximum is caused by vortex lattice melting, taking place at the same local magnetic induction Bm throughout the sample.

Vortex dynamics in a ring-like irradiated Bi2Sr2CaCu2O8 crystal

Abstract A Bi 2 Sr 2 CaCu 2 O 8 crystal selectively irradiated near the edges is studied using a Hall-sensor array. Vortices penetrating into the central J c = 0 region are “focused” in the center of the sample on increasing the applied field. In decreasing field, vortices leave the center and a large vortex accumulation is observed on the inner rim of the irradiated region. Spatially resolved magnetization measurements confirm the developed theoretical model.

Vortex matter phase transitions in Bi2Sr2CaCu2O8

Abstract Vortex-lattice phase transitions in Bi 2 Sr 2 CaCu 2 O 8 crystals are studied using local magnetization measurements. The vortex matter is found to exhibit at least three distinct phases. A rather ordered quasilattice phase is present at low fields. At elevated temperatures the quasilattice melts (or sublimates) through a first-order phase transition, whereas at lower temperatures a transition to a highly disordered vortex solid occurs at the second magnetization peak. A very low density of columnar defects results in significant pinning of the quasilattice, demonstrating the finite shear modulus of this phase. Local ac measurements reveal that in the presence of columns the pinned quasilattice still melts through a first-order phase transition with an observable step in equilibrium magnetization at elevated temperatures.