M. Baziljevich

Dendritic magnetic instability in superconducting MgB2 films

Magneto-optical imaging reveal that below 10 K the penetration of magnetic flux in MgB2 films is dominated by dendritic structures abruptly formed in response to an applied field. The dendrites show a temperature-dependent morphology ranging from quasi-1D at 4 K to large tree-like structures near 10 K. This behaviour is responsible for the anomalous noise found in magnetization curves, and strongly suppresses the apparent critical current. The instability is of thermo-magnetic origin, as supported by our simulations of vortex dynamics reproducing the variety of dendritic flux patterns.

Real-time magneto-optical imaging of vortices in superconducting NbSe2

We present here a new experimental tool for the direct observation of magnetic vortices in type-II superconductors. The magneto-optical imaging technique has been improved to enable single vortex observation at low flux densities. The main advantage of the new method is its high temporal resolution combined with the applicability to any superconducting sample with a flat surface. We give a short description of the experimental set-up and show examples of results obtained for a NbSe2 single crystal at 4.0 K.

Faraday rotation spectra of bismuth-substituted ferrite garnet films with in-plane magnetization

Single crystal films of bismuth-substituted ferrite garnets have been synthesized by the liquid phase epitaxy method where gadolinium gallium garnet substrates are dipped into the flux. The growth parameters are controlled to obtain films with in-plane magnetization and virtually no domain activity, which makes them excellently suited for magnetooptic imaging. The Faraday rotation spectra were measured across the visible range of wavelengths. To interpret the spectra we present a simple model based on the existence of two optical transitions of diamagnetic character, one tetrahedral and one octahedral. We find excellent agreement between the model and our experimental results for photon energies between 1.77 and 2.53 eV, corresponding to wavelengths between 700 and 490 nm. It is shown that the Faraday rotation changes significantly with the amount of substituted gallium and bismuth. Furthermore, the experimental results confirm that the magnetooptic response changes linearly with the bismuth substitution.

Faraday rotation and sensitivity of (100) bismuth-substituted ferrite garnet films

We have investigated the Faraday rotation of in-plane magnetized bismuth-substituted ferrite garnet films grown by liquid phase epitaxy on (100) oriented gadolinium gallium garnet substrates. The Faraday spectra were measured for photon energies between 1.7 and 2.6 eV. To interpret the spectra, we use a model based on two electric dipole transitions: one tetrahedral and one octahedral. Furthermore, the Faraday rotation sensitivity was measured at 2.3 eV, and found to be in good agreement with the theoretical predictions. In particular, we find that the sensitivity increases linearly with the bismuth content and nonlinearly with the gallium content.

Dendritic flux patterns in MgB2 films

Magneto-optical studies of a c-oriented MgB2 film with a critical current density of 107?A?cm-2 demonstrate a breakdown of the critical state at temperatures below 10?K. Instead of conventional uniform and gradual flux penetration in an applied magnetic field, we observe an abrupt invasion of complex dendritic structures. When the applied field subsequently decreases, similar dendritic structures of the return flux penetrate the film. The static and dynamic properties of the dendrites are discussed.

Magneto-optical imaging setup for single vortex observation

A recently developed high-resolution magneto-optical imaging (MOI) setup is reviewed. It is the first MOI system capable of resolving the individual vortices in a type-II superconductor. We give a detailed description of the whole setup, and discuss its measured properties in terms of magnetic sensitivity and signal-noise characteristics. A simple model for the image intensity distribution due to a vortex lattice is developed, and for the intensity profile across a single vortex, we find good agreement between model calculations and experimental data. The minimum vortex spacing resolved experimentally is 1.3 μm. Our analysis shows that increased resolution can most easily be achieved by increasing the light input intensity, but maximum resolution is ultimately limited by the effective extinction ratio through the optical system and mechanical vibrations in the setup.

Origin of dendritic flux patterns in MgB2 films

The dendritic patterns of magnetic flux motion formed during field penetration into an MgB2 film were observed using magneto-optic imaging. To investigate the origin of the dendrites, experiments were performed where the sample was partially covered with an thermally conducting foil serving as an efficient heat sink. We observed that the dendrites are formed only in areas lacking the thermal conductor. When dendrites develop in the uncovered part they never invade into the covered region. The results strongly suggest that the dendritic instability is thermal in origin. 2001 Published by Elsevier Science B.V.

Relaxation of transport current distribution in a YBaCuO strip studied by magneto-optical imaging

The dynamics of magnetic flux distributions across a YBa2Cu3O7−δ strip carrying transport current is measured using magneto-optical imaging at 20 K. The current is applied in pulses of 40–5000 ms duration and of magnitude close to the critical one, 5.5 A. During the pulse some extra flux usually penetrates the strip, so the local field increases in magnitude. When the strip is initially penetrated by flux, the local field either increases or decreases depending on both the spatial coordinate and the current magnitude. Meanwhile, the current density always tends to redistribute more uniformly. Despite the relaxation, all distributions remain qualitatively similar to the Bean-model predictions.The dynamics of magnetic flux distributions across a YBaCuO strip carrying transport current is measured using magneto-optical imaging at 20 K. The current is applied in pulses of 40-5000 ms duration and magnitude close to the critical one, 5.5 A. During the pulse some extra flux usually penetrates the strip, so the local field increases in magnitude. When the strip is initially penetrated by flux, the local field either increases or decreases depending both on the spatial coordinate and the current magnitude. Meanwhile, the current density always tends to redistribute more uniformly. Despite the relaxation, all distributions remain qualitatively similar to the Bean model predictions.

Interaction between a magnetic domain wall and a superconductor

memory device based on active control of generation and annihilation of vortices by means of one or more domain walls. In recent years superconducting circuits based on single-flux-quantum pulses have been shown to provide a family of digital electronics with ultrahigh speed and very low-power dissipitation. At clock rates exceeding 10 GHz and an operation speed of many hundred GHz, these devices can in the future outrun any semiconductor device. 7 Using domain walls as active ‘‘vortex gates,’’ we may add an additional degree of freedom in these devices. It is known that bismuth-substituted ferrite garnet films with in-plane magnetization have domain walls with very low coercivity that can be moved without ambiguity at frequencies up to several GHz. 6 Furthermore, in such materials Bloch walls are easily formed by external magnetic fields or stress patterns, and these could be manipulated in numerous ways suitable for a memory device.

Magneto-optical study of magnetic-flux penetration into a current-carrying high-temperature-superconductor strip

The magnetic-flux distribution across a high-temperature superconductor strip is measured using magneto-optical imaging at 15 K. Both the current-carrying state and the remanent state after transport current are studied up to currents of