We report a detailed comparison of experimental data and theoretical predictions for the dendritic flux instability, believed to be a generic behavior of type-II superconducting films. It is shown that a thermomagnetic model published very recently [Phys. Rev. B 73, 014512 (2006)10.1103/PhysRevB.73.014512] gives an excellent quantitative description of key features like the stability onset (first dendrite appearance) magnetic field, and how the onset field depends on both temperature and sample size. The measurements were made using magneto-optical imaging on a series of different strip-shaped samples of MgB2. Excellent agreement is also obtained by reanalyzing data previously published for Nb.

A recently developed high-resolution magneto-optical imaging ͑MOI͒ 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 m. 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.

In a type-II superconductor, the magnetic field penetrates in the form of thin filaments called vortices. The controlled behavior of these vortices may provide the basis for a new generation of nanodevices. We present here a series of experiments showing simultaneous manipulation and imaging of individual vortices in a NbSe 2 single crystal. The magnetic field from a Bloch wall in a ferrite garnet film ͑FGF͒ is used to manipulate the vortices. High-resolution magneto-optical imaging enables real-time observation of the vortex positions using the Faraday effect in the same FGF. Depending on the thickness of the sample, the vortices are either swept away or merely bent with the Bloch wall.The vortices in a type-II superconductor consist of a normal core surrounded by supercurrents creating a magnetic field along the vortex. Recent developments in magnetic pinning by nanoengineered pinning arrays 1-7 suggest the possibility of developing a new generation of devices based on the controlled behavior of vortices. Creation and manipulation of single vortices were recently demonstrated using a miniature field coil mounted on a scanning superconducting quantum interference device microscope. 8 Magnetic domain walls produce an alternative magnetic pinning potential. These walls can be shaped and controlled by external stress patterns and magnetic fields. For low coercivity ferrite garnet films ͑FGFs͒ with in-plane magnetization, domain walls can be manipulated at frequencies in the GHz regime, 9 which makes them suitable for use in potential devices. Moreover, the strong Faraday rotation in FGFs can be used for direct realtime imaging of vortices, as recently demonstrated. 10 Such simultaneous manipulation and imaging of vortices without any external mechanical motion may provide a useful tool in the development of vortex based nanodevices.In this work, we directly image pinning and manipulation of vortices in NbSe 2 by a movable magnetic Bloch wall in a bismuth-doped FGF. The Faraday effect in the same FGF is used simultaneously for imaging of vortex positions. We estimate the strength of the domain wall-vortex interaction and find that for sufficiently thin samples, the domain wall can sweep a region clear of vortices. Figure 1 shows the principle of magneto-optical imaging ͑MOI͒. When passing through the FGF, linearly polarized light will have its polarization plane rotated an angle proportional to the strength of the magnetic field present. When using a crossed polarizer/analyzer setting, maxima in rotation will give maxima in recorded intensity, hence, a vortex will appear as a bright spot. The main improvements in our setup compared to conventional MOI 11 involves minimizing the gap between the superconductor surface and the FGF ͑only 0.14 m͒, using a very thin and sensitive FGF, and building an open modular microscope optimized for polarization contrast.In an FGF with in-plane magnetization, two domains with an opposite magnetization direction will be separated by a Bloch wall. 12 The domain wall will give rise to a...

Articles you may be interested inA comparative study of the dendritic avalanche in MgB 2 thin films synthesized by pulsed laser deposition and hybrid physical chemical vapor deposition methods

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