Abstract
The intermediate state of a type-I superconductor Pb film is studied by a scanning Hall probe and scanning ac-susceptibility microscopies under static and oscillating applied magnetic fields. The structure of the typical flux patterns during magnetic field penetration/expulsion shows a strong hysteresis. Under the action of an ac field, the multiply quantized flux tubes in a type-I superconductor reveal a dynamical reordering similar to what is observed in the Campbell regime for vortices in a type-II superconductor. Most strikingly, after shaking, higher density flux tube patterns demonstrate a reorganization from a superheated metastable tubular pattern to a stable stripe pattern. We provide direct experimental evidence that the flux reorganization behavior is a dynamical transition. The local distribution of the potentials providing pinning to intermediate state patterns is mapped out, which is, as far as we know, the first direct visualization of confinement of intermediate state domains by pinning centers.
Highlights
By using a low-temperature scanning Hall probe and scanning ac-susceptibility microscopies, we have studied the IS of a type-I superconducting lead film
Contrary to pinning-free samples, in our case the flux stripe pattern has a slightly lower energy than the flux tubular pattern in most of the H –T phase diagrams
The flux tubular patterns can be further divided into two regions (LDFTP and higher densities of flux tube patterns (HDFTP)), which have different dynamics
Summary
The SHPM images were obtained using a modified low-temperature scanning Hall probe microscope from Nanomagnetics Instruments with a temperature stability better than 1 mK and a magnetic field resolution of 0.1 G [19,20,21,22]. The chosen location for the SHPM experiments is close to the center of the film, and the scanned area covers a surface of 16 × 16 μm. Flux patterns formed during magnetic field penetration into the sample were imaged upon sweeping up the magnetic field after the sample was cooled down (T < Tc) in zero magnetic field. Flux patterns formed during magnetic field expulsion were investigated while sweeping down the magnetic field, after the sample was field cooled to a given fixed temperature close to Tc(H )
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