Flux penetration in a superconducting film partially capped with a conducting layer

Abstract

The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides an additional velocity-dependent damping mechanism for the flux motion that helps to protect the superconducting state when thermomagnetic instabilities develop. If the flux advances with a velocity slower than w = 2/μ 0 σ t, where σ is the cap layer conductivity and t is its thickness, the flux penetration remains unaffected, whereas for incoming flux moving faster than w, the metallic layer becomes an active screening shield. When the metallic layer is replaced by a perfect conductor, it is expected that the flux braking effect will occur for all flux velocities. We investigate this effect by studying Nb samples with a thickness step. Some of the observed features, namely the deflection of the flux trajectories at the border of the thick center, as well as the favored flux penetration at the indentation, are reproduced by time-dependent Ginzburg-Landau simulations.

Selective triggering of magnetic flux avalanches by an edge indentation

“…3(f)), on the other hand, the first avalanche nucleates away from the indentation, along a border and at a field of 2.4 mT. The preferential triggering of avalanches away from the defect was previously observed in Nb films [11,21]. This situation was interpreted as resulting from a reduction of surface barriers near the indentation, inducing a release of the magnetic flux pressure and a smooth flux penetration through the defect.…”

Magnetic Recording of Superconducting States

“…Besides the electromagnetic damping produced by a conducting layer [28][29][30][31], in 2007 Palau and co-workers experimentally demonstrated that an additional damping mechanism appears due to the interaction of magnetic flux lines with ferromagnetic particles [32]. In a sample consisting of ferromagnetic Gd nanoparticles (crystallite size of <10 nm) embedded in an Nb film, marked hysteresis with substantial temperature dependence was observed in transport critical current vs magnetic field (J c vs H).…”

“…Magneto-optical imaging (MOI) is a magnetic field mapping technique which has proven to be useful for investigation of ferromagnets as well as superconductors [46][47][48]. It is characterized by limited invasiveness [30,31,49], short acquisition time [50,51], and reasonable spatial and magnetic field resolution. In particular, a major advantage of MOI over scanning probe magnetic imaging techniques (such as scanning Hall probe microscopy (SHPM) and magnetic force microscopy (MFM)) is the ability to quickly map the magnetic field profile over extended areas (up to few mm).…”

“…Figure 5 summarizes observations following a protocol similar to those discussed in Figure 4, at T = 4 K, for which the Nb sample is in the thermomagnetic instability regime. In addition to velocity-dependent electromagnetic damping [30,31], in this case, flux avalanches are observed to be damped due to the energy cost of flipping in-plane moments in Py when propagating anti-parallel to them (cf. in-plane magnetization orientation in Py shown in panels (a) and (d) and corresponding propagation of flux avalanches in Nb shown in panels (b) and (e) of Figure 5, respectively).…”

Guiding thermomagnetic avalanches with soft magnetic stripes

“…Various attempts have been made to manipulate TMAs using periodic arrays of nanoholes or magnetic nanodots [19][20][21][22], depositing metal layers on top of a superconductor [23][24][25][26], modulating the thickness of the SC film [27], constructing multilayered architectures of SC stripes [28], irradiating SC films [29] and forming indentations at the film edge [30][31]. These studies showed that extending branches of dendritic shaped TMAs in SC films can be partially directed along symmetry axes of the patterned defect arrays and surface steps, or can be deflected from their initial direction upon entry under a capping metal layer.…”