Imprinting superconducting vortex footsteps in a magnetic layer

Abstract

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit cards and computer hard drives. Here we experimentally show that a similar principle can be applied for imprinting the trajectory of quantum units of flux (vortices), travelling in a superconducting film (Nb), into a soft magnetic layer of permalloy (Py). In full analogy with the magnetic drawing board, vortices act as tiny magnetic scribers leaving a wake of polarized magnetic media in the Py board. The mutual interaction between superconducting vortices and ferromagnetic domains has been investigated by the magneto-optical imaging technique. For thick Py layers, the stripe magnetic domain pattern guides both the smooth magnetic flux penetration as well as the abrupt vortex avalanches in the Nb film. It is however in thin Py layers without stripe domains where superconducting vortices leave the clearest imprints of locally polarized magnetic moment along their paths. In all cases, we observe that the flux is delayed at the border of the magnetic layer. Our findings open the quest for optimizing magnetic recording of superconducting vortex trajectories.Quantum magnetic flux units in type-II superconductors can be regarded as small solenoids, producing localized magnetic field variations extending over distances of 2λ perpendicularly to the applied field, where λ is the magnetic penetration depth. These flux lines interact with inhomogeneities inside the superconducting material, leading to a plethora of well-known pinning mechanisms. Furthermore, the static and dynamic response of individual vortices can also be modified if their stray field outside the superconducting volume is altered. This is precisely the reason why multilayered hybrid systems combining superconducting and non-superconducting materials have been an active field of research in the last decade.It has been shown, for instance, that the stray field of an isolated moving vortex can induce eddy currents in a nearby metallic layer, leading to a velocity dependent damping force 1,2 . This effect can be exploited to provide better stability and quench protection in superconducting cables by reducing the speed of flux jumps 3,4 . More spectacular is the possible use of the monopolar magnetic field emanating from a vortex as a submicron-sized magnetic tweezers to locally polarize spin carriers in a diluted magnetic semiconductor 5 . Among these hybrid devices, the heterostructures combining superconductivity and magnetism, two antagonistic quantum macroscopic states, arguably represent the most investigated systems so far [6][7][8][9] . A simple attempt to classify the superconductor (S-layer) -ferromagnet (F-layer) hybrids, where the F-layer and the S-layer are in close proximity and interact only through their stray fields, consists of comparing the relative field strength scales of each subsystem. Indeed, if the coercive field, H coe , of the ferromag...

Magnetically induced anisotropy of flux penetration into strong-pinning superconductor/ferromagnet bilayers

“…A protection layer of 10 nm MgO is furthermore deposited between superconductor and ferromagnet to reduce pair-breaking effects. As previously reported, the interaction across the interface is dominated by magnetic dipolar coupling [18][19][20].…”

Advanced magneto-optical Kerr effect measurements of superconductors at low temperatures

“…1,2 Prominent results can be extracted from magneto-optical Faraday microscopy based on visible light [3][4][5][6][7] or scanning x-ray microscopy (SXM) respectively photoemission electron microscopy (PEEM) based on polarized x-rays featuring higher spatial resolution. [8][9][10] In this paper we show that a magneto-optical Kerr effect (MOKE) set-up in combination with a soft-magnetic sensor layer and an elaborate measurement scheme allows direct access to supercurrents present in thin superconducting films.…”

“…11 The magnetic stray field of the flux density of the superconductor induces a local magnetization in the sensor layer which maps the signal from the superconductor and is then accessible for optical measurements. 6,12,13 MOKE measurements are a sensitive method to obtain the magnetization signal, however, in a non-absolute way. The Kerr rotation is superimposed by a background signal, which consists of a Faraday rotation and nonlinear optical effects in the optical elements.…”

Manipulating Abrikosov vortices with soft magnetic stripes

“…References on recent studies of FM/SC hybrids can be found in [14][15][16][17][18]. In this work, we focus on the effects of soft magnetic stripes on vortex dynamics, which we posit can lead to the manipulation of single Abrikosov vortices.…”