Guiding superconducting vortices with magnetic domain walls

Abstract

We demonstrate a unique prospect for inducing anisotropic vortex pinning and manipulating the directional motion of vortices by using the stripe domain patterns of a uniaxial magnetic film in the superconducting/ ferromagnetic hybrid. Our observations can be described by a model, which considers interactions between magnetic charges of vortices and surface magnetic charges of domains resulting in the enhanced pinning of vortices on domain walls. Superconducting/ferromagnetic ͑SC/FM͒ hybrids offer a variety of exciting new phenomena, which have been extensively discussed in recent years. 1-3 They are defined by short-range and long-range interactions between the competing SC and FM components responsible for the paramagnetic or proximity effects ͑due to FM exchange͒ and orbital coupling ͑due to magnetic stray field͒, respectively. In general, the phenomena of superconductivity and ferromagnetism are considered incompatible because singlet superconducting Cooper pairs carry opposite spins, while in the ferromagnets, all the spins are aligned. However, they can coexist in the bulk under a short-range proximity effect if they form fine scale inhomogeneous structures. Superconductivity could survive under conditions of a weak ferromagnetic exchange interaction by forming a spatially oscillating order parameter, the Fulde-Farrell-Larkin-Ovchinnikov ͑FFLO͒ structure. 4,5 In turn, the ferromagnetic component could form oppositely magnetized domains at length scales way below the SC penetration depth ͑cryptomagnetic structure 3,6-8 ͒, which would support "antiferromagnetic" Cooper pairs. Neither FFLO nor cryptomagnetic structures were convincingly confirmed so far for bulk materials. However, proximity phenomena such as the appearance of and intermediate phase Josephson junctions, have been experimentally demonstrated in FM/SC hybrids ͑see review 9 ͒. The short range of these effects is defined by the oscillating penetration of the SC pairs into the FM and is usually of the order of the coherence length. These scales are similar to the size of cryptomagnetic domains envisioned in bulk FM/SCs. 1 A special case of the proximity effect should appear in FM/SC bilayers due to the FM domain walls where the reduced effective exchange at the wall could lead to a locally enhanced superconducting transition temperature T c in the SC layer. 10 The long-range effects in FM/SC bilayers are associated with interactions of the FM stray fields with the SC screening currents. Due to such interactions, more robust superconductivity just above the FM domain walls is expected when domains are magnetized perpendicular to the SC/FM interface and the stray fields at the domain wall are at a minimum ͑see Ref. 11 and references therein͒. In contrast, domain walls between in-plane magnetized domains carry enhanced stray fields and should locally suppress the critical temperature. Similarly, the suppression of superconductivity above the domain walls is expected for magnetic layers with perpendicular anisotropy when the domain width exceeds the fil...

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