Magnetic-domain-controlled vortex pinning in a superconductor/ferromagnet bilayer

Abstract

Vortex pinning in a type-II superconducting Pb film covering a Co/Pt multilayer with perpendicular magnetic anisotropy is investigated. Different stable magnetic domain patterns like band and bubble domains can be created in the Co/Pt multilayer, clearly influencing the vortex pinning in the superconducting Pb layer. Most effective pinning is observed for the bubble domain state. We demonstrate that the pinning properties of the superconductor/ferromagnet bilayer can be controlled by tuning the size, density and magnetization direction of the bubbles.Magnetic flux penetrates type-II superconductors in the form of quantized vortices, which have the tendency to form a periodic lattice. These vortices move when a current is sent through the superconductor, thus causing dissipation and limiting the critical current density j c of the superconductor. To enhance j c , vortex motion must be prevented. The latter can be achieved by introducing different pinning centers such as defects created by ion [1] or neutron irradiation [2], lithographically introduced holes [3,4,5], or magnetic dots [6,7,8,9].Recently it was proposed that vortices in a superconductor/ferromagnet (SC/FM) multilayer are strongly pinned if the FM has a stripe domain structure, which typically exists in thin magnetic films with perpendicular anisotropy [10]. Enhanced pinning was observed in SC/FM bilayers compared to SC reference films [11,12]. However, in these studies the domain structure of the FM was unknown. In this Letter, we systematically investigate the influence of the domains in a ferromagnetic Co/Pt multilayer with perpendicular anisotropy on a type-II superconducting Pb film grown on top of the FM. Stable domain structures can be produced in a controlled way by magnetizing the sample before measuring the superconducting properties of the Pb film. We show that the strongest vortex pinning is obtained when localized domains (bubble domains) are present in the FM, and that by changing size and density of the bubbles one can control and optimize the vortex pinning.The Co/Pt multilayer is deposited on a Si substrate with amorphous SiO 2 top layer in an MBE apparatus by e-beam evaporation. The multilayer has a [Co(0.4 nm)/Pt(1.0 nm)] 10 structure on a 2.8 nm Pt base layer. A 10 nm Ge film, a Pb film with thickness d P b = 50 nm and a 30 nm Ge capping layer are subsequently evaporated on the Co/Pt multilayer (see Fig. 1) at a substrate temperature of 77 K. The Pb film has a critical temperature of T c = 7.23 K. The penetration depth λ(0) = 42 nm and the coherence length ξ(0) = 41 nm are estimated from measurements of the temperature dependence of the upper critical field. The Ge film between Pb and Co/Pt is insulating at low temperatures, so that the proximity effects between Pb and Co/Pt are suppressed.The Co/Pt multilayer has perpendicular magnetic anisotropy [13], which is confirmed by hysteresis loop (Fig. 3). measurements using the magneto-optical Kerr effect (MOKE). Fig. 2 shows the magnetization M f m of the Co/Pt multilayer, normalized...

Origin of pinning enhancement in a ferromagnet-superconductor bilayer

“…Our results indicate a large, 2.5 times, enhancement of pinning which occurs exclusively in the final stage of the magnetic reversal process of the FM layer, and we show that it is caused by residual uninverted dendrite-shaped magnetic domains. While the details of this behavior differ from the effect reported for Pb film, 7 the origins are similar, in both cases related to pinning on isolated domains, suggesting that this type of pinning may be generally the most effective pinning in the FSB structures.…”

“…There are theoretical predictions that the magnetic flux of the vortex in the SC layer may be pinned by the stripe domains of the ferromagnetic (FM) layer. 4 Recent magnetic [5][6][7] and transport 8 studies of the FSB's confirm that the enhancement of pinning occurs. However, the origins of the enhancement are not evaluated in detail in most of these studies, except for the case of 50 nm Pb film [weak type II superconductor with the Ginzburg-Landau parameter Ϸ 1], deposited on the top of Co/ Pt multilayer with perpendicular magnetic anisotropy.…”

“…However, the origins of the enhancement are not evaluated in detail in most of these studies, except for the case of 50 nm Pb film [weak type II superconductor with the Ginzburg-Landau parameter Ϸ 1], deposited on the top of Co/ Pt multilayer with perpendicular magnetic anisotropy. 7 In this case, the enhancement is found to be caused not by the stripe domains but by the isolated bubble domains nucleated in the FM layer during the reversal of magnetization. The question arises if similar origins are behind the pinning enhancement in the FSBs consisting of typical type II superconductors with well above 1, which are the materials for applications.…”

Magnetic confinement of the superconducting condensate in superconductor-ferromagnet hybrid composites

“…Thus by choosing the appropriate H ret value the desired domain distribution can be readily prepared. 13 To control the magnetic state of the dots the same procedure can be applied since the diameter of the dots exceeds the typical size of the domains. Accordingly they are in a multidomain state 7 and any intermediate remanent magnetization can be reached.…”

Nanoengineered Magnetic-Field-Induced Superconductivity

“…This new field compensation effect is not restricted to specific superconductors, so that FIS could be achieved in any superconducting film with a lattice of magnetic dots. To implement the idea of the nanoengineered FIS, we have prepared a sample, which reminds us of other systems used during the last decade for studying flux pinning by periodic arrays of magnetic dots [7,8,9,10,11], and by magnetic domains [12]. The sample consists of a 85 nm superconducting Pb film evaporated on a 1 nm Ge base layer on an amorphous Si/SiO 2 substrate, which is held at liquid nitrogen temperature during deposition.…”