The perpendicular critical fields of a superconducting film have been strongly enhanced by using a nanoengineered lattice of magnetic dots (dipoles) on top of the film. Magnetic-field-induced superconductivity is observed in these hybrid superconductor / ferromagnet systems due to the compensation of the applied field between the dots by the stray field of the dipole array. By switching between different magnetic states of the nanoengineered field compensator, the critical parameters of the superconductor can be effectively controlled. When the applied magnetic field exceeds a certain critical value, superconductivity is suppressed due to orbital and spin pair breaking effects. This very general property of superconductors sets strong limits for their practical applications, since, in addition to applied magnetic fields, the current sent through a superconductor also generates magnetic fields, which can lead to a loss of zero resistance. Materials that are not only able to withstand magnetic fields, but in which superconductivity can even be induced by applying a magnetic field, are very rare and up to now only (EuSn)Mo 6 S 8 [1, 2], organic λ-(BETS) 2 FeCl 4 materials [4,5] and HoMo 6 S 8 [3] show this unusual behavior. The appearance of magnetic-fieldinduced superconductivity (FIS) in the former two compounds was interpreted in terms of the Jaccarino-Peter effect [6], in which the exchange fields from the paramagnetic ions compensate an applied magnetic field, so that the destructive action of the field is neutralized. Here we report that FIS can also be realized in hybrid superconductor / ferromagnet nanostructured bilayers. The basic idea is quite straightforward (see Fig. 1): a lattice of magnetic dots with magnetic moments aligned along the positive z-direction is placed on top of a superconducting film. The magnetic stray field of each dot has a positive z-component of the magnetic induction B z under the dots and a negative one in the area between the dots. Added to a homogeneous magnetic field H, see Fig. 1(b), these dipole fields enhance the z-component of the effective magnetic field µ 0 H ef f = µ 0 H + B z in the small area just under the dots and, at the expense of that, reduce H ef f everywhere else in the Pb film, thus providing the condition necessary for the FIS observation. 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
Superconductivity and magnetism are two antagonistic cooperative phenomena, and the intriguing problem of their coexistence has been studied for several decades. Recently, artificial hybrid superconductor-ferromagnet systems have been commonly used as model systems to reveal the interplay between competing superconducting and magnetic order parameters, and to verify the existence of new physical phenomena, including the predicted domain-wall superconductivity (DWS). Here we report the experimental observation of DWS in superconductor-ferromagnet hybrids using a niobium film on a BaFe(12)O(19) single crystal. We found that the critical temperature T(c) of the superconductivity nucleation in niobium increases with increasing field until it reaches the saturation field of BaFe(12)O(19). In accordance with the field-shift of the maximum value of T(c), pronounced hysteresis effects have been found in resistive transitions. We argue that the compensation of the applied field by the stray fields of the magnetic domains as well as the change in the domain structure is responsible for the appearance of the DWS and the coexistence of superconductivity and magnetism in the superconductor-ferromagnet hybrids.
Domain-wall superconductivity is studied in a superconducting Nb film placed between two ferromagnetic Co/Pd multilayers with perpendicular magnetization. The parameters of top and bottom ferromagnetic films are chosen to provide different coercive fields, so that the magnetic domain structure of the ferromagnets can be selectively controlled. From the dependence of the critical temperature Tc on the applied magnetic field H, we have found evidence for domain-wall superconductivity in this three-layered F/S/F structure for different magnetic domain patterns. The phase boundary, calculated numerically for this structure from the linearized Ginzburg-Landau equation, is in good agreement with the experimental data.
The pinning of flux lines by two different types of regular arrays of submicron magnetic dots is studied in superconducting Pb films; rectangular Co dots with in-plane magnetization are used as pinning centers to investigate the influence of the magnetic stray field of the dots on the pinning phenomena, whereas multilayered Co/Pt dots with out-of-plane magnetization are used to study the magnetic interaction between the flux lines and the magnetic moment of the dots. For both types of pinning arrays, matching anomalies are observed in the magnetization curves versus perpendicular applied field at integer and rational multiples of the first matching field, which correspond to stable flux configurations in the artificially created pinning potential. By varying the magnetic domain structure of the Co dots with in-plane magnetization, a clear influence of the stray field of the dots on the pinning efficiency is found. For the Co/Pt dots with out-of-plane magnetization, a pronounced field asymmetry is observed in the magnetization curves when the dots are magnetized in a perpendicular field prior to the measurement. This asymmetry can be attributed to the interaction of the out-of-plane magnetic moment of the Co/Pt dots with the local field of the flux lines and indicates that flux pinning is stronger when the magnetic moment of the dot and the field of the flux line have the same polarity.Comment: 7 pages including figures; submitted for publication in Physica C (Proceedings ESF-Vortex Conference, 18-24 Sept. 1999, Crete, Greece
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...
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