Periodic arrays of magnetic structures are well known to give rise to commensurate pinning of superconducting vortices in adjacent superconducting films. In this work we compare the pinning effects due to magnetic dots with either single-domain or magnetic-vortex magnetization configuration. We observe a clear correlation between the magnetoresistance in the superconductor and the magnetization configuration of the magnetic dots indicating that the pinning of superconducting vortices is strongly enhanced for the magnetic vortex state. The origin of this enhanced pinning is due to the locally larger stray magnetic fields produced by the magnetic vortex cores.Although the use of periodic magnetic structures for pinning of superconducting vortices has already been suggested in the early 1970s, 1 only during the last two decades has it been possible to fabricate hybrid structures of superconductors and ferromagnets with well controlled lateral dimensions necessary to observe commensurate pinning effects. 2,3 Since then periodic magnetic structures have been used to tune vortex dynamics in many various ways, such as elastic transitions due to geometrical distortions of the vortex lattice with rectangular pinning arrays 4 and rectifying vortex motion for noncircular magnetic structures. 5-7 At the same time the influence of the magnetization state on the vortex pinning properties has been extensively studied. [8][9][10][11][12] In particular, it has been shown that a perpendicular magnetization component can give rise to asymmetric pinning depending on the relative orientation of the magnetization with respect to the magnetic flux of the superconducting vortices 8,12 and that pinning effects are generally more pronounced for magnetic structures in a single-domain vs multidomain state. 9 The same advancement in lithographic fabrication of submicrometer magnetic structures that enabled the investigation of superconducting vortex pinning also paved the way for the observation of magnetic vortices in circular magnetic structures. 13-16 Namely, for small enough circular magnetic structures the magnetostatic energies may stabilize a vortex state at remanence, such that the magnetization curls up along the edges of the circular structure in order to reduce lateral stray fields. Concurrently, the hysteresis loop can be characterized by two critical fields for vortex nucleation and annihilation ͑see Fig. 1͒. In contrast, for hysteresis loops dominated by domain wall motion there is generally only one critical field due to either domain wall nucleation or domain wall depinning. 17 While the magnetization of the vortex state remains mostly oriented in the plane of the magnetic dot, there is, at the vortex center, a singularity where the magnetization points out of plane. 14 The size of this socalled vortex core is given by the magnetic exchange length; thus it is typically a few nm in diameter. 18,16 In this work, we utilized periodic arrays of magnetic dots to study how the formation of magnetic vortices influences the pinning o...

We have designed and characterized a magnetic template which can be switched between chains of parallel and antiparallel field distribution by applying an in-plane magnetic field. The parallel field profile creates highly mobile vortex channels in a superconducting film deposited on top, reproducing the behavior of a weak link as evidenced by the presence of Shapiro steps in the current-voltage characteristics under rf excitation. The Josephson coupling can be fully suppressed by changing the field distribution to the antiparallel state. As a result, a reversible ON/OFF switch for magnetically induced weak links has been demonstrated.

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high-resolution imaging techniques: Bitter decoration and scanning Hall-probe microscopy ͑SHPM͒. The periodicity and strength of the magnetic pinning potential generated by the square microrings are controlled by the magnetic history of the template. When the square rings are in the magnetized dipolar state, known as the onion state, the strong stray field generated at the domain walls prevents the decoration of vortices. SHPM images show that the stray field generated by the dipoles is much stronger than the vortex field, in agreement with the results of simulations. Real-space vortex imaging has revealed that in the onion state, the corners of the square rings act as effective pinning centers for vortices.

We have demonstrated nanofabrication with commercialized cellulose acetate. Cellulose acetate is used for bulk nanofabrication and surface nanofabrication. In bulk nanofabrication, cellulose acetate reacts with an e-beam and permanent patterns are formed in it instead of being transferred to other substrates. We have studied the nano relief modulation performance of cellulose acetate before and after development. The depth of the nanopatterns is magnified after development, and is varied by exposing dosage and line width of the pattern. The thinnest 65 nm wide line is achieved in the bulk fabrication. We also demonstrate a binary phase Fresnel lens array which is directly patterned in a cellulose acetate sheet. Because of its unique mechanical and optical properties, cellulose is a good candidate for a template material for soft imprinting lithography. In the surface nanofabrication, cellulose acetate thin film spin-coated on silicon wafers is employed as a new resist for e-beam lithography. We achieved 50 nm lines with 100 nm pitches, dots 50 nm in diameter, and single lines with the smallest width of 20 nm. As a new resist of e-beam lithography, cellulose acetate has high resolution comparable with conventional resists, while having several advantages such as low cost, long stock time and less harmfulness to human health.

We investigated experimentally the nucleation of superconductivity in a mesoscopic hybrid structure, consisting of a thin superconducting disk covered with a ferromagnetic layer with an in-plane magnetic moment. By applying a magnetic field in the plane of the structure, the remanent magnetic state of the ferromagnet can be switched from a flux-closure state where field lines are confined inside the ferromagnet to a polarized state with nonzero stray fields at the edges. This change in the magnetic state causes a drastic modification on the superconductor/normal-state phase boundary of the hybrid sample. In the polarized state a re-entrant transition line and a strong broadening of the phase boundary are observed.