A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.

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

A high resolution scanning Hall probe microscope has been used to investigate flux line dynamics and pinning phenomena in a thin Pb film containing a square array of artificial pinning sites ͑antidots͒. We observe directly that a maximum of two flux quanta can be pinned at a single antidot at low temperatures ͑5.5 K͒, in reasonable agreement with theoretical predictions. Using the scanning Hall probe to measure the ''local magnetization'' as a function of applied field, as well as image flux line structures in the same sample, allows us to directly correlate the two data sets. Peaks in the local magnetization at rational fractional matching fields can generally be linked to ordered commensurate vortex structures that maximize the mean vortex spacing. We find the commensurate vortex structures to be very stable, but excess vortices or vacancies, formed when the applied field deviates slightly from integer filling, are mobile on the time scale of image acquisition. Data are also presented where a domain wall between two ordered commensurate domains could be clearly resolved.

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A high-resolution scanning Hall probe microscope was used as a noninvasive technique to visualize the magnetization reversal in an array of micron-size Co rings. Two stable “onion” states at remanence and “vortex” states at switching fields were found. To rule out a possible influence of dipole–dipole interaction between ring elements on remagnetization processes, an isolated Co ring was deposited on top of a Hall magnetometer and extremely sharp transitions from onion to vortex and from vortex to onion state of opposite polarity were resolved. Our results were supported by MOKE magnetization measurements and micromagnetic simulations.

The dependence of the vortex penetration and expulsion on the geometry of mesoscopic superconductors is reported. Hall magnetometry measurements were performed on a superconducting Al square and triangle. The stability of the vortex patterns imposed by the sample geometry is discussed. The field-temperature H − T diagram has been reconstructed showing the transitions between states with different vorticity. We have found that the vortex penetration is only weakly affected by the vortex configuration inside the sample while the expulsion is strongly controlled by the stability of the vortex patterns. A qualitative explanation for this observation is given.

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A graphoepitaxy directed self-assembly process using cylindrical phase block copolymers is regarded as a promising approach for patterning irregularly distributed contact holes in future integrated circuits. However, control over cylinder profile and open hole rate, among others, needs to be proven before this technique can be implemented in device fabrication. Computational simulation studies predict that selective control over the surface energy of the template bottom and sidewall is crucial for achieving perpendicular cylinders in an adequate range of template dimensions and block copolymer fill levels. This work offers an experimental investigation of the influence of the surface energy on the morphology of the assembly inside the template. For this study, a dedicated surface energy modification is implemented in our process flow. Selective control over the surface energy of the template bottom and sidewall is achieved by using random copolymer brushes. The optimization of surface energy prior to the directed self-assembly allows an improvement of the three-dimensional morphology of the assembly as well as larger process windows in terms of template dimensions and template fill. In addition, a sidewall that has an affinity for the majority block allows for smaller prepattern templates.