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.

Critical current densities ${(j}_c)$ and pinning forces ${(f}_p)$ in superconducting Pb/Ge multilayers and single WGe films are strongly enhanced by introducing regular arrays of submicron holes (“antidot lattices”) acting as artificial pinning centers. Comparative measurements of $j_c$ and $f_p$ for several well-defined antidot diameters $D$ have shown that pinning centers with a size considerably larger than the temperature-dependent coherence length $\xi \mathrm{}\left(T\right)\mathrm{}$ are much more efficient than those with a size close to $\xi \mathrm{}\left(T\right).$ Moreover, the antidot size realizing the optimum pinning is field-dependent: we need smaller antidots to optimize pinning in lower fields and larger antidots for optimum pinning in higher fields. Crossover between different pinning regimes is controlled by the saturation number $n_s$ that defines the largest possible number of flux lines trapped by an antidot. In dependence upon the $n_s$ value, we have observed various composite flux lattices with vortices at antidots and interstices ${(n}_s\approx 1),$ multiquanta vortex lattices ${(n}_s>1),$ and finally we have reached the limit of superconducting networks at $n_s\gg 1.$

We have studied sharp cusplike magnetization (M) anomalies, appearing at matching fields ${\mathit{H}}_{\mathit{m}}$=m${\mathrm{\phi }}_0$/S in superconducting films with sufficiently large antidots, forming a regular lattice with a unit-cell area S. Exactly at H=${\mathit{H}}_{\mathit{m}}$ each antidot pins the same quantized flux m${\mathrm{\phi }}_0$. This m${\mathrm{\phi }}_0$-flux-line lattice has magnetization M(${\mathit{H}}_{\mathit{m}}$) ∝-m${\mathrm{\phi }}_0$/${\mathrm{\Lambda }}^2$, where Λ is the penetration depth in the film. Between the matching fields ${\mathit{H}}_{\mathit{m}}$<H<${\mathit{H}}_{\mathit{m}+1\mathrm{}}$ the M(H) curve follows a simple M∞-ln(H-${\mathit{H}}_{\mathit{m}}$) dependence. As a result, the whole magnetization curve M(H) can now be successfully described by the simple expression, derived for interacting multiquanta vortices in the London limit. In higher fields H≳${\mathit{H}}_{{\mathit{n}}_{\mathit{s}}}$, when the occupancy of the antidots reaches the saturation number ${\mathit{n}}_{\mathit{s}}$=r/2ξ(T), determined by the antidot radius r and temperature-dependent coherence length ξ(T), the ${\mathrm{\phi }}_0$ vortices begin to fill interstices, thus forming composite flux-line lattices with m${\mathrm{\phi }}_0$ vortices strongly pinned by antidots and ${\mathrm{\phi }}_0$ vortices weakly pinned by interstices. © 1996 The American Physical Society.

We report on the magnetic properties of elongated submicron magnetic islands and their influence on a superconducting film. The magnetic properties were studied by magnetization hysteresis loop measurements and scanning-force microscopy. In the as-grown state, the islands have a magnetic structure consisting of two antiparallel domains. This stable domain configuration has been directly visualized as a $2\times 2$-checkerboard pattern by magnetic-force microscopy. In the remanent state, after magnetic saturation along the easy axis, all islands have a single-domain structure with the magnetic moment oriented along the magnetizing field direction. Periodic lattices of these Co islands act as efficient artificial pinning arrays for the flux lines in a superconducting Pb film deposited on top of the Co islands. The influence of the magnetic state of the dots on their pinning efficiency is investigated in these films, before and after the Co dots are magnetized.

Shapiro voltage steps at voltages $V_n{=nV}_0$ $(n$ integer) have been observed in the voltage-current characteristics of a superconducting film with a square lattice of perforating microholes (antidots) in the presence of radiofrequent radiation. These equidistant steps appear at the second matching field $H_2$ when the flow of the interstitial vortex lattice in the periodic potential created by the antidots and the vortices trapped by them, is in phase with the applied rf frequency. Therefore, the observation of Shapiro steps clearly reveals the presence of mobile interstitial vortices in superconducting films with regular pinning arrays. The interstitial vortices, moved by the driving current, coexist with immobile vortices strongly pinned at the antidots.