We report detailed studies on ferromagnet-superconductor bilayer structures. Epitaxial bilayer structures of half metal-colossal magnetoresistive (HMCMR) ${\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}{\mathrm{MnO}}_3$ and high-$T_c$ superconducting (HTSC) ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ are grown on ${\mathrm{SrTiO}}_3\left(100\right)$ single-crystalline substrates using pulsed laser deposition. Magnetization $M\left(T\right)$ measurements show the coexistence of ferromagnetism and superconductivity in these structures at low temperatures. Using the HMCMR layer as an electrode for spin polarized electrons, we discuss the role of spin polarized self injection into the HTSC layer. The experimental results are in good agreement with a presented theoretical estimation, where the spin diffusion length ${\xi }_{\mathrm{FM}}$ is found to be in the range of ${\xi }_{\mathrm{FM}}\approx 10\mathrm{nm}$.

Anisotropic penetration of magnetic flux in ${\mathrm{MgB}}_2$ films grown on vicinal sapphire substrates is investigated using magneto-optical imaging. Regular penetration above 10 K proceeds more easily along the substrate surface steps, the anisotropy of the critical current being 6%. At lower temperatures the penetration occurs via abrupt dendritic avalanches that preferentially propagate perpendicular to the surface steps. This inverse anisotropy in the penetration pattern becomes dramatic very close to 10 K where all flux avalanches propagate in the strongest pinning direction. The observed behavior is fully explained using a thermomagnetic model of the dendritic instability.

The magnetic properties of heterostructures of high-temperature superconducting $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ and highly spin polarized, ferromagnetic ${\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_3$ that are electronically decoupled by a thin $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ layer are investigated by means of SQUID magnetometry. Below the transition temperature of the superconducting component the detected signal is a measure for the critical current in the superconductor. The obtained magnetic hysteresis loops at low temperatures exhibit a substantial asymmetry with respect to $H=0$. This can be understood in terms of pinning of flux lines in the superconducting film due to magnetic interaction with the ferromagnetic manganite layer.

Thin epitaxial films of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ grown on vicinal cut substrates exhibit a substantial anisotropy of the critical current density $j_c$ in the film plane. By means of a quantitative magneto-optical analysis it is possible to investigate the current densities along different directions independently from each other. We performed a detailed analysis of the temperature dependence of the critical currents in the range $T=5–90\mathrm{K}$ and found a completely different behavior of $j_c\left(T\right)$ along different directions. Describing the data by a step-wise power-law ansatz allows us to distinguish between different current limiting mechanisms.

The penetration of magnetic flux into a superconductor above the lower critical field does not necessarily happen in a regular way. It is found that under particular conditions, a chaotic penetration in form of magnetic avalanches occurs. These avalanches are closely related to the so-called thermomagnetic instability, which identifies local heating due to flux line movement as the origin of the effect. In case of $\mathrm{Mg}{\mathrm{B}}_2$ thin films, these avalanches are found only below $T=10\mathrm{K}$ and are suppressed by a covering metallic layer with high thermal conductivity and sufficient thickness. These avalanches are observed in the case of partly gold covered $\mathrm{Mg}{\mathrm{B}}_2$ films by the magneto-optical Faraday effect. The investigation of avalanches propagating into a gold-covered region revealed a change of the propagation direction depending on the incident angle of these avalanches.