Seminarium Fizyki Materii Skondensowanej

The superconductor-ferromagnet bilayer (SFB) is a model system to study the effects of inhomogeneous magnetic fields on superconductivity. Any magnetic texture in the F layer (such as magnetic domains) affects the nucleation of superconductivity and the vortex pinning in the S layer. This modifies the influence of the external magnetic field, H, on the basic parameters of thesuperconductor, the phase transition line, T_c(H), and the critical current density (T_c is the superconducting transition temperature). In this talk I will describe our recent experiments in which we study these effects in the SFB built from 20 nm thick niobium film as the S-layer, and the Co/Pt superlattice with perpendicular magnetic anisotropy which serves as the F-layer. We manipulate the size of the magnetic domains by the angle-dependent demagnetization procedure, and observe a continuous tuning of the phase transition line, from a conventional linear T_c(H) with a single maximum at H=0, to a reentrant behavior with multiple T_c peaks. These results are qualitatively consistent with theoretical predictions [1]. In addition, we study the influence of domain sizes on the vortex matter,and find an unusual nonmonotonic dependence of the critical current density on the magnetic field, which results from the enhancement of the flux flow in the presence of domains.

[1] A. Yu. Aladyshkin et al., Phys. Rev. B 68, 184508 (2003).

This work has been done in collaboration with L. Y. Zhu and C. L. Chien (The Johns Hopkins University).