Self-organized mode-locking effect in superconductor/ferromagnet hybrids

Abstract

The vortex dynamics in a low-temperature superconductor deposited on top of a rectangular array of micrometer size permalloy triangles is investigated experimentally. The rectangular unit cell is such that neighboring triangles physically touch each other along one direction. This design stabilizes remanent states which differ from the magnetic vortex state typical of individual noninteracting triangles. Magnetic force microscopy images have revealed that the magnetic landscape of the template can be switched to an ordered configuration after magnetizing the sample with an in-plane field. The ordered phase exhibits a broad flux-flow regime with relatively low critical current and a highly anisotropic response. This behavior is caused by the spontaneous formation of two separated rows of vortices and antivortices along each line of connected triangles. The existence of a clear flux-flow regime at zero external field supports this interpretation. The density of induced vortex-antivortex pairs is directly obtained using a high-frequency measurement technique which allows us to resolve the discrete motion of vortices. Strikingly, the presence of vortex-antivortex rows gives rise to a self-organized synchronized motion of vortices which manifests itself as field independent Shapiro steps in the current-voltage characteristics.

Magnetically controlled superconducting weak links

“…The difference between p and a state indicates that the dissipative state is highly sensitive to the relative orientation of the neighboring magnetic poles. 13 Figure 3͑a͒ summarizes the field dependence of the different dynamical regimes, using the same criterion as Fig. 2, obtained for the magnetic structure in the p state.…”

“…Note that the existence of a flux flow regime even at zero applied field is a clear indication of the presence of internally induced vortex channels in the p state. 13,14 There are two characteristic behaviors resulting from the phase coherence across a weak link, the Fraunhofer-like pattern of the critical current as a function of field, and the presence of Shapiro steps in the V͑I͒ characteristics. The fact that the dynamics of our system is dominated by the strongly inhomogeneous stray field generated by the magnetic template suggests that the Fraunhofer-like pattern in I c ͑H͒ can be severely modified.…”

Stroboscopic phenomena in superconductors with dynamic pinning landscape

“…It is actually a well-known fact that when a superconductor in the presence of periodic potential is driven with the superimposed dc and ac force, Shapiro steps can be experimentally found in the I – V characteristics 35 36 37 38 . In our case Shapiro steps appear due to fact that we apply dc Lorentz force via magnetic field and current, and on the other side the time-dependent pinning potential is contributing with the ac component.…”

Field polarity dependent nucleation of superconductivity in quasi-one-dimensional magnetic templates

“…One of the most fascinating properties of these system is the possibility to control the nucleation of the superconducting order parameter [3][4][5][6][7], the vortex pinning properties [8][9][10][11][12][13], or even to obtain switchable superconducting weak links [14,15] by simply modifying the magnetic state of the F layer.…”