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
Self-organized mode-locking effect in superconductor/ferromagnet hybrids
VAVASSORI, Paolo;
2009
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 characteristicsI documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
