The pinning of vortices in structures consisting of a superconducting layer on the top of magnetic dots or holes, or ferromagnet/superconductor bilayers (FSB) attracts much attention [1-4]. The interest is generated by both the technological promise of devices in which small magnetic field can tune the magnitude of the critical current by adjusting the flux pinning, and by the general scientific interest in properties of "vortex matter". In this study the influence of the magnetic domain reversal on the flux pinning in two different types of FSBs is investigated. The ferromagnetic (FM) layer in both types of FSBs is a cobalt/platinum multilayer with perpendicular magnetic anisotropy, grown by magnetron sputtering. The superconducting layers are either niobium, which is grown on the top of FM layer, or YBa2Cu3O7 (YBCO). In this second case the YBCO layer is grown first by the pulsed laser deposition, and the FM layer is grown on the top of it. The global magnetization is measured with the SQUID magnetometer, and the local flux profile in the SC film is probed using an array of Hall sensors. A strong enhancement of the vortex pinning is observed when the magnetic domain reversal of the FM layer is about 80% advanced. Using magnetic force microscopy imaging we demonstrate that the pinning is related to the dendrite-shaped domains left behind during the reversal process which proceeds primarily via the domain-wall motion. The details of the flux profile evolution with the FM domain reversal will be discussed.
[1] M. Baert, et at., Phys. Rev. Lett. 74, 3269 (1995).
[2] M. Lange, et at., Appl. Phys. Lett. 81, 322 (2002).
[3] D. B. Jan, et at., Appl. Phys. Lett. 82, 778 (2003).
[4] L. N. Bulaevskii, et at., Appl. Phys. Lett. 76, 2594 (2000). |