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Vortex ratchet reversal at fractional matching fields in kagomelike array with symmetric pinning centers

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2010-11-03
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American Physical Society
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Arrays of Ni nanodots embedded in Nb superconducting films have been fabricated by sputtering and electron-beam lithography techniques. The arrays are periodic triangular lattices of circular Ni dots arranged in a kagomélike pattern with broken reflection symmetry. Relevant behaviors are found in the vortex lattice dynamics: i at values lower than the first integer matching field, several fractional matching fields are present when the vortex lattice moves parallel or perpendicular to the reflection symmetry axis of the array showing a clear anisotropic character in the magnetoresistance curves, ii injecting an ac perpendicular to the reflection symmetry axis of the array yields an unidirectional motion of the vortex lattice ratchet effect as a result of the interaction between the whole vortex lattice and the asymmetric lattice of dots, iii increasing the input current amplitudes the ratchet effect changes polarity independently of matching field values. These experimental results can be explained taking into account the vortex lattice density.
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©2010 The American Physical Society. We want to thank useful conversations with Ivan K. Schuller. We want to acknowledge the support by Spanish Ministerio Ciencia e Innovación under Grants No. FIS2008- 06249 Grupo Consolidado, Consolider No. CSD2007- 00010, and No. HP2008-0032, CAM under Grant No. S2009/MAT-1726, Santander-UCM under Grant No. GR58/ 08, Principado de Asturias FICYT under Grant No. IB08- 106, and IMDEA-Nanoscience.
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