Commensurability effects in overlap Josephson junctions coupled with a magnetic dots array

Abstract

Experimental observation of the strong influence of an array of ferromagnetic nanodots on the critical current of a short overlap Josephson junction is reported. Pronounced commensurability effects are detected due to the presence of the additional peaks in the magnetic field induced diffraction pattern. The changes in the Fraunhofer pattern of the Josephson junctions are account for by the formation of Abrikosov vortices trapped in the electrodes which induce a phase inhomogeneity in the junction area. PACS numbers: 74.50.+r; 75.75.+a The role played by inhomogeneities in long Josephson junctions is known. In typical experimental realizations, periodic defects of the barrier thickness provide for a spatial modulation of the critical current density jc which results in sharp peaks of the critical current Ic on variation of the external magnetic field H [1]. These ordered peak structures are observed at fields H for which the number of enclosed magnetic flux quanta in the junction is an integer multiple of the number of defects, which is known as commensurability. Over the last years, Josephson junctions with phase inhomogeneities have been intensively investigated. In earlier studies the properties of Josephson junctions with Abrikosov vortices (AVs) pinned in the vicinity of the barrier were discussed by several authors [2, 3]. Recently investigations of unconventional Josephson junctions with a spatially alternating sign of the critical current have attracted renewed attention. The presence of Josephson phase discontinuities results in an unusual current–phase relation [4], a highly anomalous nonFraunhofer Ic(H) dependence [5], and a spontaneous generation of fractional Josephson vortices [6]. Although regular commensurability peaks similar to the found in [1] are quite expected for unconventional junctions, observing them is difficult due to the effect of randomness in the spatial distribution of jc.