Hysteresis and fractional matching in thin Nb films with rectangular arrays of nanoscaled magnetic dots

We have investigated the periodic pinning of magnetic flux quanta in thin Nb films with rectangular arrays of magnetic dots. In this type of pinning geometry, a change in the periodicity and shape of the minima in the magnetoresistance occurs for magnetic fields exceeding a certain threshold value. This was explained recently in terms of a reconfiguration transition of the vortex lattice due to an increasing vortex-vortex interaction with increasing magnetic field. In this picture the dominating elastic energy at high fields forces the vortex lattice to form a square symmetry, rather than being commensurate with the rectangular geometry of the pinning array. In this paper we present a comparative study of rectangular arrays with Ni dots, Co dots, and holes. In the magnetic dot arrays we found a strong fractional matching effect up to the second-order matching field. In contrast, no clear fractional matching is seen after the reconfiguration. Additionally, we discover the existence of hysteresis in the magnetoresistance in the crossover between the low- and high-field regimes. We find evidence that this effect is correlated with the reconfiguration phenomenon rather than to the magnetic state of the dots. The temperature and angular dependences of the effect are measured, and possible models are discussed to explain this behavior.