Multipolar effects in the hysteresis behavior of 2D arrays of compass needles: Experiments and simulations

Abstract

Despite its easy experimental implementation, macroscopic systems composed of compass needles array show a rich and complex variety of physical phenomena that are not yet fully understood. In this work we consider the experimental realization of a square lattice of equally spaced millimeter sized compass needles, as well as its theoretical approach taking into account a proper higher order moments expansion of the magnetic potential, considering the actual shape of a compass needle. A Monte Carlo simulation, done using interaction energy value given by the previous expansion, showed excelent agreement with experimental results for the squared array magnetization process. Rather than the unique commonly considered dipolar contribution, the results evidence the importance of multipole expansion in considering interactions among magnetic elements with finite size, attributing to the magnetic particle shape a critical importance that could raise higher order contributions. In addition to the clear pedagogical relevance, the conclusions deserve attention, and reopen the discussion of whether dipolar interaction is the only relevant contribution to be considered in theoretical models in which magnetostatic interactions are important, e.g. superparamagnetic nanoparticles models.