Magnetic switching fields in square monolayer and bilayer nanodots

The increasing density in recording media has augmented the interest in magnetic structures with dimensions in the range of nanometers. One of the interesting nanoscale devices for these kind of applications are the square magnetic nanodots. However a very large integration of this structures in a 2D array leads to interactions between different dots due to the existence of strong magnetic poles at the dot borders. For certain dimensions they may also become unstable due to the high magnetostatic energy and may fall into circular magnetisation distributions, usually referred to as vortex. Magnetostatically coupled bilayers exhibit a higher stability in parallel magnetisation distributions due to a good magnetic flux closure between magnetic layers. Parallel magnetisation structures appear to be stable in bilayer nanodots for a wider range of dimensions rather than in monolayers. Therefore, the higher stability of the magnetisation in this structures as well as the reduction of the magnetic poles at the dot borders make this kind of structures interesting from the high density recording point of view. In this paper it is studied the different stable configurations for square magnetic dots in materials with two magnetic layers with dimensions suitable for a good magnetostatic coupling between them. Results are compared with dots in materials with one single magnetic layer with similar dimensions. Magnetic fields necessary to switch between the different stable configurations as well as the switching mechanisms are also studied.