Abstract
High‐density magnetic recording in the Tbit/inch^2^ range is based on single bit, single domain particles in the nanometer range. In order to fulfil the prerequisites for high‐density recording the following magnetic properties must be guaranteed: Easy axis perpendicular to recording plane, thermal stability up to 200 °C, reversal fields of 1–2 T and switching times in the sub‐ns range. Composite nanoparticles composed of a soft and a hard magnetic layer are suitable to fulfil the above conditions. The magnetic ground states of exchange coupled bilayers are determined by micromagnetism and the corresponding reversal fields determined as a function of the soft magnetic film thickness. It is shown that the magnetic ground states are characterized by three configurations: in‐plane, oblique, and perpendicular magnetization depending on the thickness of the soft magnetic layer. These three ground states are characterized by reversal fields of different magnetization processes: Depinning of a charged Néel wall at the phase boundary for soft layer thicknesses L~s~ larger than the exchange length of the soft layer, spontaneous rotation of magnetization by a nucleation process for soft layer thicknesses smaller than the exchange length.