Incoherent magnetization reversal in Co–Pt nanodots investigated by magnetic force microscopy Original Research Article

The magnetization reversal behavior of a hexagonal close-packed Co–Pt nanodot (50 nm in diameter) array was studied by magnetic force microscopy (MFM). The magnetic switching field (HSW) of each dot and the switching field distribution (SFD) for the array were determined by the MFM observation. The results show that the contrast intensity of the dots in the MFM image has a strong correlation with its HSW. The angular dependence of the average HSW on the direction of the applied field deviates from the coherent rotation model, and appears to reverse by a vortex-like incoherent rotation. It is deduced that the nanodot seems to consist of a hard inner core with high magnetic anisotropy and a soft magnetic shell layer surrounding the core. This core–shell structure results in the magnetic vortex-like incoherent rotation because of an exchange coupling interaction. Shell layers of various thicknesses should result in a broad SFD. This proposal is confirmed by micromagnetic simulation using the exchange coupling model, which is in good agreement with the experimental results.