Magnetic stability of nano-particles: The role of dipolar instability pockets

We show that the magnetic stability of nano-particles, with regard to magne tization reversal or domain formation, is controlled by dipolar instability pockets. These pockets are regions in which the local dipolar fields (i.e. shape-dependent effects) favor a spin reorientation. Calculated shape-depe ndent demagnetizing fields coupled with exchange contributions combine to y ield the field at which a magnon mode attains zero frequency implying the o nset of reversal or domain formation. Calculated switching-onset fields are found to explain experimental data as a function of: material, thickness, aspect ratio, and shape.