LETTERS

Magnetization reversal in magnetic particles is one of thefundamental issues in magnetic data storage. Technologicalimprovements require the understanding of dynamical magnetization reversal processes at nanosecond time scales1. New strategies are needed to overcome current limitations. For example, the problem of thermal stability of the magnetization state (superparamagnetic limit) can be pushed down to smaller particle sizes by increasing the magnetic anisotropy2. High fields are then needed to reverse the magnetization,which are difficult to achieve in current devices. Here we propose a new method to overcome this limitation. A constant applied field, well below the switching field, combined with a radio-frequency (RF) field pulse can reverse the magnetization of a nanoparticle. The efficiency of this method is demonstrated on a 20-nm-diameter cobalt particle by using the microSQUID (superconducting quantum interference device) technique3. Other applications of this method might be nucleation or depinning of domain walls