Understanding magnetization curves of individual nanoparticles at finite temperatures

Full text: Increasing the storage density in future magnetic memory devices to the ultimate limit will require to address information at the level of individual nanoparticles. However, at small sizes thermal excitations can lead to undesired fluctuations of the magnetization and will affect their response to magnetic fields. In this contribution we use photoemission electron microscopy (PEEM) together with xray magnetic circular dichroism (XMCD) to detect the magnetization of individual Fe nanoparticles at room temperature. By varying the particle size we observe the transition from the superparamagnetic state to stable ferromagnetism at a particle size of about 12 nm. Applying a magnetic field allows us to record magnetization curves of the particles in both states and in the transition regime. The data are compared to the predictions of simple macro spin models. The role of non-collinear spin structures on the mechanism of the thermally induced magnetization reversal is also discussed. (author)