Studying individual magnetic nanoparticles with X-ray PEEM

The thesis addresses a subject with broad implications in various scientific and technical areas. It presents unique direct observations of the magnetic state of single particles of iron (Fe), cobalt (Co) and nickel (Ni) with nanoscopic dimensions by means of spatially-resolved X-ray magnetic circular dichroism (XMCD). The X-ray photoemission electron microscopy (PEEM) data are complemented with in situ reflection high energy electron diffraction (RHEED) investigations, ex situ scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. This approach enabled to correlate the magnetic character of the particles with their individual size. The experimental findings are compared with calculated magnetic anisotropy contributions of the three different types of deposited nanoparticles (NPs). It was found that despite their different atomic structure, the body-centered cubic (bcc) iron and face-centered cubic (fcc) cobalt nanoparticles have a similar behavior and can exist in a state which demonstrates an unexpected ferromagnetic (FM) behavior with sizes down to 8 nm at room temperature (RT), while nickel particles only exhibit the expected superparamagnetic (SPM) behavior. This ferromagnetic state is assigned to an energetically excited, metastable structure which has a remarkably long life time before it decays into the expected superparamagnetic state. Combining PEEM with XMCD measurements allowed for the first time to follow the spontaneous transition from ferromagnetic to superparamagnetic behavior in single nanoparticles.
Detailed calculations of all magnetic anisotropy contributions for different sizes and types of particles indicate that the reported high anisotropy state can be associated with a meta-stable structural state due to the presence of local defects within the NPs, independent of the particle atomic structure and size.
These observations shed new light on the mechanisms which establish the size-dependent evolution of magnetic properties at the nanoscale.