The role of interactions in systems of magnetic iron oxide nanoparticles in the single domain size range

Magnetic nanoparticles are interesting materials for a lot of medical and technical applications. A less experimentally investigated question is the influence of particle packing density on magnetic properties due to magnetic interactions between single particles. For this, magnetic nanoparticles of iron oxides prepared as fine dry powder by laser deposition are investigated with respect to their structural and magnetic properties as function of packing density. The particles are nearly spherically shaped single crystals in the magnetic single domain size range with a mean diameter of 21 nm occasionally exhibiting spinel growth facets. Powders of these particles are diluted by nonmagnetic silicon oxide particles in a range of volume concentrations from 0.2% up to 68% of the bulk density of magnetite. The concentration dependence of remanence, coercivity and hysteresis losses is determined by measurements of minor loops in a vibrating sample magnetometer. Results which are discussed in the frame of present theoretical models may be understood in terms of the cubic anisotropy of magnetite distorted by a small uniaxial shape contribution.