Magnetic properties of Fe-based nanoparticle assembly

The magnetic properties of an assembly of very small Fe nanoparticles with an average diameter of the order of 3–5 nm are studied both experimentally and theoretically. The particles are dispersed in a polyethylene matrix with mass concentration 5%. The stray magnetic field of the samples with sizes 1×1×0.2 mm is measured by means of scanning SQUID microscope at 77 K as a function of external magnetic field. The presence of a weak remanent magnetization indicates a magnetic ordering in the samples studied at rather high temperatures. A distribution of the particle volumes or partial agglomeration of the particles within the assembly is suggested to be responsible for this unexpected behavior.