Transition from antiferromagnetic interlayer ordering to superparamagnetic state in Fe/Cr nanostructures by varying Fe thickness

Transition from antiferromagnetic exchange coupling of the Fe layers to superparamagnetic state of the Fe/Cr nanostructures is studied experimentally and theoretically. The experimental study are performed by means of magnetoresistance and magnetization measurements as well as Mössbauer spectroscopy for the nanostructures with Fe layers thicknesses from 1.4 Å up to 16 Å alternating by 10 Å of Cr layers. It is shown that Fe layers in the nanostructures with thicknesses less than 2 Å are not continuous but consist of separate ferromagnetic clusters. Such cluster-layered nanostructures exhibit Langevin paramagnetism of the superparamagnetic clusters and the Kondo-like behavior of the resistance. For the considered nanostructures, a modeling of Fe and Cr atoms random deposition for the interface layers and self-consistent calculations of the magnetic moments distribution in Periodic Anderson model are carried out. It is shown that, for nanostructures with extremely thin Fe layers, the Fe clusters with lateral size in the range of 11 to 20 Å are self-organized. Calculations of magnetic moments distribution result in histograms which coincide with the hyperfine field distributions obtained by Mössbauer spectroscopy.