Electrical Resistivity and Magnetoresistance in Monodispersed Oxide-Coated Fe Cluster Assemblies

Abstract

We systematically studied electrical resistivity and magnetoresistance (MR) of size-monodispersed oxide-coated Fe cluster assemblies with the mean cluster sizes of d = 9–17 nm prepared by a plasma-gas-condensation-type cluster beam deposition system. The electrical resistivity and magnetoresistance strongly depend on the temperature, surface oxidization degree of the clusters (namely O2 gas flow ratio RO2), Fe cluster size d, and magnetic field. The oxide-coated Fe cluster assemblies exhibit a large negative MR effect which is further enhanced at low temperatures due to the dominant contribution of the spin-dependent tunneling process between the Fe cores through the oxide shell layers. It has been found that the magnetic field dependence of the MR ratio at all temperatures shows no saturation tendency up to a maximum field H = 50 kOe and completely disagrees with the magnetization curves which indicate a saturation tendency. These results have been interpreted by consideration of the magnetic state of the Fe-oxide shell layers, spin-dependent tunneling mechanism, and intercluster magnetic correlation. The high-field nonsaturation behavior in the magnetoresistance effect is attributed to the spin-disordered structure, which is frozen in a spin-glass-like state at low temperatures, in the surface of the Fe-oxide shell crystallites or the whole thinner Fe-oxide shell layers.