Magnetotransport, magnetic, and structural properties of TM-SiO2 (TM=Fe, Co, Ni) granular alloys

TMx(SiO2)(1 - x) (transition metals, or TM=Fe, Co, Ni) thin films were prepared in a wide concentration range with the purpose of studying the giant Hall effect (GHE). The structure was studied using transmission electron microscopy (TEM), X-ray diffraction (XRD), and small angle X-ray scattering (SAXS). Magnetic, transport, and magnetotransport properties were investigated by means of magnetization, resistivity, and Hall effect measurements. TEM images show nanometer-sized spherical structures embedded in an SiO2 amorphous matrix, with typical sizes ranging from 2 to 7 nm when TM volume concentration x is increased. SAXS measurements show a broadened peak. XRD measurements show that the system consists of amorphous SiO2 and a crystalline peak, corresponding to small TM crystallites. The observed magnetic properties are strongly dependent on x, and clearly display an evolution resulting from the progressive increase in the mean particle size. Above the percolation threshold region all samples display GHE. Relationships between structure and magnetotransport properties are discussed. (C) 2003 Elsevier Inc. All rights reserved.