Studies On The Electrical Nature And Magnetism On Co-Al₂O₃ Magnetic Granular Thin Films

The cluster size and distribution is essential for understanding the transport property in magnetic granular thin films.But little is known until now about that because of the lack of well-defined samples. The granular films usually are prepared by co-sputtering or co-evaporation of the metallic and insulating component onto a substrate or by the reactive sputtering of metallic components.Granular systems prepared by the method given above have a broad distribution in cluster size and distance between clusters is random.Therefore,it is difficult to study the influence of cluster size on physical properties of these systems.In this paper,we use Sequential deposition method to fabricate granular films,which is at the origin of the almost self-organized growth.The nominal thickness of every layer is no more than 1 nm.The large difference between the surface energies of Co and Al₂O₃ leads to a three-dimensional growth of Co granules.In contrast with the co-deposition,the samples made by the sequential deposition method have well-distributed and almost monodisperse nanoparticles in a solid matrix,which is important for studying the influence of cluster size and distribution on GMR.As far as we know,there is no report about the GMR in Co-Al₂O₃ granular films prepared by this method.We display results of an extensive study of conduction mechanism and the temperature dependence of magnetoresistance in such monodisperse granular films for temperature 13-300K.The mainly works and results are listed as following:(1)A series of Co-Al₂O₃ granular films were made by sequential deposition method based on self-organized growth,and the microstructure was studied by TEM.(2)The relation of GMR and resistivity at room temperature was studied and it is found that the GMR takes a maximum of 5.2%at room temperature and 9.4%at 13 K and 5700 Gs when the resistivity of sample is 4^*10⁵-7^*10⁵μΩcm. (3)our samples follow the mechanism fluctuation-induced tunneling for a network of tunnel junctions of uniform size from 13K to room temperature,which is different to the hopping transport in co-deposition samples(4)The magnetization curve of samples show no hysteresis and follow well the Langevin function, which suggests the superparamagnetic nature.(5)The temperature dependence of GMR was special studied and it is found that GMR-Tdependence is linear for temperature 13-300 K,which is related to the special microstructure of the samples.