Magnetic metal-insulator granular films have been expected to be applied to magnetic storage devices as read head material. Since the magnetoresistance of such films results from spin-dependent tunneling between the magnetic metal granules, it depends strongly on the property of the insulating matrix. Highly insulating, not incorporating magnetic metal atoms, forming sharp interface with magnetic metal granules are required of the insulating matrix in order to achieve high magnetoresistance ratio. In this work we choose Co-Ti-O system aiming at the formation of Co-TiO2 granular film.
The Co-Ti-O films were prepared using a two-facing-target magnetron sputter apparatus. Pure Co and Ti plates were used as the targets, and an Ar and O2 gas mixture was used as the sputtering gas. The oxygen concentration in the film was controlled by controlling the O2 flow rate.
It is found that co-sputtering of Co and Ti by Ar and O2 gases cannot result in the formation of Co-TiO2 granular film in the present work. The film is either oxygen-deficient forming a semiconducting phase, or oxygen-excess forming an insulating complex oxide. The transition between these two regions is too sharp to get an intermediate state where Co and TiO2 separate with each other. Such films do not show ferromagnetic or super-paramagentic characteristics even after annealing up to 400°C. Therefore, we considered to fabricate oxygen-concentration modulated Co-Ti-O layers, i.e. to deposite oxygen-rich and oxygen-deficient Co-Ti-O layers alternately, and to control the oxygen concentration in the films by inter-diffusion of oxygen between the layers during annealing. Bilayer, sandwich, and multilayer structures were fabricated. The as-deposited films were still paramagnetic. However upon annealing at 250°C, the films show clearly ferromagnetic characteristics and begin to show magnetoresistance, indicating the separation of Co in the films. Furthermore, the resistivity and Magnetoresistance of the films depend strongly on the thicknesses of each layer, and by optimizing the thickness of the layers, magnetoresistance as high as 9% at room temperature can be achieved after annealing. Electron diffraction results have confirmed the formation of Co and TiO2 in the films.
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