Ferrite plating facilitates the formation of polycrystalline spinel films in an aqueous solution below 100°C. The plating temperature can be extended up to 200°C when the pressure of the reaction solution is kept high (15-20 kgf/cm2). We call this "hydrothermal ferrite plating," by which solubility limit of Ni in Fe3-xNixO 4 increases much, from x=0.3 (by conventional ferrite plating at 90°C) to x=0.93.1 In this paper we describe preparation of Fe3-xCoxO4 films by the hydrothermal ferrite plating at T=120-200°C and p=15-40 kgf/cm2, and report their structural and magnetic properties. Plating for 1 h, we obtained films about 1 μm in thickness, which are polycrystalline with no preferential crystal orientation as observed by x-ray diffraction. The solubility limit of Co, which is x=0.6 at 90°C, increases to x=1.3 at 180°C. The solubility limit of Co is higher than that of Ni, which may be because Ni tends to keep 2+ state in the spinel structure, while Co can take both 2+ and 3+ states. When prepared at 200°C, the saturation magnetization of the films are smaller than that reported for bulk samples, though both agree when the films are prepared at 90°C.2 This suggests that Co is oxidized to 3+ state even when x<1.0 as large as the Fe3-xCo xO4 films are prepared in the hydrothermal conditions. We found that adding CH3COONH4 into the reaction solution as a pH buffer improves the smoothness of the film surface.
