A Soft Solution Processing Technique for Preparing Ferrite Films and Their Applications
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Processing Technique for Preparing Ferrite Films and Their Applications
plating environmentally friendly and advantageous for engineering applications, in common with the SSP techniques (see Reference 6 and the articles by Yoshimura et al. in this issue). This article describes the principle, experimental methods, and applications of ferrite plating. Special focus is placed on room-temperature ferrite plating, suggesting a close analogy between ferrite plating and biomineralization.
Principle of Ferrite Plating Figure 1 shows the principle of ferrite plating. For easier understanding, the chemical reaction in ferrite plating is divided into the following processes: Process I: Immersing a substrate with OH groups on its surface in a reaction solution containing Fe2 and other metal ions M n, the ions are adsorbed on the surface mediated by the OH groups, releasing H. Process II: When we introduce an oxidizing reagent such as NaNO2 , air (O2), or anodic current (e), some of the Fe 2 ions are oxidized to Fe3, as expressed by
Masanori Abe
Fe2 l Fe3 e.
(1)
Process III: Then the Fe 2 and M n ions are again adsorbed on the surface of the layer of the pre-adsorbed ions of Fe2, Fe3, and M n ions. Process III: This causes the formation of a ferrite layer, accompanying the hydrolytic dissociation and the release of H, which is expressed by the following reaction:
Introduction “Ferrite plating” is a typical “soft solution processing” (SSP) application; it enables the formation of oxide ferromagnetic films from an aqueous solution at T 24–100C under atmospheric pressure.1 Using ferrite plating, we can grow crystallized ferrite films of spinel-type (MFe)3O4 (where M Fe, Co, Ni, Zn, Al, Cr, etc.) in one step, requiring no heat treatment. This opens the door to fabricating novel ferritefilm devices using substrates of such nonheat-resistant materials as plastics and GaAs integrated circuits;2–4 conventional ferrite-film preparation techniques, such as sputtering, vacuum evaporation, molecularbeam epitaxy, liquid-phase epitaxy, and so on, require high temperatures (600C) for the crystallization of ferrites, which deteriorates the non-heat-resistant substrates. Ferrite plating is a unique technique that allows us to synthesize ferrite “films” by means of a wet chemical process. There are many techniques, for synthesizing ferrite “particles” from aqueous solutions, but no technique, to our knowledge, enables ferrite-film synthesis by a wet chemical process. In the early 1980s, dry processing was the only way to prepare ferrite films for such applications as magnetic recording media. At that time, I began thinking about a wet process that would be applicable to ferrite-film synthesis. In my institute, laboratory wastewater was treated on trial by
MRS BULLETIN/SEPTEMBER 2000
a “ferrite process,” which is now a conventional, widely used technique for wastewater treatment. In the ferrite process, heavy-metal ions are incorporated into the spinel-ferrite lattice by a wet process, forming powders t
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