Preparation and Properties of Well-Defined Magnetic Particles
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ar y-Ferric Oxide Particles. Particulate magnetic materials are required to hâve anisotropic structures in order to hâve high coercive force due to their shape anisotropy. However, no method is known for preparing asymmetric yFe 2 0 3 particles because of cubic crystal structure. Therefore, acicular •y-Fe 2 0 3 particles are usually prepared from other kinds of compounds through transformation in the solid phase. Although acicular •y-Fe203 can be prepared from a-, /3-, or y-FeOOH, most commercially available particles are produced from a-FeOOH by solid phase transformation. The original shape and size of a-FeOOH (goethite) particles are kept essentially unchanged during this process. For high-density recordings, particles must be small, and to achieve large coercive force, they should be single domain with large aspect ratio. Therefore, the size and shape of the starting a-FeOOH particles are carefully controlled. The goethite particles are first dehydrated by heating, then reduced by hydrogen into magnetite and, finally, oxidized into maghemite at optimum températures and réduction and reoxidation reaction times. Many techniques hâve been developed to achieve good dispersibility a n d orientability. For example, Homola and Lorenz covered surfaces of magnetic particles with monodispersed silica particles by colloid chemical procédures. 16 A similar technique was employed by Ishikawa and Matijevic.17 Maghemite particles on the market are 0.2-0.4/x long with an aspect ratio of 7:10, as illustrated in Figure 1.
Préparation Particles for Magnetic Recording Media To ensure sufficiently strong output and permanency of the signal with low noise, particulate magnetic recording materials are required to hâve (1) large saturation magnetization, (2) high coercivity, (3) narrow distributions in size and shape, (4) good dispersibility and
In usihg a-FeOOH as the starting material, it is very difficult to completely avoid pore formation. It was assumed that nonporous solids could be formed, if elongated hématite particles were used instead. I n d e e d , particles p r e pared directly from hématite, obtained by transformation of iron hydroxide, exhibited good magnetic properties. 18 - 19 Finely dispersed solids of good magnetic properties could also be produced
Masataka Ozaki Introduction Magnetic particles are important not only in the technology, but also in the function of some biosystems. In addition, they are of great scientific interest in developing a better understanding of magnetic phenomena. Ever since magnetic recording média were first prepared, extensive efforts hâve been made to produce improved magnetic dispersions. 1 " 3 The particles for m a g n e t i c média must be of single domain, high saturation magnetization, and proper coercive force. However, the magnetic interactions between such particles are very strong, and stable dispersions are difficult to obtain. Originalîy, their use was limited to audio tapes, but presently they are employed in a variety of applications. Thus, small particles of différent magn
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