Materials for Magnetic-Tape Media
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MRS BULLETIN/SEPTEMBER 1996
media and the recent development of both monolithic thin-film media and double-coated particulate metal media. Particulate Media Structure of the Magnetic Tapes Figure 1 shows a transmission-electron-microscopy (TEM) micrograph of the cross-sectional view of particulate media. The coating formulation (binder, magnetic particles, solvents, crosslinker, lubricant, and abrasives), which is mixed by a disperser such as a sand mill, is coated onto the poly(ethylene terephthalate) (PET) substrate. A back coat is optionally applied to improve runability. Thicknesses of the coating are commonly about 6 yam and 3 /u,m for audio tape and videotape, respectively. Surface roughness of the tapes, which affects both electromagnetic properties and friction, is determined by the extent of the dispersion and particle size. Its center-line average, or Ra, is about 15 nm by the optical method (Zygo) for audio use. It becomes smoother for video and data-processing applications as higher recording density is required. It is 5-7 nm for Hi8MP videotape (metal particulate tape for 8-mm video recorder). Materials Used for Tapes The magnetic particles (Figure 2) play an important role in determining the electromagnetic properties while the polymer binder and the other dispersants are critical to the dispersion of the particles. The interaction of the particles and the organic compounds becomes important as particles are made smaller. Mechanical properties depend on the substrate, and the lubricant is a key technology in determining the tribological performance.
Magnetic Particles. y-Fe2O3 and Co-Modified y-Fe2O3.
The first commercially available magnetic-recording tapes used iron oxide particles. y-Fe2O3 is still the most widely used magnetic-recording material because of its great chemical and physical stability, and it has an important role in audio tapes. y-Fe2O3 has been produced by the following synthesis scheme (Scheme I). First needle-shaped goethite FeOOH is grown as a precipitate from a solution of iron salts and then dehydrated into the nonmagnetic hematite a-Fe2O3, which is then reduced to magnetite or Fe3O4. Magnetite seems to be a desirable recording material.1 However it has some chemical and magnetic instabilities. Therefore it is oxidized to y-Fe2O3 for most recording applications. The hydroxide has three different acicular modifications thac lead to y-Fe2Oj particles of different morphological properties (Scheme I).2 Further requirements in magnetic tapes call for higher values of coercivity than can be obtained from pure oxide. Particles can be made containing 2-3-wt% cobalt having coercivity up to 60 kA/m.3 The mechanism of coercivity enhancement resulting from Co2+ ions in the oxide lattice has been explained.4 Co-doped oxides are characterized by strongly temperature-dependent coercivity with time, and show a progressive loss of short-wavelength signal amplitude with repeated playback as a result of mechanical stresses. To avoid these shortcomings, Co is impregnated just near the surface o
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