Overview
Magnetic powder, the core component of magnetic coatings, is the main factor that determines the magnetic properties of magnetic recording media. It should have enough coercivity to effectively improve the demagnetization effect, but not so high that it is difficult to demagnetize, and its magnetization should have the same magnitude as that of ferromagnetic metal in order to provide sufficient magnetic flux to the magnetic head; It should be uniform, no sintered mass, complete crystals, and generally less than 1 µm in particle size; it should have good dispersibility and high packing density; its magnetic properties should be stable and not affected by time, temperature, humidity, and pressure.
Since the use of carbonyl iron powder in 1935, the development of magnetic powder has been continuously enriched. Later, it developed from oxides to metal alloy magnetic powders, and the magnetic powder particles continued to shrink and the coercivity continued to increase.
Requirements
The requirements that the magnetic powder should have are:
1. Specific saturation magnetization: As much as possible than saturation magnetization High to improve the output sensitivity of the recording medium. In order to increase the number of feet of the magnetic recording medium, the residual magnetization should also be as high as possible.
2. Curie temperature: Curie temperature must reach a certain value or more, so that the magnetization changes slowly with temperature.
3. High coercivity Hc: In order to overcome the demagnetizing effect of the magnetic powder itself, it must have sufficient coercivity to ensure the improvement of magnetic recording information. But it should not be too high, so that the magnetic field generated by the writing and erasing head is not enough to reverse the magnetization of the magnetic powder.
4. High orientation, high packing density and good dispersibility
5. Narrow particle size distribution
6. High mechanical strength
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7. High stability
To sum up, the granular magnetic powder should have the characteristics of single domain particles, complete crystallization, smooth surface, appropriate size, uniform particle size, and stable performance.
Category
Oxide magnetic powder
The largest type of magnetic powder, there are three main types:
Iron oxide magnetic powder< /b>
Fe3O4 (the main component of magnetite) is one of the earliest magnetic materials. Its σs and Hc are higher than the most used γ-FeZrO₂O3, but due to its instability and copying (copying refers to the characteristics of interference between the magnetic tape layers and the layers) Such shortcomings are gradually replaced by γ-Fe2O3. γ-Fe2O3 has been put into production since the 1950s, and it still occupies the leading position in magnetic materials. The production of γ-Fe2O3 in various factories basically still uses hydrated iron oxide FeOOH (ie, iron yellow, short for Fe2O3·H2O) as the starting material for the following heat treatment:
The quality of the product is very important. Much depends on the starting material. Therefore, how to obtain iron yellow with good crystal shape and narrow particle size distribution, and to keep it from being damaged in the subsequent processing (such as pores and sintering to destroy the needle shape, etc.) is the key to improving product performance. In recent years, a lot of work has been done for this purpose, such as adding nickel, chromium, zinc, strontium and other elements to the reaction solution; changing the traditional process; using γ-FeOOH (γ-iron yellow) as the starting material; and The surface of α- or γ-iron yellow is coated with a layer of anti-sintering agent; finally, the finished γ-Fe2O3 is densified and surface treated, so that the final product has good dispersibility and so on.
Chromium dioxide magnetic powder
In 1961, DuPont of the United States published a hydrothermal method to synthesize single-phase ferromagnetic chromium dioxide. Commercial production began in 1967. Chromium dioxide has high Hc and other properties are better than γ-Fe2O3. It is mainly used for high-end audio tapes and video tapes. Chromium dioxide is obtained by decomposing chromium trioxide under high temperature (400～525℃) and high pressure (50～300MPa). Adding a catalyst can reduce the reaction temperature and pressure. This kind of magnetic powder has not been widely used due to the disadvantages of high cost and large wear on the magnetic head. Currently, work is being done to improve chromium dioxide, such as preparation under normal pressure and research on coating cobalt.
Cobalt-iron oxide magnetic powder
In order to improve the Hc of iron oxide magnetic powder, people have long wanted to use the method of adding cobalt to it, and it has been the most successful so far. The one is coated with cobalt-type magnetic powder. This method was first proposed by the United States in 1971. Cobalt coating can be divided into two types: using γ-Fe2O3 as a raw material to disperse in water and then coating Co(OH)2 on the surface or forming cobalt ferrite Cox i>Fe3-xO4. The latter's Hc can be about twice as high. In 1973, the Avi-lyn magnetic powder developed by Tokyo Electric Chemical Industry Co., Ltd. belongs to this category. Its Hc is high and can be changed within a certain range, and the wear on the magnetic head is only 1/5 of that of chromium dioxide. The tape made of cobalt-coated magnetic powder is not only completely interchangeable with chromium dioxide tape, but also the color signal output level and signal-to-noise ratio surpass that of chromium dioxide tape.
In recent years, due to the special needs of high Hc replication master tape, magnetic card and perpendicular recording, high Hc magnetic powder, hexagonal barium Ferrite (Hc>2000Oe) and other high Hc permanent magnet materials are also used as recording materials and are being valued. In 1982, Japan developed barium ferrite single-domain fine powder by the glass crystallization method and made it into a coated vertical magnetic tape.
Metal magnetic powder
Its high σs (twice γ-Fe2O3) and high Hc(>1000 Oe) make it a high-density record The material has long attracted people's attention, and has not been put into practical use due to the disadvantages of poor stability and difficulty in dispersing in the magnetic slurry. In 1978, the metal powder commercial tape was successfully developed, and the development in this area was extremely rapid. The manufacturing methods mainly include: ①reduction of needle-shaped iron oxide in hydrogen; ②reduction of metal salts in aqueous solution with a strong reducing agent under the action of a magnetic field; ③vacuum evaporation and condensation, etc.