This invention relates to a process for the production of a ferromagnetic metal powder and more particularly, it is concerned with a process for the production of a ferromagnetic metal powder for magnetic recording media, in particular, suitable for high density recording.
Examples of the ferromagnetic powder which has hitherto been used for magnetic recording media are .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4 and CrO.sub.2. However, these ferromagnetic powders are not so suitable for magnetic recording of a signal of short magnetic recording, for example, less than about 10 microns. That is to say, the magnetic properties such as coercive force (Hc) and maximum remanence magnetization (Br) are not sufficient for the so-called high density recording. Of late, it has been desired to develop a ferromagnetic powder having properties suitable for high density recording. One example of such a material is a ferromagnetic metal powder.
The following are known methods for producing the ferromagnetic metal powder:
1. A method comprising heat-decomposing an organic acid salt of a metal capable of forming a ferromagnetic material, for example, oxalate and reducing with a reducing gas. This method is described in, for example, Japanese Patent Publication Nos. 11412/1961, 22230/1961, 14809/1963, 3807/1964, 8026/1965, 8027/1965, 15167/1965, 16899/1965 (U.S. Patent No. 3,186,829), 12096/1966, 14818/1966 (U.S. Patent No. 3,190,748), 24032/1967, 3221/1968, 22394/1968, 29268/1968, 4471/1969, 27942/1969, 38755/1971, 38417/1972, 41158/1972 and 29280/1972.
2. A method comprising reducing a needle-like iron oxyhydroxide or a substance containing metals other than iron as well as above oxyhydroxide or needle-like iron oxide derived from the oxyhydroxide. This method is described in, for example, Japanese Patent Publication Nos. 3862/1960, 11520/1962, 20335/1964, 20939/1964, 24833/1971, 29706/1972, 30477/1972 (U.S. Pat. No. 3,598,568), 39477/1972, Japanese Patent Application (OPI) Nos 5057/1971 and 7153/1971, Japanese Patent Publication No. 24952/1973, Japanese Patent Application (OPI) Nos. 79153/1973 and 82395/1973 and U.S. Pat. Nos. 3,607,220 and 3,702,270.
3. A method comprising evaporating a ferromagnetic metal in a low pressure inert gas. This method is described in, for example, Japanese Patent Publication Nos. 25620/1971, 4131/1972, 27718/1972, Japanese Patent Application (OPI) Nos. 25662/1973 to 25665/1973, 31166/1973, 55400/1973 and 81092/1973.
4. A method comprising heat-decomposing a metal carbonyl compound. This method is described in Japanese Patent Publication Nos. 1004/1964, 3415/1965 and 16868/1970 and U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882.
5. A method comprising electrodepositing a ferromagnetic metal powder using a mercury cathode and then separating the metal powder from mercury. This method is described in Japanese Patent Publication Nos. 12910/1960, 3860/1961, 5513/1961, 787/1964, 15525/1964, 8123/1965, 9605/1965 (U.S. Pat. No. 3,198,717), 19661/1970 (U.S. Pat. No. 3,156,650) and U.S. Pat. No. 3,262,812.
6. A method comprising reducing a solution containing a metal salt capable of forming ferromagnetic metal powder by adding a reducing agent thereto. This method is described in, for example, Japanese Patent Publication Nos. 20520/1963, 26555/1963, 20116/1968, 9869/1970, 14934/1970, 7820/1972, 16052/1972, 41718/1972 and 4.719/1972 (U.S. Patent 3,607,218), Japanese Patent Application (OPI) Nos. 1353/1972 (U.S. Pat. No. 3,756,866), 1363/1972, 42252/1972, 42253/1972, 44194/1973, 79754/1973 and 82396/1973 and U.S. Pat. Nos. 3,206,338, 3,494,760, 3,535,104, 3,567,525 3,661,556, 3,663,318, 3,669,643, 3,672,867 and 3,726,664.
The present invention relates to a method for the after-treatment of ferromagnetic metal powders obtained by the above described reducing reactions by dry prcess (1), (2) and (3) and reducing reactions by wet process (4), (5) and (6). In particular, the method (6) is preferable wherein a metal salt capable of forming a ferromagnetic metal powder is reduced in its solution.
The ferromagnetic metal powders obtained by the above described dry process or wet process reducing reactions have the following problems:
These ferromagnetic metal powders lack magnetic stability, are particularly weak under humid conditions and, even at normal temperature, are gradually oxidized by the presence of humid conditions, and the powders consisting mainly of Fe tend to readily lose their magnetism often. In the step of after-treatment after the reducing reaction, therefore, the oxidation takes place gradually and the high Bm property, one of the excellent properties of ferromagnetic metal powders, often deteriorates.
Various studies have lately been made in order to solve this problemm. For example, it has been proposed to add non-magnetic elements to reaction baths as disclosed in U.S. Pat. Nos. 3,535,104, 3,669,643 and 3,672,867, Japanese Patent Publication Nos. 7820/1972 and 20520/1963 and Japanese Patent Application (OPI) Nos. 78896/1975 (corresponding to U.S. Patent Application Ser. No. 524,853, filed Nov. 18, 1974) and 78897/1975 (corresponding to U.S. Pat. Application Ser. No. 524,860, filed Nov. 18, 1974). Another method has been proposed comprising adding additives such as organic compounds to reaction baths as disclosed in Japanese Patent Publication Nos. 20116/1968, 14934/1970 and 41719/1972 (U.S. Pat. No. 3,607,218) and 7820/1972 and Japanese Patent Application (OPI) Nos. 42253/1972, 44194/1972, 79754/1973 and 82396/1973. Furthermore, a method for solving such a problem by heat treatment of ferromagnetic metal powders obtained by the wet process reducing reaction is reported in Japanese Patent Publication Nos. 26555/1963 and 16052/1972 and Japanese Patent Application (OPI) No. 42252/1972.
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