1.  Field of the Invention
This invention relates to ferromagnetic metal powders and to a method of preparing the same.  More particularly, this invention relates to ferromagnetic metal powders which are used for a magnetic recording medium suitable for high density recording and to a method of preparing the same. 
2.  Description of the Prior Art
Ferromagnetic powders such as .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4 or CrO.sub.2 have hitherto been used for a magnetic recording medium.  However, these ferromagnetic powders are not suitable for magnetic recording of a signal having a short recording wavelength (about 10 microns or less), i.e., their magnetic characteristics such as coercive force (Hc) and maximum residual magnetic flux density (Br) are insufficient for use in high density recording.  In recent years, extensive efforts have been made to obtain ferromagnetic powders having characteristics suitable for high density recording.  Ferromagnetic metal powders are one material which have been the object of such research.  The metals mostly used include iron, cobalt and nickel, and, if desired, others such as chromium, manganese, zinc or rare earth elements are added. 
The following methods of producing ferromagnetic metal powders are known. 
(1) Thermally decomposing an organic acid salt of a ferromagnetic metal and then reducing with a reducing gas (as described, e.g., in Japanese Patent Publication Nos.  11,412/61, 22,230/61, 14,809/63, 3,807/64, 8,026/65, 8,027/65, 15,167/65, 16,899/65 (U.S. Pat.  No.  3,186,829), 12,096/66, 14,818/66 (U.S. Pat.  No.  3,190,748), 24,032/67, 3,221/68, 22,394/68, 29,268/68, 4,471/69, 27,942/69, 38,755/71, 38,417/72, 41,158/72, 29,280/73, etc.). 
(2) Reducing needle-like oxyhydroxides, needle-like oxyhydroxides having a metal contained therein, or needle-like iron oxide obtained from these oxyhydroxides (as described, e.g., in Japanese Patent Publication Nos.  3,862/60, 11,520/62, 20,335/64, 20,939/64, 24,833/71, 29,706/72, 30,477/72 (U.S. Pat.  No.  3,598,568), 39,477/72 and 24,952/73, Japanese Patent Laid-Open Application Nos.  5,057/71, 7,153/71, 79,153/73 and 82,395/73, U.S. Pat.  Nos.  3,607,220 and 3,702,270, etc.). 
(3) Evaporating a ferromagnetic metal in an inert gas under low pressure (as described, e.g., in Japanese Patent Laid-Open Application Nos.  25,620/71, 4,131/72, 27,718/72, 25,662/73, 25,663/73, 25,664/73, 25,665/73, 31,166/73, 55,400/73 and 81,092/73, etc.). 
(4) Thermally decomposing a metal carbonyl compound (as described, e.g., in Japanese Patent Laid-Open Application Nos.  1,004/64, 3,415/65 and 16,868/70, U.S. Pat.  Nos.  2,983,997, 3,172,776, 3,200,007 and 3,228,882, etc.). 
(5) Electrodepositing ferromagnetic metal powders using a mercury cathode and then separating the powder from mercury (as described, e.g., in Japanese Patent Publication Nos.  12,910/60, 3,850/61, 5,513/61, 787/64, 15,525/64, 8,123/65, 9,605/65 (U.S. Pat.  No.  3,198,717) and 19,661/70 (U.S. Pat.  No.  3,156,650), U.S. Pat.  No.  3,262,812, etc.). 
(6) Adding a reducing agent to a solution containing a salt of a ferromagnetic metal to reduce the salt (as described, e.g., in Japanese Patent Publication Nos.  20,520/63, 26,555/63, 20,116/68, 9,869/70, 14,934/70, 7,820/72, 16,052/72, 41,718/72 and 41,719/72 (U.S. Pat.  No.  3,607,218), Japanese Patent Laid-Open Application Nos.  1,353/72 (U.S. Pat.  No.  3,756,866), 1,363/72, 42,252/72, 42,253/72, 44,194/73, 79,754/73, 82,396/73 and 41,899/74, 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, etc.). 
This invention is concerned with ferromagnetic powders obtained by the afore-said various methods, and particularly concerned with an after-treatment of ferromagnetic metal powders obtained by the above method (6) which comprises reducing a salt of a ferromagnetic metal in solution. 
In general, the ferromagnetic metal powders obtained by the above method (6) are treated, for example, in such a manner that they are separated from the reaction mother liquid, washed with water to remove unreacted metal ions and anions such as SO.sub.4.sup.-- and Cl.sup.-, and then subjected to a solvent exchange or dried in an inert gas (in a non-oxidizing atmosphere).  The ferromagnetic metal powders obtained by these procedures have the following disadvantages: the ferromagnetic metal powders are poor in magnetic stability.  Particularly, they are so unstable to moisture that they are gradually oxidized even at ambient temperatures in the presence of moisture.  For example, powders consisting of Fe as a main component often lose their magnetic properties.  Therefore, in an after-treatment step after reduction, oxidation gradually proceeds, and thus the ferromagnetic metal powders are liable to lose their high Bm property, one of their excellent features. 
Various research have been performed to eliminate the above defect. 
For example, the addition of a non-magnetic element to the reaction solution to impart oxidation resistance to the composition is described in U.S. Pat.  Nos.  3,535,104, 3,669,643 and 3,672,867, Japanese Patent Publication Nos.  7,820/72 and 20,520/63, Japanese Patent Application Nos.  128,988/73 and 12,898/73, etc. 
Moreover, the addition of an organic compound to a reaction bath is described in Japanese Patent Publication Nos.  20,116/68, 14,934/70, 41,719/72 (U.S. Pat.  No.  3,607,218) and 7,820/72, Japanese Patent Laid-Open Application Nos.  42,253/72, 44,194/72, 79,754/73 and 82,396/73, etc. 
In addition, there is known adding a higher aliphatic acid in the reaction step as described in Japanese Patent Publication No.  20,116/68, adding a compound having a sulfone group on the aromatic ring in the reaction step as described in Japanese Patent Publication No.  14,934/70, and adding a surface active agent in the reaction step as described in Japanese Patent Publication No.  7,820/72. 
In any of the above methods, a surface active agent or the like is added to a reaction bath in the reaction step, but it was found that when the reaction was carried out by adding such materials in an amount sufficient to improve Bm and oxidation resistance, ferromagnetic metal powders having a high coercive force and high squareness ratio (Br/Bm) could not be obtained.  In other words, when the reaction was carried out by adding a surface active agent or the like to the reaction bath, ferromagnetic metal powders having high Bm and good oxidation resistance could not be obtained without deteriorating magnetic characteristics (coercive force and squareness ratio). 
We have studied various approaches of preventing oxidation in an aftertreatment step after a reduction reaction, and as a result, found that oxidation was highest in a washing step.  We thus proposed a method of washing with an aqueous solution containing sacchrin, an aqueous solution containing a reducing substance, an aqueous solution containing an anionic surface active agent, or the like, as disclosed in Japanese Patent Application Nos.  10,441/74, 11,678/74 and 106,901/74. 
Japanese Patent Publication 2,612/65 discloses a method of incorporating a volatile corrosion inhibitor into a magnetic layer or a backing layer for the prevention of corrosion of a magnetic metal thin film or a magnetic alloy powder tape. 
In the above method, the volatile corrosion inhibitor is used together with a binder and magnetic powders.  However, according to later made detailed investigations, it was found that an anticorrosive compound shows much more effect when added immediately after the reaction producing the magnetic powders, i.e., in the step of washing the surface of the magnetic powders, than when added during kneading the magnetic powders.