It is known that that ferromagnetic iron oxide useful for high density magnetic recording should possess, a high coercive force. There are several techniques for increasing the coercivity of a ferromagnetic oxide powder. These include, for example, adding cobalt to acicular ferromagnetic oxide.

U.S. Patent No. 4,188,302 teaches the use of an aqueous suspension of γ-Fe ₂ 0 ₃ containing cobalt and iron ions to form a single mixed coating on a ferromagnetic oxide core. The coercivity of the coated particles can then be increased by heating the ferromagnetic oxide particle for a sustained period. The properties of coercivity as well as remanent magnetism are important characteristics of magnetic tapes for they directl reflect the recording density which a magnetic tape can achieve.

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Ever increasing demands to improve the areal density of recorde and retrievable information have put a great challenge on the recording media as well as magnetic particulate manufacturers to design, produce, and use materials which are capable of increasing recording density without sacrificing other perform¬ ance aspects such as magnetic stability. This burden is par¬ ticularly acute when one considers that about 80-85% of the tonnage and material costs of magnetic recording media coating is attributable to the magnetic particulates. As alluded to above, one principal method of particle modi fication to improve coercivity is adding cobalt to Fe.,0. or

Y-Fe ₂ 0 ₃ which results in substituting cobalt for iron ions in

_F e _O ^. -

3 4 or by filling vacant lattice positions in γ-Fe ₂ 0 ₃ with cobalt ions. Prior art modification methods have employed the use of FeOOH, Fe.,0., γ-Fe ₃ 0., and partially reduced γ-Fe ₂ 0 ₃ to serve as core materials. To generalize, the materials in particulate form are suspended in aqueous solutions of cobalt, iron, or other transition metal ions traditionally as divalent salts and, in some cases, with complexing and reducing agents. A chemical reaction is carried out by adding equal or greater than stoichiometric amounts of a base such as NaOH, KOH or NH.OH at specified conditions of temperature, pressure, concentration of modifying metal salt solutions and pH for a specific period of time. The core materials so treated are then separated from the aqueous phase, dried, and usually subjected to a heat-treatment which is carried out in oxidizin

reducing, or inert environmental conditions at a specified temperature for a specified period of time. The extent of increase or enhancement of magnetic properties specifically in coercivity and specific saturation and remanent magnetization which is achieved depends primarily on (1) the amount of cobalt (2) the amount of other metallic ions, if present, (3) condi¬ tions of modification of the recited treatment reactions, and (4) the post heat treatment program.

There are, however, certain disadvantages associated with the above-described particle modification methods. For example there is a certain lack of chemical as well as processing flexibility. Cobalt, an extremely expensive element, is not used efficiently while post heat treatment steps cause magnetic instability and provide for gas-solid reactions which are difficult to control.

From the material user's standpoint, the prior art process are very complicated and require extensive equipment and manpower to justify an in-house effort to produce and use high-performance materials. These methods lack the flexibility " for direct incorporation of modified materials into the coating formulations resulting in an invariable need for drying and post-treatment steps. Lastly, precise control of FeO content and, hence, of saturation and remanent magnetization, which increases low frequency response and sensitivity of the recordi media, is very difficult.

It is thus an object of the. present invention to produce high-performance ferromagnetic oxides without the difficulties experienced by prior art methods.

It is yet another object of the present invention to convert low-performance ferromagnetic oxides into high-perform ferromagnetic oxides with increased coercivity without the nee for post-treatment heating steps. It is still another object of the present invention to produce high coercivity ferromagnetic iron oxides using lesser amounts of cobalt than utilized in prior inventions or previou practiced methods.

It is yet another object of the present invention to produce a high-performance ferromagnetic oxide particle whereb the divalent iron oxide (FeO) content and hence the saturation magnitization can be precisely controlled.

These and other objects of the present invention will be more fully appreciated when considering the following discussi

SUMMARY OF THE INVENTION

The present invention deals with the preparation and manufacture of high coercivity recording materials, specifical high coercivity ferromagnetic oxides by which low coercivity iron oxides are modified by depositing two or more distinct layers of metal compounds, pref rrably, compounds of cobalt, iron, mixed compounds of the same, or compounds of other transi tion metal ions such as manganese, nickel and zinc or mixed compounds thereof to yield high coercivity and high saturation magnetization materials suitable for high density analog and digital magnetic recording members.

The method of the present invention comprises suspending low coercivity ferromagnetic oxide particles in an aqueous medium, forming a layer of the first metal on the surfaces of t particles, forming a layer of the second metal on the surfaces t- of the first metal, and forming a layer of the third metal on the surfaces of the second metal. The first and third metals can be the same. Ideally, the first, second and third metal layers are formed by providing individual solutions of each metal separately and adding a sufficient quantity of a base to 1 ₀ cause metal adsorption reactions to take place on each surface. This multi-layer growth approach is based on the principle of "surface renewal". Ideally, each metal is added in an amount t saturate the surface to which the metal is. applied.

As an example of practicing the present invention, a ferro 15 magnetic oxide of low coercivity and magnetic moment composed of Y-Fe ₂ 0 ₃ or F ^O. or the intermediate or pretreated forms thereof and being of suitable particle size, surface area and overall morphology can be milled and suspended in water with or without any base or dispersing agents. According to 20., prior art techniques, transition metal ions such as cobalt and ^" iron would be added together in a single step in order to modify the ferromagnetic core particle. By contrast, the present invention splits the amounts of transition metal ions into various portions and adds them during different steps according 25 to a predesigned scheme.

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If cobalt were to be used as the first transition metal ion, it would be added to the aqueous suspension and then subjected to an adsorption reaction to completely form' a layer of cobalt upon the ferromagnetic core materials. Then, to the c same suspension, would be added a solution of another transitio metal ion such as ferrous ions and a base, such as NaOH to grow an iron containing layer atop the cobalt layer. The iron so formed acts as a new core or surface which is more susceptible to adsorption of a third layer, such as cobalt. A third

1 ₀ solution containing, for example, cobalt, is formed and added t the suspension which is again subjected to a base such as NaOH to form a third layer similar to the first. This process can b continued further with other surface renewal reactions.

The above-recited process results in a multi-fold enhance-

- _j ς ment of the magnetic properties of a ferromagnetic particle from a fixed amount of transition metal ions. Maximizing the use of these materials is extremely important for cobalt, if used as one of the transition metal ions, is extremely expensiv particularly when used on a commercial basis. Also, all of the

20" reactions involve only the employment of wet chemistry in a single reaction vessel. After the reactions are complete, the suspension can be filtered, washed and dried to recover the modified oxides which do not require any post heat treatment to achieve high coercivity and magnetic moment. EJ However, post heat treatment can still be applied to furth increase the coercivity of modified oxides prepared according t the present invention.

EXAMPLE I Single Layer Structure 20.0 g of accicular gamma ferric oxide powder having a surface area of ca.30m /g, a coercivity of 340 oetsteds, and a saturation magnetization of 72.3 emu/g was dispersed in 2 liter of DI water. The suspension was bubbled with nitrogen for 30 minutes to purge air from the system and 2.8 g CoS0 ₄ '7H ₂ 0 and 7 g FeSO.-7H ₂ 0 dissolved in 200 m of DI water were added to th reaction vessel. This was followed by the addition of 5.9 g of 50% NaOH over a ten-minute period. The above suspension was heated with vigorous stirring at 100 ^β C and maintained at that temperature for an additional 3.5 hours. The suspension was allowed to cool ^' to room temperature, then washed with deionized water until the conductivity was less than 500 ymhos. The oxide was then filtered and dried at 100 ^β C for 2.5 hours.

Coercivity of the modified oxide was 480 Oe, while the saturation magnetic moment was 78.-5 emu/g. Analysis for Co and FeO gave respective values of 2.7% and 4.8%. After subsequent "heating at 200 ^C C for one hour, the coercivity was measured at 610 Oe and the saturation magnetic moment at 75.4 emu/g. This powder was then processed into a polyurethane binder and coated onto a polyethylene terephthalate base in the convention manner known in the manufacture of magnetic tape. The resulting tape properties are as follows:

Table I

Example I - Properties on Audio Tape

He 668 Oe

MS 1636 G

Squareness 0.79

Orientation Ratio 2.12

MOL - 400 Hz -0.7 dB

Signal/Print Ratio

24 hrs/50 ^β C _40.5 dB

EXAMPLE II

Two Layer Structure 100 g of the core material of Example I was dispersed in 2 liters of deionized water and bubbled with nitrogen as in Example I. 50.0 g FeSO.- 7H ₂ 0 was dissolved in 200.0 ml deionized water and added to the same suspension as in the previous example. This was followed by the addition of 29.3 g 50% NaOH over a 15-minute period with vigorous stirring.

The suspension was heated to 100 ^β C and maintained at that temperature for 3.5 hours. The suspension was then allowe to cool to room temperature. A small sample of modified oxide

'_

.was withdrawn, processed, and analyzed for magnetics, and this sample was denominated Sample A.

To the above suspension was added 14.3 g CoS0 ₄ *7H ₂ 0 and 35 g FeS0 ₄ - 7H ₂ 0 which was followed by the addition of 29.0 g 50 NaOH under vigorous stirring over a 15-minute period. The slur was then heated and maintained at 100 ^β C for an additional 3 hou

and then cooled. The oxide was washed with deionized water until a conductivity of 400 μmhos was obtained in the slurry. The oxide was then filtered and dried for 3 hours at 100°C and the sample denominated Sample B. 5 Coercivity values for Samples A and B were 297 Oe and

441 _O e, respectively. Saturation magnetic moments were measure at 78.5 emu/g and 84.5 emu/g, respectively. Potentiometric analysis for Co and FeO for Sample A gave values of .07% and 4.9 _% . While _. for Sample B, Co and FeO values were 2.8% and 15.2 0 respectively.

EXAMPLE III Three Layer Structure 100 g of the core oxide of Example I was dispersed, added to the reaction vessel and purged with nitrogen as described in 5 the previous examples. 7.1 g CoS0 ₄ .7H ₂ 0 was dissolved in 100 ml deionized water and added to the reaction vessel with stirring. To this was added 4.2 g of 50% NaOH over a 10-minute period. The slurry which formed was then heated to 100 ^β C and maintained at that temperature for 3 hours and then cooled to 0' room temperature. A 5.0 g sample labeled X, was withdrawn, filtered, washed and dried as in the previous example.

To the remaining cooled slurry, 35.0 g FeS0 ₄ • 7H-0 dissolved in 200 ml deionized water, was added. Thereupon, 20.5 g 50% NaOH diluted to 150 ml was added to the reaction vessel <- over a 15-minute period with vigorous stirring. The slurry whic formed was heated to 100 ^β C and maintained at that temperature

for 16 hours, and then allowed to cool. A 5.0 g sample, labele Y, was withdrawn and processed and analyzed as described above.

To the remaining cooled slurry, 7.1 g CoS0 ₄ -7H ₂ 0 dissolved in 100 ml deionized water with stirring, was added. Then, 4.25 50% NaOH which was diluted to 100 ml was added over a 15-minute period with vigorous stirring. The slurry was then heated to 100°C while stirring and maintained at that temperature for 3 hours and then cooled to room temperature. The oxide, labeled Sample Z, was washed until a conductivity of 500 mhos was obtained, then filtered and dried for 2.5 hours.

A portion of Sample Z was post-heat treated exactly in a manner described in Example I. The oxide obtained was labeled Sample ZZ. "

The pertinent magnetic and chemical analysis for Samples X Y, Z and ZZ are as follows:

Powder Sample X Sample Y Sample Z Sample ZZ

He (Oe) 405 399 596 779 σs (EMU/g) 72.7 78.1 78.9 77.6

Co 1.5% 1.4% 2.9% 2.9% FeO 0.2% 6.0% 7.8% 6.3%

The modified oxide of Example III, Sample Z was processed with a polyurethane binder system and coated onto polyethylene terephthalate base in the conventional manner known in the manufacture of magnetic tape. The magnetic properties of this tape are as follows:

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Table II

Example III - Properties of Three Layer Non- Post Heat Treated Modified Oxide on Audio Tape

He 619 Oe

M 1936 G

Squareness 0.84

Orientation Ratio 2.82

MOL - 400 Hz +3.7 dB

Signal/Print Ratio

24 hrs/50 ^β C 43.6 dB

EXAMPLE IV

Five Layer Structure

Exactly the same preparation procedure of Example III was followed until the three-layer oxide slurry was obtained. Thi slurry was further processed by repeating the procedure for depositing the iron and cobalt layers as recited in the same example. The five-layer non-post-heat treated-modified oxide powder had coercivity and saturation magnetic moment of 674 Oe and 81.7 emu/g, respectively. The chemical analysis of the sam powder for Co and FeO gave values of 4.1 and 13.5%, respectivel A comparison of modified oxides obtained by the methods of Example _* s I through III is shown in Table III. In each case, th ςores were modified with 3% cobalt ion and 7% iron ion. The improvements in the magnetic properties of the 3-layered structure of the present invention are quite evident by viewing Table III.

Table III

Modification Experiments Using 3% Cobalt Ion and 7% Iron Ion on Core (No post heat treatment)

Method as in

Experiment Examples He (Oe) σs (EMU/g)

3%Co++//7%Fe++ (2 layer) II 457 79.1

7%Fe+-+//3%Co++ (2 layer) II 449 75.7

Expitaxial (Co-precipitation) I 480 78.5 1.5%Co++//7%Fe++//1.5%Co++ III 600 78.5

(3 layers)

In order to further demonstrate that the method of the present invention produces an improved magnetic particle, the metal ion precipitation as described in Example III was used to obtain various distributions of cobalt and iron ions in respec¬ tive layers, although a total of 3% cobalt and 7% iron was maintained. The results are shown in Table IV.

Table IV Splitting of Cobalt Ion or Iron Ion Amounts Experiment He (Oe) σs (EMU/g)

'HCo+V/7%Fe++//2%Co++ 556 77.6

2%Co++//7%Fe++//* _| %Co++ 543. _78#0

1.5%Co++//7%Fe-f+/1-5%Co++ 600 78.5

3.5%Fe++//3%Co++//3.5 _% Fe ₊ + _{501 74<7}

The figure which is attached depicts the efficiency of the modification procedures of the present invention as a graph of coercivity versuses % cobalt on a ferromagnetic core. More

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specifically, γ-Fe ₂ 0 ₃ cores were modified according to prior art and inventive methods shown herein. Curve 1 shows the relationship between coercivity and % cobalt when only cobalt is added to the γ-Fe ₂ 0 ₃ particles. Curve 2 shows a c similar relationship when cobalt and iron ions are epitaxially applied to the core as a single layer. By contrast. Curve 3 depicts the relationship between coercivity and % cobalt when cobalt/iron/cobalt three or more layered structure is produced according to the present invention. As stated previously, prior art particles can approach the magnetic properties of the multi-layered particles of the present invention only if a post formation heating step is carried out. By contrast, magnetic particles of the,present invention can be conveniently produced by employing only wet chemistry methods. This method of particle modification also lends itself to direct utilization of the final slurry in maki of magnetic recording members. For example, one needs only remove excess salts from a finished suspension of ferromagneti particles and then use the same slurry in making the final " product either by solvent replacement followed by conventional solvent borne formulation technology or by using the more direc water borne formulation technology without resorting to essenti filtration, washing, drying and post heat treatment process steps. Therefore, the method described and claimed herein directly overcomes prior art manufacturing flexibility problems poor cobalt efficiency usage, magnetic instability and costly post treatment process steps.

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