DE2221264B2 - COBALT AND SILICON OXIDE CONTAINING NEEDLE-SHAPED FERRIMAGNETIC IRON OXIDES - Google Patents

Description

45

The present invention relates to acicular ferrimagnetic iron oxides containing cobalt and silicon. These needle-shaped iron oxides, cobalt and siliceous show, after being heated to 150 ⁰ C for 30 minutes, a reduced Remanenzverlust.

The iron oxides, Fe ₃ O ₄ and V-Fe ₂ O ₃ with or without doping with Co and other elements are suitable for the production of magnetogram carriers. The literature usually speaks of ferromagnetic iron oxides. However, since the work of L. Neel at the latest, it has been known that the underlying phenomenon leading to magnetic behavior differs from that in ferromagnetic materials and can be described as ferrite-magnetic. The analogy of ferrimagnetism to ferromagnetism remains limited to the macroscopic behavior in the external magnetic field fts.

In the Ferromagnelisnuis, all are spin moments of the individual atoms coupled and in individual areas, the so-called Weiss districts or domains strictly aligned in parallel. This creates a resulting total moment for each de domain, which corresponds to the sum of all magnetic atomic moments.

The ferrimagnetism, on the other hand, is a special case of antiferromagnetism, in which the atomic spin moments are of the same size and aligned antiparallel are. In ferrimagnetism, atomic moments of different strength act magnetically against each other and thus lead to a total moment of the elementary areas, which, as in ferromagnetism, is different from zero. The atoms with oppositely directed spins occupy different ones Crystal lattice sites and form so-called sublattices. The resulting magnetic moment corresponds to then no longer the sum of all atomic moments, but the difference between the resulting ones magnetic moments of the individual sublattices.

In the case of the magnetic iron oxides that crystallize in the spinel lattice, the sublattices are formed by the tetrahedral and octahedral gaps of the cubic closest oxygen packing. The spin moments of all ions on tetrahedral sites couple parallel to one another and are in turn antiparallel to the spin moments of the ions on octahedral sites, which in turn are also aligned parallel to one another.

The resulting total moment of the domains results from the difference in the magnetic moment of all ions on octahedral sites minus the magnetic moment, all ions on tetrahedral sites. In order to there is ferrimagnetism in magnetic iron oxides. As a result, in the present application the magnetic iron oxide is no longer identified as ferromagnetic, but rather as ferrimagnetic.

The invention now relates to acicular ferrimagnetic iron oxides with a cobalt content of about 0.5-10 atomic percent and reduced remanence loss, characterized in that they additionally contain 0.05-3 percent by weight SiO ₂ .

The invention also relates to a process for the production of these ferrimagnetic iron oxides.

The ferrimagnetic iron oxides produced according to the present process have a somewhat lower remanence than the SiOj-free material, but the loss of remanence is usually less than 10% under thermal stress, while the remanence of SiO ₂ -free material with the same treatment according to the cobalt content in iron oxide and decreases by more than 23%. Magnetic pigments are thus obtained with a remanence which is sufficient for most areas of application and, above all, largely constant. The ferrimagnetic cobalt-containing iron oxides, which have been stabilized with SiOi, are suitable for magnetic recording and playback in and on all materials, such as B. tapes, plates, foils, papers in printing inks, encryption substances, etc.

Iron oxides containing cobalt are particularly suitable for recording because of their high coercive force of high-frequency acoustic and optical signals or of digital information.

It is known from the literature that magnetic tapes with cobalt-containing cube-shaped iron oxides

“Show a drop in the original signal when walking past a record and playback head on a tape recorder. For example, JR Morrison and DE Speiiotis, IEEE Transactions on Electronic Computers, Volume EC 15, No. 5, page 782 [1966) found an output signal drop of 55% after 6000 runs on a recording head at a speed of 76 cm / sec. This effect is known as mechanical instability. An effect of a similar magnitude is also obtained if the tape is heated ^{to temperatures around 150 ° C. with cobalt-containing iron oxide as the recording material.} Here, too, a decrease in the previously recorded signals by a percentage similar to that observed when the tape frequently passes the tape head is observed. This effect is known as the thermal instability of cobalt-containing iron oxides. Both phenomena are based on an instability of the remanent magnetization of iron oxides containing cobalt. It is also known that acicular cobalt-containing iron oxides, although to a lesser extent, have thermal and mechanical instability of the remanence.

In DT-OS 1907 236 a magnetic cobalt-containing acicular iron oxide is described, which on the aligned magnetic tape a remanence loss of at most 20% parallel to the alignment of the acicular particles and perpendicular to the alignment at least 1.5 times the remanence loss after heating the magnetic tape to 150 for 30 minutes ⁰ C. The best stabilization of the remanence of the cobalt-containing iron oxides is achieved with an FeO content of 5 to 17% by weight FeO.

The FeO content necessary to stabilize the remanence cannot be avoided in technology adjust greater technical effort. Even with the slightest fluctuations in the reduction conditions changes in the FeO content will occur, so that the production of a product with constant Loss of remanence is made very difficult. On the other hand, it is known that with decreasing Particle size of FeO-containing iron oxides the risk of oxidation during storage and processing increases in air. With certain particle sizes of the FeO-stabilized cobalt-containing iron oxides the oxidation can even proceed so quickly that they burn off.

The production of the needle-shaped, cobalt-containing iron oxide or iron oxide hydrate takes place in and of itself known methods.

According to DT-AS 12 26997, a mixture of an iron (2) and cobalt (2) salt solution at temperatures of 0-3O ⁰ C by adding a basic precipitant until a pH value of about 4.5-6 is reached, 5 staggered. The reaction mixture is then treated with oxidizing agents and the growth of the a- (Fe, Co) OOH particles is in the pH range of 4.5-6.5 at temperatures of 30-65 ° C with simultaneous addition of iron / cobalt - Saline solution carried out. The conversion of i, ₀ thus prepared cobalt-containing iron (3) oxide-hydroxide containing cobalt in V-Fe ₂ O ₃ is carried out by known processes.

In a strongly alkaline medium (pH value> 13), according to DT-AS 1204 644, an iron hydroxide containing cobalt is used or, according to a previously unpublished proposal, a cobalt-containing iron hydroxide or carbonate suspension oxidized in excess alkali hydroxide solution to the acicular σ-FeOOH.

According to the German Orfenlegungsschrift 20 22 013, acicular ff-FeOOH, Fe ₂ O ₃ or Fe ₃ O ₄ is mixed with a cobalt-containing liquid, the sludge is dried and the mass is converted into y-Fe ^ by known methods. Nitrate, acetate, hydroxide or formate are used as cobalt compounds.

The SiO _{2 is} preferably deposited on acicular ff-FeOOH containing cobalt with a length-to-width ratio of 5: 1 to 50: 1. This process can be carried out directly during or immediately after the production of the ff-FeOOH without any major additional work.

Surprisingly, it was found that acicular ferrimagnetic iron oxides containing cobalt have a remanence loss of less than 20% after the powder has been heated at 150 ^° C. for 30 minutes while the chemical composition remains the same and contains 0.05 to 3% by weight of SiO _2. Magnetic tapes which are produced with the iron oxide containing cobalt and silicon oxide according to the invention show an improved thermal stability of the remanence and consequently also an improved ability to store high-frequency information without excessive output signal drop at higher temperatures and / or mechanical stress, so that an improved reproduction of Signals becomes possible.

The novel, cobalt and silicon oxide-containing acicular ferromagnetic iron oxides are produced by depositing 0.05 to 3% by weight SiO ₂ , preferably 0.1 to 1%, on finished acicular cobalt-containing iron oxide or iron oxide hydroxide, or the SiO ₂ during the production process Pigment coincides.

The amount and concentration of the SiO ₂ -containing solutions depends on the size of the batch and on the desired SiO ₂ content of the finished pigment. In general, SiO ₂ solutions with a content of 0.5 to 10% by weight SiO ₂ are used.

The SiO ₂ is made from solutions of SiO ₂ (silica sols), alkali silicates, e.g. B. sodium or potassium water glass, or guanidine silicate deposited on the cobalt-containing acicular iron oxide hydroxide. The solution, which contains the SiO ₂ in dissolved form, can be added to the suspension either during the last 20% of the pigment formation time in the α-FeOOH preparation, during and after pigment formation or immediately after the oxidation to the «--FeOOH has ended so that SiO _{2 is} deposited on the cobalt-containing a-FeOOH needles. If the cobalt-containing a-FeOOH in the acidic medium by the method, for. If, for example, DT-AS 12 26 997 is produced at pH 4.0 to 6.5, a sodium silicate solution can be added during the last 10 to 20% pigment formation time or immediately after pigment formation has ended. In this pH range, SiO _{2 is} deposited directly on the needles. Another possibility of SiO ₂ deposition consists in adjusting the cobalt-containing a-FeOOH suspension to a pH value> 12, adding the water glass solution and then slowly lowering the pH value of the suspension to pH 7 with hydrolysis of the water glass solution .

Another way of depositing the SiO ₂ on cobalt-containing α-FcOOH at a pH value> 12, with a suspension of freshly prepared needle-shaped

cobalt-containing cr-FeOOH at a pH value> 12, treated with Table I of a water glass solution. A test is made that even under these conditions SiO _{2 is} deposited on the ff-FeOOH. For this reason, the cobalt-containing acicular a-FeOOH formed in a strongly alkaline medium (pH value 13), e.g. according to DT-AS 12 04 644, can be used during the last 20% of the oxidation time, during and / or after complete oxidation to 0-- (Fe ₁ Co) OOH, _{treat with a solution containing SiO 2} at pH> 13. Here, too, SiO ₂ is deposited.

According to another, as yet unpublished proposal, those containing cobalt oxide and silicon dioxide can be used After dehydration, pigments are initially tempered at temperatures of about 650 to 800 ° C before they are subjected to reduction and reoxidation. Depending on the applied Temperature is sufficient for a period of about 0.25 to about 2 hours for relaxation. Time and temperature are inversely related. In any case, the tempering must be carried out in such a way that that a coarsening of the pigment grain does not occur as far as possible.

The acicular cr-FeOOH containing cobalt and silicon oxide produced according to the invention is dehydrated to ff-Fe ₂ Oj and then converted in a known manner by reduction and possibly reoxidation into magnetic iron oxide containing cobalt and silicon oxide. A product is thus obtained which has a reduced loss of remanence after heating at 150 ° C. for 30 minutes.

However, in another variant of the process, the SiO ₂ can also be deposited _{on cobalt-containing a-Fe 2} O _3. The method can e.g. B. be carried out as follows:

Acicular cobalt-containing Cr-Fe ₂ O ₃ , produced by dehydrating cobalt-containing or-FeOOH according to the method described above, is suspended in water and the pH is adjusted to 12. The SiO ₂ -containing solution is then added and the pH is gradually lowered to 7 with acid so that the SiO _{2 is} deposited on the acicular cobalt-containing a-Fe ₂ O _3. There is no other possibility that one can rely on the dry cobalt-containing ff-Fe ₂ O _? before the conversion into the V-Fe ₂ O _3, the SiO ₂ -containing solution is sprayed onto the material.

The great advantage of the acicular ferromagnetic iron oxide containing cobalt and silicon oxide described in accordance with the invention is that it has been possible to produce a product with a constant composition in a simple manner that has a remanence loss of less than 20% on the powder after heating to 150 'for 30 minutes. C and in which there is no change in remanence due to change in chemical composition, even during storage and processing, / .. B. by oxidation in air.

In Table 1, the loss of remanence on the powder and on the magnetic tape and the calculated mean value are given for some cobalt-containing acicular iron oxides obtained according to the following examples by grinding for 3.5 hours in a pearl mill and then applying 15 g / m ² (> s of the lacquer with a layer thickness of 12 μ on a 23 -! - (.- FoIiC. The loss of remanence was produced after 30 minutes of heating to 150 "C determined.

sample Parallel to 1.3 Remanence Average powder No. Alignment 3.1 loss (%) 6.2 perpendicular 5.6 to the off 12.7 direction A. 19.7 B. (A + B) -2 1 26.0 9.0 5.2 3.1 2 9.3 6.2 5.35 3 16.1 11.2 8.85 4th 21.8 13.7 11.5 5 26.6 19.7 17.0 6th 35.5 27.6 19.0 7th 38.5 32.3 32.2

It can be seen from Table 1 that the mean value of the remanence loss from columns A and B and the remanence loss of the powder show a parallel course. For example, products with a high remanence loss on the powder also show unfavorable values on the magnetic tape, while low values on the powder indicate a lower remanence decrease of the same size on the tape.

It is thus possible, through a relatively simple measurement of the remanence drop in cobalt-containing iron oxides Powder received a statement about the decrease in remanence on the magnetic tape. This is why this method is used in the following examples, which are intended to further explain the present invention, used to assess the properties of cobalt-containing iron oxides.

example 1
a) Production of the cobalt-containing e-FeOOH

According to DT-AS12 04 644, a 25 l glass vessel is added to a receiver of 2.81 sodium hydroxide solution with 1218 g NaCH, with vigorous stirring and simultaneous introduction of 250 l air / hour at 50 ° C. 9.4 l of a solution containing 1910 gFeSO ₄ 7H ₂ O and 50.4 g CoCl ₂ · 6H ₂ O. The cobalt-containing iron (II) hydroxide is precipitated in this way. It is oxidized for 30 minutes while maintaining the introduction of air and stirring at 50 "C, then heated to 70" C and a- (Fe, Co) OOH formation ends with 500 l air / hour. If 75% ff- (Fe, Co) OOH is present in the suspension, 200 ml of a dilute waterglass solution containing 2.22 g of SiO ₂ , added dropwise and after complete oxidation the suspension was stirred for a further 2.5 hours at this temperature.SiO ₂ deposition takes place during and after complete oxidation of descr-FeOOH in a strongly alkaline medium at pH> 13. A total of 6 hours after the precipitation is filtered off, washed alkali-free and dried at 130C.
The ff- (Fc, Co) OOH has a needle width, determined radiographically at the (OII) -reflex, of 115 Å.

b) Conversion to cobalt and silicon oxide containing V-Fc ₂ O ₃

The conversion of the acicular iron oxide hydroxide in magnetic cobalt-containing needle-shaped

Total V-Fc ₂ O ₃ takes place as follows: dewatering table of C- (Pe ₁ Co) OOH, if necessary tempering at 680 C to 750 C, reduction at 400 C to cobalt-hally Fe ₃ O ₄ . The mixture is then allowed to cool in a stream of nitrogen and oxidized at 260 ° C. with air to give cobalt-containing J) -Fc ₂ Ov

The loss of energy of the cobalt-containing V-Fc ₂ O ₃ samples is determined after heating the powder at 150 ° C. for 30 minutes.

Example I, sample 1 Loss of credit (%) Vertical medium powder 2.6 at% Co for evaluation Parallel direction to the off B (A + B): 2 direction A.

Table 2

The ff- (Fe, Co) OOH comparison _{samples are produced as described in Example 1a, but without addition of SiO 2} , and converted into cobalt-containing samples by the usual method.

a) without SiO ₂ 19.7 35.5 27.6 19.0

b) with 0.36% SiO ₂ 12.7 26.6 19.7 17.0

It can be seen that the SiO ₂ -containing product in the strip direction (A) has a greatly reduced remanence loss of only 12.7% compared to 19.7 for the comparison product.

rehearse No. 2 b) 1 a) a) b) 67.2 lot 56.0 CoCl ₂ -OH ₂ O per ff-FeOOH approach 3.9 Atomic% Co im 3.2 0.37 V-Fe ₂ O ₃ without Wt .-% SiO ₂ im without 0.37 SiO ₂ 531 V-Fe ₂ O ₃ SiO ₂ 691 Coercive force l _Hc 538 511 423 (Oe) 568 Retentivity, BrAi 533 460 15.6% (GcmV ¹ ) 39% Loss of remanence 29% 13% after 30 minutes 357 150C 346 Remanence, BrAj 378 400 (GcmV ¹ ) Heat at 150 ° C

Example 3

^P.

a) In a 30-1 boilers are to 4.21 with sodium hydroxide 177Og NaOH while stirring and introducing nitrogen of 1001 / hour at 50 ⁰ C within 3 minutes 14.21 with 2860g solution FeSO ₄ .7H ₂ O and 74, 3g CoCl ₂ · 6H ₂ O given. After the precipitation has ended, 100 l air / hour are passed in for 30 minutes while maintaining the temperature and continuing stirring. The suspension is then heated to 70 ° C. and air is passed in until the oxidation is complete. After 80% has been oxidized to ff- (Fe, Co) OOH, 200 ml of a dilute waterglass solution with 1.65 g are added during the oxidation in 60 minutes SiO _{2 was} dropped into the alkaline suspension (pH> 13).

6 hours after the precipitation the «- (Fe, Co) OOH · formation is complete. It has a radiographically determined needle width of 155 Å.
The conversion into cobalt and silicon oxide-containing V-Fe ₂ O ₃ results in the powder after tempering the 0- (Fe ₁ Co) ₂ O ₃ at 750 ° C. and reduction at 400 ^° C. for 30 minutes

a coercive force, / _{W (} . = 540Oe,
a remanence, BrA / = 407 Gladly ³ g ~ ',
a decrease in remanence
from

40

45

It can be seen from Table 2 that in the samples containing cobalt and silicon the percentage loss of remanence after heating at 150 ° C. for 30 minutes is always less than the loss of remanence of the corresponding pure 50 b) cobalt-containing comparison samples. The final value of the remanence Br / », which is essential for magnetic recording, is considerably higher for the samples containing SiOj than for the products without deposited SiO ₂ after the thermal treatment.

55

Example 2

From the acicular cobalt and silica-containing V-Fe ₂ O ₃ with 2.6 atomic% Co produced in Example 1, magnetic tapes directed in a magnetic field were cast by a known method (GB-PS 10 80 614). After 3.5 hours of grinding in a Perl mill, a 12 μ thick layer at 15 g / m 2 is applied to a 23 ^ film. The blinders are heated for 30 minutes at 150 C and then the loss of remanence is determined parallel and perpendicular to the alignment.

9.1% to 370 gcm after 30 minutes
150 C.

The cobalt-containing V-Fc ₂ O ₃ contains 0.10% SiO ₂ .
The - (Fe, Co) OOH is produced as described in Example 3 a), but a water glass solution containing 3.3 g of SiO _{2 is} added dropwise.
The conversion to cobalt and silicon oxide-containing y-Fe ₂ O 3 results in the powder after 30 Minutei annealing at 700 ⁰ C and reducing at 400 ° (

a coercive force, // ,,. - 556Oe,
a remanence, Br / »= 417 Gcm ³ g
a loss of remanence of

-1

8.4% on 383 Gern ^J g after 30 minutes
150 ⁶ C.
The cobalt-containing V-Fe ₂ Oj contains 0.18% SiO ₂

Example 4

Under the conditions described in Example 3b, while adding 33 g of CoCl ₂ · 6H ₂ O in 5 hours, a cobalt-containing c-FeOOH with a radiographic

709 827/3

5

specific needle width of 250A. The water glass is added as described above. The conversion into cobalt and silicon oxide-containing V-Fe ₂ Oi after 30 minutes of tempering at 700 ° C. and reduction at 400 ° C. results

a coercive force, l, _lt . = 437Oc, a remanence, Br / .. = 445Gcnv'g ', a loss of remanence of 3.1% to 431 Gem V

after 30 minutes

at I5O "C.

The cobalt-containing ^ -Fe ₂ O ₁ contains 0.22% SiO ₂ according to the analysis.

A magnetic tape is produced from this material according to the information in Example 2 and the loss of integrity measured after heating the tape to 150 "C for 30 minutes.

Parallel to perpendicular to the mean powder alignment alignment (A + B): 2 AWAY

5.1

3.2

Example 5

3.1

To 8.21 with NaOH NaOII 1770g of a solution containing 1860g FeSO., 7 · H ₃ O, were added 74.3 CoCI ₂ · 6 HO and the subsequent oxidation, as described in Example 3 carried out under the conditions described in Example 3. 101 . After 90% of the suspension has been oxidized to - (Fe, Co) OOH, a solution containing 3.3 g of SiO _{2 is} added dropwise in 60 minutes. The oxidation is complete after a total of 6 hours. The conversion into cobalt and silicon oxide-containing 1'-Fe ₂ Oj with 3 atom% Co after 30 minutes' tempering at 700 ° C. and reduction at 400 ° C. results.

a coercive force, I.
a remanence, BrAi
a loss of remanence of

536Oc, 429Gcm'g ',

8.8% to 391 Genv'g ' after 30 minutes 150 'C ".

Example 6

From a template of 14.1 1 solution 2860 g FeSO., · 7HjO and 74,3g CoCI ₂ ■ 6H ₂ O, with 4.21 sodium hydroxide solution (177Og NaOH) at 50 C under stirring and simultaneous introduction of 100 I Nitrogen / hour the cobalt-containing iron (II) hydroxide is precipitated. 100 liters of air / hour are then passed in at 50 ° C. for 30 minutes with stirring, then 200 liters of air / hour until the end of the oxidation (7 hours). After 90% has been oxidized to the cobalt-containing ar-FeOOH, a dilute waterglass solution with 0.8 g SiO _{3 is added} to the suspension in 3 minutes so that SiO _{2 is} deposited in the strongly alkaline medium.

The α (Fe ₁ Co) OOH has an X-ray determined needle width of 150 A.

The conversion into cobalt and silicon oxide y-FejOj after 30 minutes of tempering at 700 C and Reduction at 400 C delivers

a coercive force, l _lh = 524Oe,
a remanence, BrAi = 446
a loss of income of

12.3% on 391 gem
after 30 minutes
150 C.

The corresponding comparison _{product without SiO 2} has a significantly higher loss of remanence of 24% from 479 to 364 degrees ⁽ g '.

Example 7
a) germination

According to DT-AS 12 26 997wcrdcn3040gFeSO _{4 and} 346g CoSO _{4 dissolved} in 201 water and stirred while stirring! 20-25'C with 915g of technical NaOII in 2.51 Wassci. A Schlitzflügclrühres at 1500 rpm are in the llydroxidsuspcnsion with Helps added 100 liters of air / hour, whereby the temperature in the Vcrlaul of 4Stundcn at 55 C "rises sink in this time. The pH to 5.0, and the color of the Suspcnsior changes from dark green to dark blue and green to orange-brown.The resulting iron oxide hydroxide suspension (germ) now contains 50.2 g / l FeSO. and 38.2 g / l FeOOH. The degree of precipitation was then 56.6%.

b) Pigmentation

201 germ suspension are heated to 60 to 70 C / and 4.6 of a 12.65% NaOII were added over the course of 5 hours while stirring. Simultaneously with clei NaOH addition, 201 air per hour are introduced. According to this process, the germs grow at pH 4;

vs up to 6.5 by oxidation of the II-valent metal ions to the III-valent stage and are then precipitated by subsequent hydrolysis. After the dci (Fe, Co) SO ₄ concentration has been broken down, a solution of Fc and Cosulfa is produced at the same time as the sodium hydroxide solution

• registered io. After adding 14.4MoI FeSO ₄ to 1.6MoI CoSO _{4 dissolved} in 15.41 water and 15.41 a; 8.3% sodium hydroxide solution with an air supply of 30 l / hour, the pigment formation is complete after 25 hours. The pigment is nitrided off, washed and in 4-pore

■ is divided into tioncn. The cobalt-containing a-FeOOII shows needle shape with a needle width of about 200 Å and a length: width ratio of 5: 1 to 40: 150 g of this <HFc, Co) OOH are resuspended before drying and as a 6% igo suspension

heated to 80 ° C. with stirring, the pH value was adjusted to 5.5 with dilute sulfuric acid and 6.5 g of sodium water glass corresponding to 1.75 g of SiO _{2 were} added over the course of 80 minutes. The pH increases to 5.8. After a further 10 minutes, stirrer

the pigment is suctioned off and ^{dried at 120 °} C. The a- (Fe, Co) ₂ O> at 680 ° C. for one hour, reduction at 400 ° C. and Rcoxidation at 260 ° C. results

a coercive force, 1 " _r " 570Oe, a remanence, Br /-.- -465Gcm ^J g ", a remanence loss of 10.5% aur417GcmV after 30 minutes at 150 ° C.

The acicular ferromagnetic iron oxide containing cobalt contains 6.5 atom% Co and 0.46% by weight / SiO.

Example 8

175 g cobalt-containing iron oxide hydroxide from Example 7 are dispersed, heated to 80.degree. C., the pH value is adjusted to 12.2 with NaOH, 3.5 g SiO ₂ added as water glass and then the pH value with H. ₂ SO ₄ lowered to pH = 7, filtered off and dried. The SiO ₂ deposition leads to a more difficult growth of the pigment during the conversion. Jf- (Fe ₁ Co) ₂ O ₃ , that after heating at 730 ° C for one hour before the reduction is obtained at 460 "C and reoxidation at 29O" C, shows a residual loss of less than 10%,

a coercive force, / _{W (} . = 433Oc,

a remanence, Br / .j = 332GcnvV, ' ⁵

a reminder

loss of 8% to 306 Gcm ³ g '

after 30 minutes

150'C.

20th

The coball-containing Jf-Fc ₂ O ₃ contains 2.1% by weight SiO ₂ .

Example 9

The a- (Fc, Co) OOII is coated according to Example 7, but the pH is kept at pH = 12.2 throughout the entire time. The ff- (Fe, Co) OOH obtained contains 0.19% SiO ₂ after drying. The conversion into cobalt and silicon oxide-containing J) -Fc ₂ O ₁ resulted after tempering for one hour at 730 "C, reduction at 430" C, reoxidation at 260 "C

a coercive force, I _IU = 456Oe,
a remanence, BrA. = 396Gcm'g ', one Br /. -Loss

of '10% aur356Gcm ¹ g'

of 30 minutes 150'C.

Example 10
a) nucleation

In a 30-1 reactor with a slotted blade stirrer, a 14.7% solution containing 21.375MoI FeSO ₄ and 1.125MoI CoSO _{4 is converted} into basic (Fe, Co) with 915g of technical NaOH in 2.51 of water at 35 ° C II sulfate is precipitated and oxidized with 100 liters of air / hour while stirring at 1500 rpm. The pH value falls to 4.2 over the course of 4 hours, while the temperature rises from 35 to 66 "C. The nucleation is complete when the initially blue-green suspension has turned brown-yellow; it then contains 58 g / l FeSO ₄ and 38.3 g / l FeOOH, corresponding to an initial degree of precipitation of 53%.

b) Pigment formation without SiO ₂

221 of the bacterial suspension according to Example 10a are oxidized at 6O ⁰ C to 70 ⁰ C by bubbling 201 air / hour and stirring at 1500 rpm, and the resulting trivalent ions by the addition of 126,5g NaOH in one liter of water in the course of 8 hours aushydrolysiert keeping the pH between 4.2 and 5.2. The resulting coball-containing iron oxide hydroxide has a needle width of about 0.023 μm and a length: width ratio of 5: 1 to 40: 1.

The pigment is washed and divided. _One part is converted directly into X- (Fc 1 Co) ₂ O ₁ , another part is redispersed and _{aftertreated with 0.5% by weight SiO 2} according to the examples.

Both samples are converted into y- (Fc, Co) ₂ O ₁ with and without tempering, reduction conditions 400 "C to 460" C, reoxidation at 290 "C.

The cobalt-containing K-Fc ₂ O ₁ contained 3.5 atom% Co.

The magnetic values are summarized in Table 4.

Table 4

Tcmpcrung
1 h ("C ¹ ) Coercive force
// ,, (Oe) Br /. 1 8th ') Br /.,
(Gcm'g '
30min at me
150 Ο Rcmunen / vcrlust without SiOj 630 495
513 487
483 397
416 18.5
14.0 with 0.55% SiO; 730 477
415 462
449 402
422 13.0
6.0

It can be seen that both with and without tempering the cobalt and silicon oxide-containing Fe ₂ Oi described according to the invention exhibits a lower loss of romance.

Example 11

To 100 g of cobalt-containing, SiO j-free, needle-shaped Of-Fe ₁ Oj, with 2.2 atom% Co, enough water is added that a 10% suspension is formed, after which the pH is adjusted to 12.2. A water glass solution containing 0.4 g of SiO _{2 is} added to this suspension and, as in Example 8, the SiO _{2 is} deposited on the a- (Fe, Co) _{7 O.}

The conversion into cobalt and silicon oxide shells Jf-Fc ₂ O ₃ resulted after 30 minutes of tomerification at 700 ° C. and reduction at 400 ° C.

a coercive force, / _{W (} . - S19Oe a remanence, BrA- »446GcmV a residual loss of 12.1% to 392Qcm ³ g '

after 30 minutes

150-C.

The corresponding SiO ₂ -freje Vorglelchsproduk hold a remanence loss of 24%.

Claims (6)

Patent claims: 1. Acicular ferrimagnetic iron oxides with a cobalt content of about 0.5 to 10 atomic percent and reduced remanence loss, characterized in that they additionally contain 0.05 to 3 percent by weight SiO ₂ . 2. Acicular ferrimagnetic iron oxides according to claim 1 with an additional SiO ₂ content of 0.1 to 1 percent by weight. 3. Process for the production of cobalt and silicon oxide. Acicular ferrimagnetic iron oxides with reduced remanence loss according to claim 1 or 2, characterized in that SiO _{2 is} deposited on acicular cobalt-containing iron oxide hydroxide during and / or after its production, the product obtained is dehydrated and then converted into ferrimagnetic iron oxide by reduction and possibly reoxidation. 4. A process for the production of acicular ferrimagnetic iron oxide containing cobalt and silicon oxide with reduced remanence loss according to claim 1 or 2, characterized in that the SiO ₂ is deposited on acicular cobalt-containing primordial Fe ₂ O ₃ , which is then converted into ferrimagnetic by reduction and optionally reoxidation Iron oxide is converted. 5. The method according to claim 3 or 4, characterized in that the cobalt and silicon oxide-containing iron oxide or oxide hydrate is tempered ^{at temperatures of 650 to 800 0 C after the dehydration before the reduction.} 6. Use of the acicular ferrimagnetic iron oxides according to claim 1 or 2 for Manufacture of magnetic recording media and magnetic printing inks. 40