magnetic pigments for recording information

Journal of Magnetism and Magnetic Materials 74 (1988) 193-202

North-Holland, Amsterdam 193

MAGNETIC PIGMENTS FOR RECORDING INFORMATION

Hartmut HIBST

BASF Aktiengesellschaft, Ammoniaklaboratorium, D-6700 Ludwigshafen, Fed. Rep. Germany

Received 11 March 1988; in revised form 20 April 1988

Dedicated to Professor Helmut DGfel on the occasion of his 60th birthday

The magnetic pigments used for longitudinal audio, video and data recording have undergone steady development from the

invention of particulate magnetic tape in the 1930s to the present day. The current types are acicular y-Fe,03, Co- y-Fe,O,,

CrO, and a-Fe magnetic pigments, and they differ from their predecessors, i.e. the comparatively coarse spherical carbonyl Fe

and cubic ferrite powders, in that they have only one magnetic axis, are very finely divided and have a narrow particle-size

distribution. Finely divided, magnetic, multiaxial solids for combined longitudinal/vertical recording attract interest for future

high-density mass storage systems. In addition, finely divided, plate-like shaped Ba ferrite pigments hold out prospects for

increasing the current values for the storage density by a substantial factor. This applied particularly to vertical recording.

1. Introduction

Magnetic storage media in the form of tapes, flexible disks and rigid disks are used for analog audio and video recording and for digital audio, video and data recording. Present-day information storage is based on the principle of magnetic longitudinal recording, in which case the magnetized domains are directed parallel to the surface of the substrate. The storage medium is led past a magnetic head that is usually toroidal and has a gap close to the recording medium. The head consists of a soft magnetic material, e.g. Mn-Zn or Ni-Zn ferrite or a crystalline or amorphous Fe or Co alloy, and is wound with a coil. In analog storage, the signal is transformed into a corre- spondingly varying head current. In digital recording, the binary information is stored by means of a steplike change in the head current. The stray field induced in the head gap magnetizes the storage medium in the same rhythm as that in which the information arrives. The magnetization in the medium is retained even after the stray field has gone. During inductive reading, the sequence de- scribed is reversed.

The field strength of the magnetic recording head imposes an upper limit on the coercivity H, of the magnetic layer. On the other hand, H, must be sufficiently large to prevent the magnetic do-

mains from being demagnetized by the self-demagnetization field. Since the strength of this field rises with increase in storage density, i.e. with decrease in the length of the magnetized domains, media with very great storage densities must have high coercivities in order to resist demagnetization. The remanent magnetization A4, must not be too high, because the demagnetizing field increases with the magnetization, but must be high enough to yield an adequately high inductive signal, particularly if the magnetic layer is very thin. The switching field distribution should be as small as possible in order to ensure that the transition zones between the opposing magnetizing domains are very narrow. One means of realizing this aim is to ensure a high relative remanence, i.e. a large ratio of remanent to saturation magnetization. Thus, a high coercivity together with a high relative remanence and a low switching field distribution leads to a high storage density and saturation output level at high frequences. In order to ensure

0304~8853/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

194 H. Hibst / Magnetic pigments for recording information

permanent magnetization with little dependence on temperature, magnetic storage media must not have any parts that are instable to heat. Another demand is minimum magnetostriction.

2. Longitudinal magnetic recording with wires and tapes

In 1897, the Telegraphon invented by Poulsen [l] recorded sound on a ferromagnetic steel wire. Thus, the principle of magnetic sound recording devised by Smith in 1888 was realized for the first time. The steel wire used had a C content of 0.9%. Its linear velocity was 2.13 m/s, and its coercivity was 40 Oe (= 3.2 kA/m), i.e. it was a comparatively soft magnetic material. Later, W steel with a similarly low coercivity was used. In 1940, Kelsall [2] developed the Mirrophon in the USA. This instrument recorded speech on a metal tape that was 1.27 mm wide and was composed of an Fe alloy with a Co content of 32-62% and a V content of 6-16%. By variation of the alloy com- position and by subsequent heat treatment, rustproof magnetic information storage media could be produced with coercivities of up to 80 kA/m, which is the value attained in the storage media of today.

3. Longitudinal magnetic recording with magnetic pigments

Shortly before the Second World War, a new type of magnetic tape was developed in Germany, and it was subsequently adopted throughout the whole world. The work performed by Pfleumer led to the joint development of the Magnetophon sound recording system by AEG and BASF in 1934 [3]. The magnetic tapes in this instrument consisted of a plastic film coated with a composi- tion in which ferromagnetic or ferrimagnetic pigment particles were embedded.

3.1. Spherical carbonyt Fe powder

The magnetic powder first used in 1934 was carbonyl Fe (fig. 1). This Fe metal powder consists of spherical particles and is produced by decomposing gaseous Fe(CO), at 250-300 o C. The only impurities in the Fe are traces of C and 0, which originate from the Fe-catalyzed decomposition of CO and the associated secondary reactions. On the addition of small amounts of NH, during the thermal decomposition reaction, the C and 0 contents can be significantly reduced, but Fe nitride is thus formed. C and N are inter- calated within the particles formed from the gas phase, with the result that the spheres of carbonyl Fe powder consist of concentric layers [4]. The embedded carbide is responsible for mechanically

Fig. 1. Left: Heap of spherical particles of carbonyl Fe powder (1 cm 2 3 pm); right: microtome section of a spherical particle of carbonyl Fe powder (1 cm L 0.5 pm) [4].

H. Hibsr / Magnetic pigments for recording information 195

hard, brittle particles. The following properties and thus the coercivity can be judiciously altered by varying the conditions under which thermal decomposition takes place: the particle size from 1 to 10 pm, the particle size distribution, the C content from 0.8 to 1.5%, the 0 content from 0.3

to 0.9% and the N content from 0.7 to 1.6%. The carbonyl Fe powder in the first magnetic

recording tapes consisted of spheres of 3-5 pm diameter. Since it was not sufficiently finely divided, it was responsible for an undesirably high background noise level.

3.2. Cubic (Co-doped) Fe,O, and y-Fe,O,

A few years later, a much more finely divided magnetic powder was used, viz. cubic magnetite (Fe,O,) with a particle diameter of about 0.5 pm (fig. 2). However, it has a comparatively low relative remanence, a high switching field distribution and magnetostriction, and its magnetic properties depend greatly on temperature. Consequently, the recording tapes produced from it had an exces- sively high “print-through” and poor erasure. For this reason, a change-over was made shortly afterwards to cubic y-Fe,O,.

Cubic ferrite powder usually is produced by what is known as the oxidation process [5,6]. In the first stage, an aqueous dispersion of M(I1) hydroxide (M = Co, Zn, . . .) and Fe(I1) hydroxide is precipitated (reaction I). In a second stage, the

Fig. 2. Cubic Fe,O, obtained by the oxidation process [ll] (1 cm ^ 0.2 pm).

dispersion is oxidized to (doped) cubic Fe,O, by passing a gas containing oxygen through it at a defined pH value and at temperatures higher than 50 o C (reaction II):

xM2+ + (3 - x)Fe2+ + 60H-

+ 3(M,Fe,-,)1,3(OH)Z(s), (I)

3(M,Fe,-,),,,(OH),(s) + :0,(g)

+ M,Fe,_,O,(s) + 3H@. (II)

The (doped) magnetite particles obtained in the synthesis are in the form of small cubic single crystals. After filtration and drying, they are oxidized topotactically at 300” C to metastable y-Fe,O, powder with a gas containing oxygen, e.g. air (reaction III). The cubic crystal habit is re-

tained:

M,Fe,-P,(s) + $(l -x)&(g)

+ y-M, Fe 3-XQ.5-0.5X(s). (III)

Fe,O, has an inverse cubic spine1 structure which is fundamentally retained in the y-Fe,O,. Only 21: of the entire 24 cation locations in the elementary cell of the spine1 lattice are occupied by Fe(II1) ions, with the result that there are voids in the lattice. On account of their cubic symmetry and the resultant cubic crystalline form, ferrite powders

with spine1 structure display multiaxial magnetization.

The particle size and magnetic properties of cubic ferrites produced by the oxidation process depend on the reaction parameters, i.e. the pH, temperature and cation concentration [6]. The minimum particle diameter obtained is 0.15 pm, and the minimum particle volume is thus 6 X 1O-4 rJ,m3. However, in order to achieve the low background noise level specified, the maximum particle volume of magnetic pigments obtained from y-Fe,O, ought to be only 2 x lop4 pm3. If the particles are cubic, this corresponds to a diameter of 0.1 pm.

In addition, cubic y-Fe,O, with a H, value of only 10 kA/m has a too low coercivity for use in sophisticated magnetic storage media. The magnetocrystalline anisotropy and thus the coercivity could be increased by volume substitution of


Co(I1) ions for y-Fe,O, to give y-Co,Fe,_,

04.5-0.5x* However, this also exerts an adverse

effect, i.e. greater dependence of the magnetic properties on the temperature.

3.3. Acicular y-Fe,O,

After 1949, a significant improvement was achieved in magnetic tape recording, when a change-over was made to acicular y-Fe,O,. Acicu- lar y-Fe,O, with needles of about 0.5 urn length and a length/thickness ratio of about 10 also proved to be eminently suitable for the profes- sional video recording system developed by Ampex in the 1950s and for data recording on rigid disks, which was introduced by IBM in 1955.

4). If an external magnetic field is applied, the acicular pigment particles can be aligned in the pigment/resin dispersion while it can still flow so

that their axes are parallel to that of the tape. As a consequence, there is a further increase in the coercivity, a considerable rise in the relative remanence and a reduction of the “switching field distribution” (SFD) in the direction in which the tape runs. (Coercivity is an average switching field of many particles. SFD is a measure of the vari- ance of the switching field.)

Owing to its low magnetocrystalline anisotropic energy, y-Fe,O, with cubic crystal habitus has low H, values, viz. only about 10 kA/m. On the other hand, y-Fe,O, with acicular particles of one-domain size, has what is known as shape anisotropy owing to demagnetization effects. Together with the crystal anisotropy, this gives rise to coercivities of up to 30 kA/m. Another consequence of pro- nounced shape anisotropy is slight uniaxial magnetization in a direction parallel to the axis of the needle, as compared to the multiaxial magnetization observed in cubic y-Fe,O,. An advantage of the uniaxial magnetization becomes evident during the production of magnetic tapes (cf. section

Owing to the cubic crystal symmetry, no single-stage synthesis is known for acicular Fe,O, or y-Fe,O,, such as that developed for the cubic ferrites. Thus, the starting material for the produc-

tion of acicular y-Fe,O, is Fe oxyhydroxid (FeOOH), which is readily accessible chemically and is also acicular. When it is converted chem-

ically into magnetic y-Fe,O,, it retains its crystal habit (fig. 3). The LY- or y-FeOOH crystals are grown in aqueous solution by judicious oxidation with air of precipitated Fe(I1) hydroxide at a defined pH and at temperatures less than 50 a C. The reaction conditions must be controlled to ensure that acicular Fe oxyhydroxide is obtained with optimum particle size, the desired length/thickness ratio and an extremely narrow particle size distribution. The acicular, nonmagnetic and orthorhombic Fe oxyhydroxide obtained is dehydrated by heat treatment to yield a-Fe,O,,

Fig. 3. Left: y-FeOOH (1 cm P 1 pm); right: acicular y-Fe,O, produced from y-FeOOH (1 cm ^ 1 am).

H. Hibst / Magnetic pigments for recording information 197

32-

20-

Fig. 4. Rise in coercivity H, with increase in crystallite diame-

ter d in acicular y-Fe,O, (derived from y-FeOOH).

which is also acicular and nonmagnetic but trigo- nal. When the cY-Fe,O, is subsequently reduced to Fe,O,, it retains its needle shape. In the next stage, the Fe,O, is topotactically oxidized at 300 o C to give acicular y-Fe,O,.

Unlike the cubic y-Fe,O, previously discussed, the y-Fe,O, aciculae obtained are not single- crystals, but consist of small crystallites. As the size of the crystallites in the 20-50 nm range

increases, the coercivity of the y-Fe,O, obtained from y-FeOOH rises from about 20 to 30 kA/m (fig. 4). The conditions under which the three reactions take place must be set so that - despite the considerable differences in density between the individual intermediate products - the aciculae suffer as little damage as possible and, at the same time, crystallites of the desired size and thus with the necessary magnetic properties are obtained.

A new, very promising synthesis consists of obtaining y-Fe,O, direct from acicular a-Fe,O, produced hydrothermally from Fe(II1) salts and chelating agents. By this means, the dehydration stage in the method previously discussed is eliminated. Since it is precisely this stage that has an adverse effect on the shape of the needles, the new synthesis offers great advantages. The elon- gate y-Fe,03 particles produced from hydrothermal a-Fe,O, features an extremely narrow particle size distribution and very low porosity.

3.4. Acicular CrO,

In the 1960s the classical magnetic recording tape was ousted by compact cassettes with less recording width and lower tape speeds, and in the

1970s compact video recorders were developed for the home. These trends initiated a worldwide search for new types of magnetic pigments with improved recording properties and higher storage densities. In 1966, the acicular, ferromagnetic CrO, pigment appeared on the market. By virtue of its greater coercivity, viz. 40-60 kA/m, it had superior recording properties, particularly at shorter wavelengths. The very smooth surface and the virtual absence of pores in the CrO, aciculae were responsible for greater shape anisotropy than that in acicular y-Fe,O, and thus for greater coercivi-

ties. CrO, is thermodynamically metastable. At

about 350” C, it gives off oxygen to yield Cr,O,. CrO, can be produced by various routes at atmo-

spheric pressure:

CrO,Cl z + CrO, + Cl,, (IV)

4Cr(OH)s + 0, - 4Cr0, + 6H,O, (V

4CrOOH + 0, -+ 4Cr0, + 2H,O, (VI)

H ,CrO, + CrO, + HZ0 + :O,. (VII)

However, these methods are not feasible for the production of magnetic pigments, either because they do not proceed quantitatively, or they yield CrO, products of unsuitable particle shape or with a wide particle size distribution. As opposed to this, CrO, pigments that conform to the re- quirements on particle shape and size can be

produced by various hydrothermal syntheses:

CrO, + CrO, + +O,, (VIII)

Cr,O, + HClO, + 2Cr0, + HClO, , (IX)

Cr,O, + CrO, + 3CrO,, VW

Cr,O, + 3Cr0, --, 5Cr0, + 0,. ow

The technical single-stage hydrothermal synthesis resorted to today is based on the simple synpro- portioning reaction expressed by reaction Xb. It yields perfectly crystallized, single-crystal and acicular chromium dioxide with a narrow particle


Fig. 5. Hydrothermally produced acicular CrO,

(1 cm B 0.5 pm).

size distribution (fig. 5). This product can be very readily dispersed and magnetically aligned in organic resins. The size and the length/thickness ratio of the CrO, needles can be engineered by additional dopants, e.g. Sb and Te. Doping with Fe increases the magnetocrystalline anisotropy and thus the coercivity.

When the hydrothermal reactor is heated up, CrO, first appears at 260 o C. At 300 o C, the intermediate product Cr,O, is formed, and single-phase CrO, is finally obtained at 325’ C. The presence of the Sb,O, dopant lowers these temperatures by about 50 o C, but CrSbO, also occurs at 140 o C. In common with CrO,, it has rutile structure and it functions as a nucleating agent in the CrO, synthesis. Since CrO, is a strong oxidizing agent, it must be stabilized by superficial reduction before it is processed to magnetic storage media. In this case, the CrO, needles are covered with a dense protective layer of nonmagnetic Cr(II1) oxides of about only 1 nm thickness [7].

CrO, has a low Curie temperature, viz. 120-155” C, the actual value depending on the doping. This fact, together with the great stability of magnetization up to the Curie temperature, permits thermoremanent copying, which is feasible only with CrO, pigment. The process can be compared with printing. The parent tape repre- sents the printing block; and the subsidiary tape,

the paper to be printed. A mirror image of the information on the parent tape is thus transferred

to the subsidiary. The parent tape is now brought into intimate contact with the subsidiary, which is heated barely above the Curie temperature. On cooling, the magnetic domains in the subsidiary tape will be aligned in the opposite direction as those on the parent tape.

3.5. Acicular y-Fe,O, superficially doped with Co* +

(Co-y-Fe,O,)

It did not take long before GO, had to face competition from Co-doped y-Fe,O, magnetic pigments. It was known that the coercivity of y-Fe,O, could be considerably increased by volume-doping with Co(I1) ions. However, Co- doped acicular y-Fe,O, has very temperature-depending magnetic properties and high magnetostriction. The breakthrough came when a change- over was made from doping the entire volume to doping the surface only. For this purpose, acicular y-Fe,O, was coated with hydroxides of Co and Fe. Subsequent heat treatment yields Co-y-Fe,O, pigment particles with a y-Fe,O, core and a Co-

doped y-Fe,O, sheath. The advantage of this com- posite structure is that the y-Fe,O, core ensures that the magnetic properties do not depend greatly on temperature and the ferrite sheath containing Co yields the desired high coercivity.

3.6. Acicular a-Fe

Ever since carbonyl Fe powder was introduced in 1935, magnetic metal pigments attracted great interest because of the high magnetization of metallic Fe, Co and their alloys. Interest was heightened by the fact that the high shape anisotropy in acicular Fe pigments can give rise to very high coercivity, viz. more than 120 kA/m. For a long time, however, difficulties were caused by the poor shape retention of the pigment aciculae and the sintering that occurred during the multi- state conversion of acicular FeOOH into the cor- responding a-Fe. This can be readily understood, because the needles must suffer several complete rearrangements of the crystal lattice and very great changes in density. The problem defied solution

H. Hibst / Magnetic pigments for recording informaiion 199

until the development of shape-stabilizing finishes, e.g. phosphates, with which the FeOOH needles can be coated before they undergo the multi-stage conversion. The coating allows the acicular shape to be largely retained and prevents more severe

sintering. Since the finely divided Fe pigment reduced with H, is pyrophoric, it must be passi- vated by controlled superficial oxidation at temperatures below 60” C before it is further processed. Under these circumstances, the Fe aciculae are coated with a Fe,O, or y-Fe,O, sheath of about 5 mn thickness [7]. By this means, a stable Fe pigment is obtained that has far superior coercivity and magnetization to those of any other common pigment. The saturation magnetization

can be further increased by alloying with Co.

4. Production of pigmented magnetic recording media

All the magnetic recording media on the market today consist of a substrate coated with a magnetic storage layer. Polyester film of a 6-80 pm gauge is the substrate for magnetic tapes and flexy disks, and A1,,Mg4 of l-2 mm thickness is the substrate material for rigid magnetic disks. The magnetic coating on all the recording media encountered on the market is 0.5-15 pm thick. The very finely divided acicular magnetic pigments, readily dispersible in organic binders, are embedded in the magnetic coating as homogeneously as possible. The length of the aciculae is about 0.5 urn, the length/thickness ratio is about 10, and the pigment volume concentration is

20-50%. The magnetic pigments used are y-Fe,O,, Co-doped y-Fe,O,, CrO, and &-Fe (fig. 6). Mag- netic particles of one-domain size and narrow particle size distribution lead to a narrow switching field distribution of the media.

In the modem production process for magnetic tapes, the pigment is dispersed in various organic solvents, binders and dispersion aids into a homogeneous dispersion, which is applied by curtain coating in a magnetic field onto a polyester film passing through a winder. The horizontal magnetic field aligns the acicular magnetic pigments and thus increases the relative remanence and the

.Fe.

‘/--Fe@,

.I;- :: 00,

b Co-y-Fe,O, BaFe,,O,s

01 I I I

0 100 200 300

H, (Wm) ---+

Fig. 6. Coercivity H, and specific remanent magnetization M,/p of acicular or platelet shaped magnetic pigments (Fe

and CrO, unstabilized).

coercivity in the direction in which the tape is running. The solvent in the coating is evaporated and recovered. Afterwards, the magnetic coating is calendered between heated rolls and thus com- pacted. Magnetic disks are produced by spincoat- ing the magnetic dispersion onto rapidly rotating

Al substrates. The disks are subsequently dried in a magnetic field that orients the magnetic aciculae in the tangential direction. Afterwards the disks

are polished.

5. Prospects of increasing storage density by new magnetic recording techniques

Consumers are currently expressing the wish for smaller video cassettes without loss of playing time. These cassettes would be a great advantage

in portable video camera-recorders. Another trend that can be expected from 1990 onwards will follow from the introduction of digital magnetic video recording. Hence future magnetic tapes will require a higher storage density than that of the recording media in current use. Since magnetic disk drives were introduced in 1955, the storage capacity per unit area of the disks has increased by a factor of 104. In future, magnetic disks of even higher storage capacity will be required.

A significant increase in the storage density of longitudinal magnetic media is feasible, if the

2ocl H. Hibsi / Magnetic pigments for recording information

current minimum for the thickness of the magnetic coating, viz. about 0.5-1.0 pm, is reduced still further, because the strength of the demagnetizing field becomes less with decrease in the thickness of the coating. Furthermore, thinner coats lead to an increase in the gradient of the recording head field and thus to a reduction in the transition zones between the opposing magnetiza- tions. However, if the bit density is extremely high, thin coats can give rise to extremely weak read output signals and thus to an adverse signal- to-noise ratio. Yet another point to consider is that further decreases in coating thickness will make it increasingly more difficult technically to produce flawless homogeneous layers of dispersion with a uniform thickness. Consequently, the mass storage systems of the future will require new magnetic recording techniques with new types of pigments that permit very high storage densities even in thicknesses of 0.5-1.0 pm or more.

5.1. Attempts to increase the storage density by combined longitudinal and vertical recording with

isotropically distributed multi-axial magnetic pig-

ments

In 1975, Lembke [8] introduced a new technique, combined longitudinal/vertical recording, which holds out the prospects of much higher storage densities than that obtainable by conven- tional longitudinal recording. This technique re-

quires comparatively thick, i.e. 5 pm, magnetic coatings with isotropically distributed, i.e. un- aligned, multi-axial magnetic particles with as high a relative remanence as possible. For this applica- tion, cubic ferrite particles have again attracted current interest, but they have the disadvantage of being too coarse if they are produced by the oxidation process (cf. section 3.2) and to fine if they are obtained by what is known as the neutralization process.

In the neutralization process [9,10] aqueous dis- persions of M(II), Fe(H) and Fe(II1) hydroxides with a molar concentration ratio of

A = [Fe3’]/[M2’ + Fe2+] = 2

(reaction XI) are heated to temperatures above 50°C with exclusion of air (reaction XII):

xM2+ + (1 - x)Fe2+ + 2Fe3+ + 80H-

--, 3(M,Fe3-,),,,(OH)s,3(s),

3(M,Fe,-,),,,(OH)s,3(s)

(XI)

+ M,Fe,_,O,(s) + 4H,O. (XII)

The ferrite powder M,Fe,_,O, obtained can again be oxidized topotactically by means of a gas con-

taining oxygen to metastable doped y-Fe,O, so that it retains its shape (reaction III). However, the cubic ferrite powder thus formed is extremely finely divided, i.e. the particle diameter is 6-20 nm. The particle volume V < 2 x lop6 pm3 is decidedly lower than the critical magnetically stable single-domain volume of 2 x lo-’ pm3 for y-Fe,O,. Owing to the resulting superpara- magnetic properties with very low values for the coercivity and relative remanence, the extremely finely divided cubic ferrite pigments yielded by the neutralization process are of no significance for magnetic storage, but they are being used on an increasing scale for magnetic fluids and inks.

Various recently developed production processes [7] have permitted cubic ferrite pigments of the requisite grain size to be obtained in the last few years. The combined neutralization/oxidation process [ll] again starts from an aqueous dispersion of M(II), Fe(I1) and Fe(II1) hydroxide (reaction XIII). But, in contrast with the neutralization process, there is an excess of Fe(I1) and the molar concentration ratio is

A = [Fe3+]/[M2+ + Fe*+] < 2.

(The conditions for the reactions in the oxidation process are y = 2 and A = 0; in the neutralization process, however, y = 0 and A = 2.) The precipitated mixture is heated with exclusion of air to a temperature higher than 50 o C. As the value for A increases, a growing number of ferrite nuclei are formed (reaction XIV). Afterwards air is passed through the dispersion in order to oxidize it, and the M,Fe, _,O, nuclei then grow homogeneously

H. Hibst / Magnetic pigments for recording information 201

XV):

xM2++(1 -x+y)Fe2++(2-y)Fe3+

+ (8 -y)OH-

-, 3(M,Fe,-,)1,3(OH)(s-Y~,3(s),

+ (1 -y/2)M,Fe,_,O,(s) + (4 - 2y)H,O

+ (3.U2)(M,Fe3-&(OH)2(s), (XIV)

(3y/2)(M,Fe3-&(OH)2(s) + (y/4)0,(g)

+ (y/2)M,Fe3-.0,(s) + (3y/2)H20. (XV)

Since the pigment becomes more finely divided with increase in the value for A, the particle size can be set to any defined value within a wide range. Thus, the combined neutralization/oxidation process gives rise to cubic ferrite pigments whose particle size lies between that obtained in the oxidation process, i.e. a diameter of 0.15 pm or more, and that obtained in the neutralization process, i.e. a value of 20 nm or less. Hence the new process allows the production of finely divided y-Co,Fe, _xO4,5 _,,5x with the high multiaxial anisotropy and coercivity desired for isotropic recording. The increased dependence on temperature may be reduced by doping with foreign ele- ments, and there undesirably high magnetostriction may perhaps be reduced by the same means. Good prospects are also held out by a change-over from volume doping to partial surface doping.

5.2. Efforts

Fig. 7. Platelet shaped BaFe,,O,, [13] produced in fused NaCl

(1 cm P 0.5 pm).

magnetization is along the longitudinal axes of the needles. Another possibility is offered by platelet- shaped pigments if their preferred magnetic direction, i.e. the direction of easy magnetization, is at right angles to the plane of the platelets. Pigments of this type are found in the class of hexagonal ferrites [13]. They are of great interest for high-

density, vertical magnetic recording, if the platelets can be aligned parallel to the plane of the storage medium. Hydrothermally produced barium ferrite pigments generally feature an extremely lamellar form, i.e. a high diameter-to-thickness ratio and can be magnetically aligned correspond- ingly well in organic binders. A disadvantage is their tendency to stacking, which results in media

with a rough surface. As opposed to this, barium ferrite platelets crystallized from various fluxes generally display a more compact crystal habit and can be more densely packed in organic binders (fig. 7).

6. Future increase in storage density by binder-free, coherent magnetic coatings

It is still uncertain whether an adequate increase in storage density of pigmented magnetic coatings can be realized by the new recording


techniques and types of pigments dealt with in sections 5.1 and 5.2. A very promising alternative means for increasing the storage density is offered by binder-free, coherent magnetic coatings that can be applied by chemical or electrochemical deposition, vacuum evaporation or sputtering. Very thin and smooth, flawless coatings with a homogeneous microstructure can thus be produced [14].

Acknowledgements

My thanks are due to Dr. Jakusch and Dr. Veitch for their critical perusal of the manuscript.

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magnetic pigments for recording information

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