Particulate magnetic recording media: a review

4314 IEEE TRANSACTIONS ON MAGNETICS. VOL 25, NO. 6. NOVEMBER 1989

Particulate Magnetic Recording Media: A Review MICHAEL P. SHARROCK

Abstract-Most recording media (tapes and disks) are made by the well-established technique of coating a support material with a dispersion of discrete magnetic particles in organic binders. The particles that are dispersed in such coatings can be chosen from a variety of compositions, to fit the needs of diverse applications. After describing briefly the principles of magnetic recording and the manufacture of media, this review focuses on the history, characteristics, and development trends of the most important particulate magnetic recording materials. These are acicular iron oxides, chromium dioxide, cobalt- modified iron oxides, acicular metal particles, and barium ferrite. All continue to be subjects of active research and development, with a common emphasis on very small particle size.

INTRODUCTION AGNETIC recording is an extremely important M technology. It is principally responsible for the

widespread, relatively inexpensive use of recorded sound and video images and is, along with semiconductor technology, one of the two foundations of the expansion in computer power. Magnetic recording products have, in fact, an economic importance that is comparable to that of semiconductor devices.

The success of magnetic recording, despite the avail- ability of other means of storing images, sounds, and numerical data (photography, phonograph records, optical recording media, punched tape), is due to many advantages. Among them are

for use in recording media. More thorough treatments are available [ 11-[5]. The information to be recorded, whether representing sound, image, or numerical data, is expressed as a time-varying electrical current. The schemes for doing such encoding are beyond the scope of this review and are discussed elsewhere [ 11-[5]. The recording head consists of a small coil, through which the current passes, and a gapped magnetic structure that intensifies and localizes the resulting field. The recording medium consists of a magnetic material, coated on the surface of a tape or disk that moves relative to the head and in close proximity to its gap. The head field magnetizes the coating according to the current in the coil, so that the time- varying electrical signal is converted into a spatially varying magnetic pattern along a track on the medium’s surface. Reversals of the current produce transitions between regions of opposite magnetization. The read-back process involves moving the surface once again past the head, or past a different head of analogous structure. When this happens, the field arising from the magnetized regions of the surface magnetizes the gapped structure, causing a magnetic flux through the coil. The relative motion between head and medium gives rise to an induced voltage in the coil in accordance with changes in the magnetization of the medium. The spatially varying pattern is thus converted back into a time-varying electrical signal.

1) simple, inexpensive recording and reading trans- ducers (heads)-the same head can serve both func- REQUIREMENTS OF MAGNETIC RECORDING MEDIA tions;

2) stable storage, combined with 3) easy erasure and rewriting; 4) high surface information density; 5 ) thin flexible media that can be rolled up to give ex-

6) inexpensive media-high-quality video tape can be

In addition to these desirable features, magnetic recording technology also demonstrates a continuing ability to grow and to advance, evidence that it is based on highly useful and versatile principles.

tremely high volume information density;

made for substantially less than $1 /m2.

PHYSICAL PRINCIPLES OF MAGNETIC RECORDING The fundamental physical principles of magnetic re-

cording will be briefly described here, in order to place in context the requirements imposed on magnetic materials

Manuscript received July 5 , 1989. The author is with the 3M Company, 236-3C-02 3M Center, St. Paul,

IEEE Log Number 8931095. M N 55144-1000.

A consideration of the above brief description illustrates some of the primary requirements placed on a magnetic material for use in recording media. First, the retained magnetization intensity of the tape or disk coating must be sufficiently high, since it determines the strength of the field sensed by the readback head. The retained intensity, or retentivity, depends on the intrinsic magnetization of the material in the coating and also on the preferred directions of magnetization in this material. A second major requirement relates to the field strength needed to cause magnetic reversal of the material in the coating. This strength is generally characterized by the coercivity ( H c ) , the median value of the fields where reversal occurs; see Fig. 1. The coercivity must not be so large as to prevent successful writing, and possibly overwriting or erasure, by available heads. It must, on the other hand, be large enough for the material to resist unwanted changes or degradation of the signal during the required storage time. A major potential cause of such changes is the internal, or self-demagnetizing, field due to the material itself; this field is proportional to the magnetization

0018-9464/89/1100-4374$01 .OO @ 1989 IEEE

SHARROCK: PARTICULATE MAGNETIC RECORDING MEDIA 4315

SFD = bK tM H. ,-, >-r

2 - y Squareness = %

M,

Fig. 1 . Hysteresis loop (plot of magnetization M versus applied field H ) for a typical recording medium. The parameter A H is the full width at half height of the differentiated hysteresis loop. H , designates the coercivity.

intensity of the coating, and therefore the required coercivity is greater for more strongly magnetizable coatings. The internal demagnetization field is important principally at high densities of magnetic transitions, where opposite magnetic poles in the recorded pattern are in close proximity to each other [ 11-[5]. Coercivity requirements therefore tend to increase with increasing recording density.

The coercivity (the median switching field) of a magnetic material does not completely characterize its properties for recording purposes. The breadth of the distribution of fields, centered on the coercivity, at which reversal occurs is also crucial. This is expressed by a variety of parameters, of which perhaps the most common is simply called the switching-field distribution (SFD) and is explained in Fig. 1. A narrow switching-field distribution clearly facilitates the writing of sharp, well-defined magnetic transitions and therefore contributes to the ability to record information at high densities. A broad SFD not only diffuses the transitions but can also lead to a variety of other problems. A distribution that extends to excessively large switching fields can lead to problems in erasure or in overwriting old information with new. This is of concern both in systems using special erase heads but needing very thorough erasure (e.g., analog audio) and in systems where some residual overwritten signal can be tolerated but no erase head is used (most digital systems). A distribution that extends too far to the low end, on the other hand, can lead to instability and is commonly blamed for the occurrence of “print-through,’’ the acquisition of a weak signal in one layer of tape due to the field arising from the recorded magnetization in an adjacent layer on the storage reel [6]. Print-through is a major concern in analog audio but is relatively unimportant in digital systems.

The magnetic properties of a recording medium must not only meet the requirements outlined above but also be stable under the conditions that will exist during its use and storage. Temperature, relative humidity, and atmo- spheric pollutants are significant threats in this regard. Al- though irreversible changes of properties as a result of, for example, exposure to elevated temperatures are ob-

viously unacceptable, reversible changes can be trouble- some too. The dependence of coercivity on temperature is especially important. The strength of this dependence varies greatly among the most important recording materials, as will be discussed below. A strong dependence can endanger signal stability, and also creates problems with regard to adjustment of the writing and erasing head fields, which should be approximately proportional to the coercivity. The latter difficulty is especially pronounced in equipment that must be used “in the field,” such as video cameras. Digital equipment such as disk drives can present temperature-related problems because of the internal heat generated by their motors and other components; the operating temperature can vary with how long the unit has been turned on. The overwriting capability can be critically affected in situations where data originally recorded at a temperature where the coercivity was relatively low (so that the pattern was recorded deeply in the coating) must be overwritten at a temperature where the coercivity is relatively high [7].

In addition to environmental factors such as temperature, mechanical stress is also a threat to the stability of a recording medium. Contact between head and medium and, in the case of tapes, passage through the transport mechanism cause a variety of stresses in the magnetic coating. These stresses can be locally quite large. Stress sensitivity, due to magnetostrictive effects, results in signal degradation after repeated reading operations and must therefore be at sufficiently low levels [8].

The characteristics discussed thus far relate to the bulk, or macroscopic, magnetic properties of a desirable medium; the microstructure is equally important. Recording media must contain small discrete magnetic units that are at least partially independent of each other, so that the transitions between different directions of magnetization in the recorded pattern are stable and not free to move around. The magnetic interactions between the units are not, however, unimportant, and models for understanding these interactions in recording media have been reviewed elsewhere [9]. Each magnetized segment in the written record must contain a large number of these units, in order that the signal-to-noise ratios be adequate. The broad-band power signal-to-noise ratio can in fact be shown to be nu- merically on the order of the number of magnetic units in each magnetized segment [ 101. It is clearly advantageous, then, for these units to be as small as practical. The unit size also determines the precision with which transitions between oppositely magnetized segments can be located, and thus can potentially limit the transition density [ 1 11. If the magnetic units correspond to physical grains or particles, a further reason exists for making them as small as possible. This is the need for a smooth tape or disk surface. The effectiveness of both the recording and reading processes depends critically on minimizing the separation of the head(s) from the medium, which in turn depends at least partly on the smoothness of both. The overall loss of output, assuming equal separation for read and write processes, can be estimated as a total of nearly 100 dB

4316 I

per wavelength of separation [12], [13]. (A recorded wavelength is given by the ratio of the relative head-to-medium speed to the signal frequency; the typical separation is a small fraction of one wavelength.)

The benefits of very small magnetic switching units (higher signal-to-noise ratios, higher maximum transition density, smoother surface) must be balanced against a factor that places a lower practical limit on their size. This is the need for adequate stability of the magnetization against the randomizing effects of thermal energy. Ex- tremely small units would exhibit superparamagnetic behavior, having no stable magnetization [ 141. A more sub- tle effect of thermal instability can, however, be seen even at sizes substantially larger than the superparamagnetic limit. This is a significant dependence of the coercivity, or field needed to cause magnetic switching, on the length of time during which the field is applied [ 1 11, [ 151, [ 161. This time-scale dependence implies that for small-grained recording media the coercivity relevant to long-term information storage stability, in particular the resistance to demagnetization by internal fields, can be substantially smaller than that relevant to high-frequency writing processes. Since the former must be adequate for the intended information density and storage life, and the latter is limited by available head technology, a lower limit to the size of the magnetic units is implied [ 111.

The magnetization changes in recording can best be thought of as involving the switching of the magnetization direction between stable (or “preferred”) directions in single-domain magnetic units. Such units are always at their saturated magnetization intensity, except perhaps during switching. Magnetization changes that occur through domain-wall motion cannot be totally ruled out, but the particle sizes used in modern magnetic recording are not conducive to multidomain behavior. Most commonly, each unit will have one preferred axis of magnetization (two oppositely directed preferred directions), although materials exist that show multiaxial behavior (for example, three mutually perpendicular preferred axes). The strength of the magnetic anisotropy, that is of the “preferredness” of the axis (or axes), determines the difficulty of switching and hence the coercivity. The uniaxial materials are commonly oriented magnetically so that the preferred axis of each unit is approximately collinear with the relative motion of head and medium; this maximizes the remanent magnetization in this direction. Magnetic recording has traditionally been viewed as involving principally magnetization in this “longitudinal” direction. Even with media whose magnetic units have their preferred axes oriented longitudinally, however, magnetization perpendicular to the surface may make a significant contribution to the output-signal amplitude [ 171. In recent years, considerable attention has focused on the possible advantages of orienting media to favor the perpendicular, or “vertical” direction; lower internal demagnetization fields and sharper transitions, compared with those in longitudinal recording, have been claimed [ 181. Media have also been constructed using multiaxial magnetic mate-

EEE TRANSACTIONS ON MAGNETICS, VOL. 25. NO. 6, NOVEMBER 1989

rials; these exhibit a high ratio of remanent magnetization to saturated magnetization (termed the squareness ratio; see Fig. 1) regardless of the direction of magnetization and are intended to use both longitudinal and perpendicular recorded magnetization [ 191. The relative merits of longitudinal versus perpendicular recording have been studied and debated; the two magnetization modes may be essentially equivalent in their ultimate capabilities, with the decision between them to be based on practical material properties and ease of manufacture [20], [21].

PRODUCTION OF MAGNETIC RECORDING MEDIA There are two major techniques for the manufacture of

magnetic recording media; they can be designated as “thin-film’’ and “particulate. ” Each creates a magnetizable layer on the surface of a nonmagnetic support material. The support can be a thin plastic film (typically polyethylene terephthalate, PET) in the case of tapes, a thicker plastic film for flexible (“floppy”) disks, or an aluminum plate for rigid disks. The newer method consists of the deposition of a thin film of oxide, pure metal, or alloy on the support [22]. This is done, depending on the material to be deposited, by thermal evaporation, sputtering, or chemical plating. The resulting magnetic layer typically can range in thickness from 0.01 to 0.5 pm. The older and still more widely used production method involves the synthesis of discrete magnetic particles, which are then dispersed in organic resins that are designed to bind them into tough, flexible coatings on the surface of the tape or disk. The suspension of particles and resin is diluted with volatile solvents, which evapo- rate after the resulting dispersion is smoothly spread on the support. Dispersing agents, to aid suspension of the particles in the organic materials, and lubricants, to as- sure good frictional properties of the finished medium, are generally added before coating. The magnetic particles usually constitute 20-50 percent of the dried coating volume. Typical dried coating thicknesses range from 0.25 pm (rigid disk) to about 12 pm (audio tape). Most recording media made commercially today are of this particulate construction. Thin-film media have, however, found significant use in rigid disks. Also, a metallic thin- film tape was recently introduced for use in advanced video recording (the “high-band” 8-mm tape).

The two production technologies (thin-film and particulate) yield media having distinctly different properties. Those of the thin films can be summarized as nearly ideal magnetic properties combined with some shortcomings relating to practical manufacture and use. These coatings can be very thin and very smooth, provided the support material has adequate surface properties. Minimizing the thickness is desirable because it restricts the recording process to the region very close to the head, where the writing field is sharply defined. Thin films can obviously have very high magnetization intensities, because of the absence of the organic components used with discrete particles. (Many practical thin films actually contain strongly magnetic grains separated by nonmagnetic, or weakly


magnetic, oxide or metal compositions and thus have a “quasi-particulate” structure. This reduces the net magnetization below the theoretically available values, but it is still higher than those of particulate coatings.) The dis- advantages of thin-film media include the tendency, in some cases, to lack the necessary durability and chemical stability unless overcoated with a protective nonmagnetic layer. In the case of flexible media, especially tapes, the thin magnetic film has elastic properties much different from those of its plastic support. This can result in concentration of stress in the thin film, which may cause cracking and separation. On rigid disks, however, the mechanical properties of thin films are probably advantageous.

The particle-in-organic construction, on the other hand, can be characterized as combining satisfactory magnetic properties (which are continually being improved) with numerous practical and economic advantages. The mechanical properties of the coating can be reasonably well matched to those of flexible plastic support films. An organic composition of proven merit can frequently be adapted to various types of magnetic particles, for different applications. Lubricants can be incorporated inte- grally in the coating rather than added superficially to the surface. The abrasivity properties of the medium can be controlled, to give an adequate degree of head-polishing effect without excessive head wear, by adding to the dispersion correct amounts of aluminum or chromium oxides. In summary, the particulate coating is a complex composite material whose properties can be engineered somewhat independently to meet the complex needs of a recording application. Two final, and very important, advantages of particulate media are the large base of existing facilities for manufacturing them at low cost and the large base of experience in their use.

The techniques for coating particulate recording media will be reviewed very briefly. In the case of flexible media, the dispersion is spread onto a moving strip of the support film by a rotating drum that carries a metered amount of the fluid or by a die that extrudes a continuous fluid curtain. If the intended product is a tape, the still- wet coating then passes through a magnetic field, to ob- tain the magnetic orientation discussed above. If flexible disks are to be punched from the resulting coating, this orientation step is not used; in fact, a magnetic environment that tends to randomize the particle preferred axes is sometimes substituted. This process is needed because most magnetic particles are elongated and tend to acquire an orientation from fluid forces occurring in coating. A circumferential orientation, which would be ideal in disks since it would be collinear with the direction of relative head-medium motion, is not at present economically fea- sible in flexible disks. Rigid disks, however, are coated individually by introducing the dispersion onto the rapidly spinning support material, and then subjected to circumferential orientation by being rotated past permanent magnets. After a tape coating has dried, it is usually subjected to pressure treating between rollers (calendering) to

achieve a smoother, denser surface before being slit into the desired width. Coatings intended to be punched into flexible disks are sometimes polished or burnished, as are rigid disks.

The foregoing paragraphs have attempted to outline the context in which particulate recording materials must function and to give the general requirements placed on them. Subsequent sections will describe the major types of materials in use, their properties, and current development trends.

IRON OXIDES

The first commercially available magnetic recording tapes were produced in the late 1940’s using iron oxide particles. Gamma ferric oxide ( y-Fe203) is the most useful of the oxides, because of its great chemical and physical stability, and retains an important position today in audio and computer tapes and in flexible and rigid disks. Particle size and formation have been greatly improved in the last four decades. The particles used today are acicular (needle-like or rod-like) in shape. This shape anisotropy is the major source of their magnetic anisotropy; the magnetostatic energy is lowest when the magnetization direction is collinear with the particle’s longest dimension. A secondary source of magnetic anisotropy in y-Fe203 is magnetocrystalline in origin, arising from the interaction of electron spins with the crystal structure of the oxide. These two types of anisotropy determine the field needed to switch the magnetization from one of the two energet- ically preferred directions to the other and therefore determine the coercivity. Neither anisotropy has a strong temperature dependence [23]. The magnitude of the coercivity can be explained by a model for magnetic reversal wherein the magnetization vectors of different parts of the particle change directions simultaneously, but do not remain parallel (or coherent) during the reversal process. This is called the “chain-of-spheres fanning” model (241,

Recent years have seen an interesting discussion re- garding the microstructure of y-Fe203 particles and how it influences their magnetic properties [26]-[28]. The best picture of these particles is probably one in which they have a substantially single-crystal character but may undergo magnetic reversal in ways that are influenced by defects and inhomogeneities [27], [29].

Acicular iron oxides are important to current magnetic recording technology not only in their own right but also as precursors for cobalt-modified oxides and metallic particles, which will be discussed later. Both of these classes of particulate recording materials are more suitable for use at high recording densities, but are also more expensive and in some respects less stable, than the pure iron oxides.

Production of y-Fe20i usually begins with the nuclea- tion and growth of particles of a-FeOOH (goethite) or y-FeOOH (lepidocrocite) . Dehydration of a-FeOOH forms particles of nonmagnetic a-Fe203 (hematite), which

~ 5 1 .

4378 IEEE TRANSACTIONS ON MAGNETICS, VOL 25 . NO. 6. NOVEMBER 1989

are then reduced to yield Fe304 (magnetite). Magnetite would seem to be a desirable recording material, and in fact serves as the core particle for a successful cobalt- surface-doped oxide [30]. Magnetite itself, however, has some chemical and magnetic instabilities and is therefore oxidized to y-Fe203 for most recording applications today. This conversion must be done carefully, because y-Fe203 is an unstable structure that converts to the nonmagnetic form a-Fe2O3 at approximately 400°C. The starting material y-FeOOH can be converted directly to y-Fe203 by dehydration, but this is difficult owing to the instability of y-Fe203; a-Fe203 can easily be the product of the dehydration, as it is when a-FeOOH is the starting material.

The major challenge in producing oxides for recording is the control of particle shape, surface quality, and size. Some intraparticle fusion, due to processing at high temperatures, is desirable in that it tends to close pores and thus yield particles of higher density. Any fusing together (sintering) of the particles, however, degrades the shape anisotropy and interferes with efficient packing of the particles in the tape coating. Various additives are used to inhibit sintering. A trend to smaller particles exists, motivated by the desire for decreased noise, but must be pur- sued with adequate control of size distribution in order to avoid an appreciable population of particles too small for thermal stability. Such thermally unstable particles will effectively have a very low coercivity and can cause print- through [6]. Particles currently used in audio tapes have typical lengths of 0.3-0.4 pm, with their width (or diameter) about an order of magnitude smaller; those intended as precursors for cobalt-modified oxides may be smaller. (In most recording materials, a significant distribution of particle sizes exists. Where “typical” dimen- sions are given in this review, they are representative of particles in the size range where most of the mass of the sample is found, rather than an unweighted average over all particles including very small ones.)

Recently, an alternate pathway for the production of y-Fe2O3 has been introduced [31], [32]. In it, particles of a-Fe203 are formed by a hydrothermal process from a slurry of Fe ( OH ) 3 , using additives that control crystal growth. The formation of FeOOH particles is bypassed, eliminating the need for a dehydration step that can result in pore formation. The resulting magnetic particles, formed from this hydrothermally produced a-Fe203 by conventional processes, can exhibit very good shape and size uniformity. It is claimed that these features will facilitate efficient packing in coatings and that the lack of irregularities and imperfections in the particles may avoid possible sources of internal demagnetization and other instabilities [31], [33]. The lack of pores or projections that could serve as sources of internal demagnetization has led to the term “nonpolar,” or “NP,” for these particles. They have not played a major role in the development of advanced media, perhaps because of difficulties in obtaining very small particle sizes, but have found acceptance in high-quality audio tapes.

Current y-Fe203 particles have typical saturation magnetization densities of about 340 emu /cm3, coercivities of 300-400 Oe, and prices of around $4/kg for high- quality (low-noise) versions. These properties, along with their great chemical and physical stability, suit them very well for a number of applications not requiring high recording densities.

CHROMIUM DIOXIDE

Despite the many virtues of iron oxides, their relatively low coercivity values (usually 300-400 Oe) proved to be a serious limitation as recording densities increased. The requirement for higher coercivities was first met in the mid- 1960’s with the introduction of chromium dioxide ( Cr02) particles. Like iron oxides, these particles are magnetically uniaxial and derive their magnetic anisotropy partly from their acicular shape and partly from magnetocrystalline sources [34]. Unlike iron oxides, however, they probably undergo magnetic reversal by the mechanism known as “curling” [25], [35], which is possible only in acicular particles of great morphological perfection. Particles of Cr02 do indeed have great perfection and uniformity of shape, qualities that aid in efficient packing and orienting in the coatings of magnetic media; see Fig. 2.

The current development of Cr02 centers on control of particle size and coercivity, both of which can be modified by the addition of Fe and Sb. Narrowing the distribution of sizes is especially important, as particles below a certain size are likely to contribute to the phenomenon of print-through [36]. Rhodium and iridium are also used as dopants. Iridium acts as a growth inhibitor to reduce particle diameter; this causes an increase in coercivity, as predicted by the curling model [37]. However, the iridium is also thought to contribute to coercivity enhancement by increasing the magnetocrystalline anisotropy [38]. Originally, chromium dioxide offered coercivities not very much above those of iron oxides, and typical particle lengths were in excess of 0.5 pm. These properties have been steadily improved, and lengths of less than 0.1 pm and coercivities of 2800 Oe have been reported [38], [39]. Particles sold commercially at this time commonly have coercivities of 500-600 Oe and magnetization intensities of 350-400 emu/cm3. Their price is around $14 / kg .

Chromium dioxide was first introduced in computer tapes and soon appeared in audio tapes. It is currently used in some video cassettes and data recording cartridges. It would probably have a broader application, were it not for the versatility and relatively low cost of the more recently developed cobalt-modified iron oxides (see next section). A further limiting factor may be that the production process for Cr02 is a difficult one, requiring high pressures. The particles are quite abrasive; this has in the past been a source of concern, but head wear has been shown to be controllable at satisfactory levels [40]. Chromium dioxide is also a somewhat reactive ma-


Fig. 2. Commercially available chromium dioxide particles.

terial; stable chemical and physical properties can be obtained through appropriate choices of the organic components of the coating [41]. The unusually low Curie temperature, about 125”C, permits tapes made with CrOz to be used in thermomagnetic duplication. Tn this process, the information is copied from a master tape, having a much higher Curie temperature, to the Cr02 tape by heating while their surfaces are pressed together [42]. Chro- mium dioxide is the only particulate material known to be useful in practice for thermomagnetic copying. Contact duplication processes, of which thermomagnetic copying is one, are of economic importance because they permit copying of video information at speeds much higher than can be done by direct head recording.

COBALT-MODIFIED IRON OXIDES By far the most successful particles for applications re-

quiring coercivities well in excess of 400 Oe are iron oxides to which cobalt has been added. Many of the practical benefits of the original oxides (e.g., modest cost, chemical stability) can be retained, and coercivities can be obtained in the range from 400 to well above 1000 Oe. The magnetization intensities are close to those of the un- modified oxides. Cobalt-modified oxide particles are the predominant material for use in video tapes today. They are also used in some audio tapes and find application in tapes and disks for high-density digital recording. These uses make them the most commercially significant of the particulate recording materials. Their prices range from about $7 /kg for materials suited to general-use video cassette tape to about $25/kg for small particles designed for advanced applications (e.g., the S-VHS video format).

In addition to being currently of great economic importance, cobalt-modified iron oxides have a long history of development that in some sense parallels the development

of magnetic recording itself. Their gradual increase in coercivity from values just above those of pure iron oxides for early applications to about 900 Oe in some of today’s video tapes represents the continuing drive to higher recording densities. The continual improvement with respect to various kinds of stability in these materials illustrates some of the nonmagnetic requirements placed on recording media. This section will attempt to outline some of the technologically interesting aspects of cobalt- modified oxide particles.

The earliest attempts at using cobalt to enhance the coercivity of iron oxides for recording uses involved uniform doping of CO*+ ions in particles of approximately cubic shape. The first commercially successful materials, however, were acicular particles with cobalt deeply diffused into them [43]. These represented a significant advance toward high-density recording and made possible a new generation of video and data recorders.

The mechanism of coercivity enhancement resulting from Co2+ ions in the oxide lattice has been explained theoretically [44], 1451. It is a magnetocrystalline anisotropy due to the interaction of the electron structure of the cobalt ion with the crystal field in the octahedral (B-type) site of the cubic spinel structure. This interaction gives the electron spin a preferred direction, which depends upon the local geometry of the site. The exchange interaction couples the spins of the cobalt ions to those of the iron ions, and thus imparts an increased magnetocrystalline anisotropy to the oxide structure. Owing to the cubic symmetry of the oxide lattice, a particle having cobalt ions distributed at random over the available sites will have a number of preferred axes of magnetization. This multiaxial anisotropy causes the squareness ratio of a sample of randomly oriented particles to be relatively high, sometimes above 0.8, while randomly oriented uniaxial particles (e.g. , undoped oxide or CrOz) have a squareness of 0.5 1461.

Cobalt-doped oxides are not totally satisfactory for recording applications; they are characterized by strongly temperature-dependent coercivity and can exhibit a pro- gressive loss of short-wavelength signal amplitude with repeated playback, probably as a result of mechanical stresses. To avoid these shortcomings, alternate methods for using cobalt to enhance coercivity were developed [301, [471-[491. These placed the cobalt on or near the surface of the particles and achieved much improved stability with respect to temperature and stress. For example, temperature dependences near 20°C of -0.2 percent/ “C to -0.5 percent/”C are possible; uniformly doped materials have ratios of about - 1 percent/ “C. The products of the newer processes are termed “surface- doped,” “cobalt-adsorbed,’’ or “epitaxial”; they show uniaxial magnetic anisotropy, having a single preferred axis of magnetization like the oxide particles from which they are made. The fact that the anisotropy due to the added cobalt appears to reinforce that of the core particle, rather than having a multiaxial character, is thought to result from the formation of the cobalt-rich surface under

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the influence of the magnetic field of the core particle 1501. This acquisition of a uniaxial magnetocrystalline anisotropy by growth in the presence of a field may be analogous to the induced uniaxial anisotropy that occurs in ferrites that are annealed in a field [5 11, 1521, with Co2+ ions occupying lattice sites favorable to the overall preferred axis. The coercivity enhancement by surface-deposited cobalt is by no means fully understood, and other mech- anisms may also be operative 1531. The uniaxial character and relatively weak coercivity temperature dependence of surface-doped material are both lost if the particles are annealed at temperatures that diffuse the cobalt into their interiors 1541.

The weaker temperature dependence of the coercivity in the surface-doped particles can be understood as resulting from the fact that cobalt exists at relatively high concentrations in their surface regions as compared with the concentrations found in volume-doped particles. The temperature dependence of the cubic anisotropy constant is weaker in CoFe204 than in cobalt-doped Fe304 ( Co,rFe3 -x04 with x much less than 1 ) 1441, [45]. Pos- sibly, the temperature dependence is also weaker in induced (uniaxial) anisotropy than in cubic (multiaxial) anisotropy, even for comparable cobalt concentrations [44].

The amount of signal decay upon repeated replays, pre- sumably stress-related, is also decreased in the surface- doped materials by having the cobalt confined at high concentrations near the particle surface. This increased stability may result from an intrinsically lower sensitivity to stress when the cobalt exists at a higher concentration. It is also possible that the presence of uniaxial, rather than multiaxial, magnetic anisotropy makes demagnetization more difficult.

The sensitivity to mechanical stress in all cobalt-containing magnetic oxides can be understood physically as due to the phenomenon of magnetostriction, which, like magnetocrystalline anisotropy, is related to the interaction of the electron structure of the Co2+ ion with the oxide lattice 1551. The presence of Fe2+ ions also causes a magnetostrictive effect but of the opposite sign 181. It is possible that the two ions could be used together in the proper proportions to eliminate the stress sensitivity, but the presence of the Fe2+ might be undesirable in other respects.

The presence of Fe2+ is a potential cause of time-dependent instabilities in oxides containing cobalt. Magne- tite and other oxides having appreciable Fe2+, if volume- doped with cobalt, are able to acquire an “induced” uniaxial magnetic anisotropy that is not stable [49]. This unstable anisotropy results from the easy motion of CO’’ ions in the lattice and is related to magnetic annealing in ferrites, where the presence of Fe2+ (in addition to Fe3+) is thought to allow easy exchange of electrons. This in turn helps to maintain charge neutrality and aids in the migration of c o 2 + ions to sites of lower magnetostatic energy 1511, 1521.

The unstable anisotropy, due to relatively high Co2+ mobility, usually manifests itself as a continuing rise in

coercivity 1561, [57]. A shift in anisotropy due to the application of a weak magnetic field can be a source of “print-through” problems; this effect has been found greater in particles that have the cobalt deeply diffused than in those that are surface doped [33], 1571, but the enhanced cobalt mobility due to the presence of Fe2+ [56], 1581 may be the crucial factor. Problems with erasure, especially a deterioration of erasability with length of storage in the recorded state, are also associated with the cobalt being dispersed throughout a large fraction of the particle volume 1331; this too might be expected to be ag- gravated by significant amounts of Fe2+ in the cobalt- doped volume. Thus the advent of surface doping, which retains the cobalt at high concentrations near the surface (perhaps in a composition close to that of CoFe204, which has no Fe2+) , has helped to minimize numerous instabilities in oxide particles for magnetic recording. Most commercially available cobalt-surface-modified oxide particles in fact contain a few percent Fe2+; their satisfactory stability probably results from this ion existing at a sufficiently low concentration in and near the Co2+-containing surface layer.

While the presence of some amount of Fe2+ is not a sufficient condition for coercivity changes with time, it also appears not to be a necessary condition. In some studies 1591, such changes were found to occur independently of Fe2+ content but requiring the presence of oxygen, so that a mechanism of coercivity instability different from that described above appears possible. The observed effects in tapes and pressed-powder samples were found to differ from those in loose powders; this is an important experimental distinction. Later experiments using sample dilution (magnetic particles mixed with nonmagnetic a-Fe203 in loose powder) showed that interparticle magnetic interactions are involved in these aging effects 1601.

The methods of surface doping are of two main types. In both, the cobalt content in particles for typical applications is 3--6 percent by weight. Also common to both methods is the importance of the surface quality of the core particle 1611, 1621. In the first process, oxide particles are suspended in an aqueous solution of Co2+ ions. A base, e.g. , NH40H or NaOH, is added and the mixture is heated, but not to a temperature that appreciably diffuses the cobalt into the oxide [48], [49]. If the Fe2+ content of the core particles is sufficiently high, the heating step can be omitted 1301; in this case, the complete lack of cobalt diffusion into the particle prevents instability due to Co2+ mobility.

The second major surface doping process consists of adding both Co2+ and Fe2+, usually in a ratio of about 1 : 2, to a suspension of y-Fe203 particles, raising the pH to alkaline levels, and heating (again, not enough to appreciably diffuse the cobalt). A material whose composition is close to that of cobalt ferrite ( CoFe204) is considered to grow on the surface, with a uniaxial anisotropy that reinforces that of the core particle [50], 1631. The conclusion that the added Fe2+ and Co2+ react to form a

SHARROCK. PARTICULATE MAGNETIC RECORDING MEDIA 438 1

composition near that of CoFez04, which has little or no Fe2+, with the concurrent reduction of some Fe3+ in the core particle [63] was supported by X-ray diffraction studies [64].

The nature of the cobalt compounds formed on the particle surface by alkaline precipitation of Co2+ (with no Fe2+ added) has been clarified by means of Mossbauer emission spectroscopy, using radioactive ”Co. For surface doping of both y-Fe203 [65] and Fe30, [66] core particles, a component closely resembling cobalt ferrite ( CoFe20,), and associated with the coercivity increase, was detected. For the Fe,O,-based particles, the measured coercivity enhancement was approximately one- third of what would be expected theoretically from the magnetocrystalline anisotropy contribution of the detected CoFe20a, perhaps indicating crystalline disorder at the particle surface [66]. Oxides and/or hydroxides, which are not magnetically ordered, were also detected in both cases; the fraction of the cobalt incorporated into these “waste” compounds tended to increase with the amount of cobalt added. In the case of the Fe30, core, a substantial amount of CO ( OH)2 was found when the amount of cobalt used was sufficient to give a significant coercivity enhancement [66]. This supports an earlier conclusion (481 that in alkaline surface doping (with no Fe2+ added) of y-Fe203, a portion of the cobalt was inserted into the oxide lattice and the remainder adhered to the particles as

Despite the success of surface-doped oxides, particles with uniform, or at least highly diffused, cobalt doping received renewed attention a few years ago. This interest was due to the multiaxial anisotropy such particles pos- sess, which can provide media with high squareness in all directions. To enhance their multiaxial character, the particles can be made with a relatively small amount of elon- gation 1671, as compared with the usual needle-like or rod- like shapes of other oxides. The “high-squareness isotropic” media made from multiaxial particles offer the possibility of using favorable combinations of longitudinal and perpendicular magnetization at high recording densities [19]. The proper combination of these two magnetization components can, in fact, produce a “one- sided” magnetic field that extends only toward the surface of the coating [68]. The absence of a magnetically preferred direction makes these materials especially attractive for flexible disk applications, since they need no magnetic process to “disorient” them after coating. Their well-known temperature and stress sensitivities are sources of concern, however. One approach to improving these characteristics is the use of various metal dopants, especially zinc [69]. Zinc probably tends to stabilize the coercivity by accelerating the decrease of the saturation magnetization with increasing temperature [70, p. 1581, to more closely match that of the magnetocrystalline anisotropy; the cobalt’s contribution to the coercivity is proportional to the ratio of anisotropy to saturation magnetization. Besides reducing the dependence of coercivity on temperature, Zn” may also provide stability against

C O ( O H ) ~ .

the migration of CO*+ ions in the lattice [71]. The contribution of Zn2+ to magnetostrictive effects appears to be, like that of Fe2+, of the opposite sign to that of Co2+ [70, p. 1691, and thus may help to decrease stress sensitivity in cobalt-doped oxides. It remains to be seen whether zinc doping or other strategies can succeed in removing the instabilities of cobalt-doped oxides without sacrificing their attractive features.

Multiaxial (‘‘isotropic”) cobalt-doped oxides are currently receiving less attention than was formerly the case. One reason for this may be the increased prominence of barium ferrite particles, to be discussed below. These can have excellent time and temperature stability and are appropriate to media that utilize the perpendicular component of magnetization.

A major direction in the further development of cobalt- modified oxide particles is size reduction. This is required by their use in demanding, high-density recording applications such as present and future video tapes; see Figs. 3 and 4. There are, however, some practical limitations to this trend. Simple thermal considerations predict that cobalt-modified oxide particles appreciably smaller than

cm3 in volume can be expected to have coercivities that are markedly “dynamic”; that is, the value depends strongly on the time scale of interest [ 151, [ 161. The most recently developed cobalt-modified oxides have volumes of about 0.5 X cm3 and show significant time-scale effects. Their lengths are typically 0.25 pm.

Two further limitations to the trend toward smaller particles exist. One is that as the particles become smaller, they become more difficult (and expensive) to manufacture. The other is that the increasing surface-to-volume ratio can bring about increasing demands for the surfactants that aid in the dispersion of the particles. Current research and development efforts seek to alleviate this dispersion problem through modification of the particle surface (see, for example, [72]).

In summary, cobalt-modified iron oxide particles have been extremely successful, both technically and commercially, in current recording applications. In order to allow them to meet future demands, development work continues in the areas discussed above and also in the further improvement of their magnetic performance; higher coercivities with narrower switching-field distributions, together with time and temperature stability, are the main objectives.

METALLIC PARTICLES

Pure metals can have magnetization intensities far in excess of those of oxides. Iron has the highest magnetization of the ferromagnetic elements, about four times that of iron oxides, and is also the least expensive of these metals. Metallic iron particles are therefore very attractive candidates for recording applications. Their disad- vantage is the need for protection against the tendency to rapid oxidation (or other reactions) that could result from the very high specific surface areas common to all fine

4382 IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 6, NOVEMBER 1989

1 pm Fig. 3. Commercially available particles of iron oxide with suface-depos-

ited cobalt. They have a coercivity in the 700-750-Oe range and are suitable for many audio and video tape applications.

Fig. 4. Commercially available particles of iron oxide with surface-deposited cobalt, typical of those used in advanced applications such as S-VHS video tape. Their coercivity is approximately 900 Oe, and they are substantially smaller than those shown in Fig. 3.

particles. Acicular iron particles for use in recording media are stabilized against corrosion by the use of alloying elements and additives, by the protective action of the organic coating components, and by a controlled oxidation of their surfaces. The last of these is the most important, but it has the unfortunate effect of reducing the average magnetization intensity of the particles to approximately one-half of the value for bulk iron, which is 1700

emu/cm3. The resulting media can still, however, have magnetic retentivity that is twice that of oxide media [73], and signal output amplitudes well above those of cobalt- modified oxides are obtainable [74].

Numerous methods exist for the production of metal particles suitable for recording [73]. One is the reduction of iron salts in solution by strong reducing agents [75]. Another, which permits the use of arbitrary alloy compositions, is condensation of vaporized metal [76]. The only commercially significant process today, however, is the reduction of acicular particles of oxides, oxalates, or oxyhydroxides. Depending upon the process used, this reduction may yield metal particles similar in shape to those of the precursor, or may create networks of small, often nearly spherical, particles. The diameters of these subunits can be controlled over the approximate range 0.01- 0.08 pm by reduction temperature or other process vari- ables. The coercivity increases with decreasing subunit size, and can in this way be varied from about 400 to well over 1000 Oe [77].

After synthesis, the particles must be passivated against oxidation. The usual means of doing this is to expose them to oxygen, initially at low concentrations and then at pro- gressively higher levels. The resulting oxidized surface layer is typically 0.002 to 0.004 pm thick and has been found to consist primarily of y-Fe203 and Fe304 [78]- [80]. These oxides may in part be present as very small superparamagnetic crystals, which reduces the net magnetization of the surface layer; antiferromagnetic compounds ( a-Fe203, FeOOH) may also make up part of the layer [go]. The process of passivation continues to be the subject of further study, aimed at characterizing and improving particle stability [8 13, [82].

An alternate approach to the preparation of stable particles of high magnetization intensity is the nitridation of iron particles to the composition Fe4N. The resulting material has been found to have magnetization intensity comparable with that of passivated iron, although the coercivity values so far achieved are generally lower (below 1000 Oe) than for iron particles. Some initial results showed chemical stability better than that of a reference iron sample [83], but subsequent work has found no ad- vantage [84], [85]. It is likely, therefore, that particles of predominantly iron composition, with a controlled surface oxidation, will remain the principal candidates for use in particulate media where very high retentivity is needed.

The magnetic properties of iron-based metallic particles continue to be improved, with coercivities exceeding 2000 Oe and particle lengths not much more than 0.1 pm having been achieved [86]. The technology of metal particle production involves a variety of additives and alloying elements. Silica and alumina are common agents used to prevent fusing together (sintering) of the particles during the required heating processes [87], [88]. Numerous other elements, both metallic and nonmetallic, have been used


to serve a variety of purposes; these additives can inhibit sintering, facilitate reduction or pore closure, alter the magnetization or coercivity, influence size or shape, or impart resistance to corrosion (see, for examples, (841,

One property of iron-based materials that has been ex- tensively studied is the origin of their coercivity. These particles are very acicular; ratios of length to width on the order of ten are common. Their coercivity is due primarily to shape anisotropy, and is therefore relatively insen- sitive to temperature and mechanical stress. The mechanism of magnetic reversal in some acicular metal particles [9 13 appears to be “chain-of-spheres fanning” [24] ; their coercivity is independent of particle width or diameter. In other preparations, the coercivity falls with increasing diameter of the elongated particle [92] or of the subunits (771, and the “curling” model may be appropriate (353. Further study continues to refine the models of magnetic reversal, especially those based on fanning [93], (941.

Metal particles were introduced in 1979 in high-performance audio cassette tapes and still find some use in that application. The tapes have coercivities well above 1000 Oe, and adequate writing on them requires the use of special recording heads having metal cores rather than the commonly used ferrite cores. These metallic heads have wear properties that are adequate for analog audio use but not for video and data applications, where head-to-medium speeds are much higher. For this reason, lower coercivity (typically 700 Oe) metal particles have been produced, by means of the microstructure control described above. These are compatible with the durable, relatively inexpensive femte heads commonly used in video and data recording, and can offer output amplitudes, at least in some range of recording densities, superior to those of oxides [84]. Full use of the high magnetization intensity of metal at high recording densities, however, requires commensurately high coercivity. Fortunately, the metal- in-gap ferrite head developed in recent years can provide the strong writing fields needed for metal particle media having coercivities in the area of 1500 Oe, while retaining good wear properties (951. With the use of these heads, metal particles have been successfully used in high-density recording media such as the rotating digital audio tape (R-DAT) and the 8-mm video tape. The metal particles prepared for these systems have typical lengths of 0.25 pm (see Fig. 5) and are available commercially at a cost of about $85/kg. The price is expected to fall with wider use.

As with other materials, the development of metal particles is moving toward smaller particle size. In this case, however, the trend to smaller size must be accompanied by adequate passivation techniques because of the increasing surface-to-volume ratio of the particles. As with oxides, the increased surface requires attention to dispersion properties. The high signal level made possible by a high magnetization intensity is of little use if accom-

1861, ~891, [901).

1 pm Fig. 5. Commercially available metallic particles, typical of those used in

advanced media such as 8-mm video tape and digital audio tape (R-DAT). Their coercivity is approximately 1500 Oe. Some of the longer particles may tend to break as a result of forces encountered in the dispersion process, so that typical lengths in the tape would be about 0.25 pm.

panied by unacceptable levels of noise due to aggregates of undispersed particles. Indeed, the high magnetization values of metal particles imply a relatively strong magnetic attraction. The properties of the oxidized surface layer are probably very important to the strength of this attraction, and the passivated particles in use today can be very well dispersed.

BARIUM FERRITE Ferrites having the hexagonal lattice structure, such as

barium ferrite, have been known for decades. Pure barium ferrite (BaFe,,O,,) has had many applications that utilize its high anisotropy (for example, the low-density recording on credit card strips [96]), but its use in high- density magnetic recording was hindered by excessive particle size and/or coercivity. The introduction of processes for the substitition of Fe with various metals, especially CO and Ti (971, however, have made platelike particles having diameters less than 0.1 pm and coercivities of, for example, 800 Oe available. These particles were orginally developed with an application to perpendicular recording media in view [98]. Potential benefits of perpendicular recording with regard to reduction of self- demagnetization and retention of sharp transitions have been claimed, and impressive results have in fact been demonstrated using special thin-film recording media (991.

Many practical and economic factors, however, dictate that the bulk of magnetic media will be made with particulate coatings in the foreseeable future. A particulate- coated medium that supports substantial perpendicular magnetization might thus be an attractive candidate for

4384 IEEE TRANSACTIONS ON MAGNETICS. VOL. 25. NO 6. NOVEMBER 1989

advanced applications. There have been a number of ap- proaches to this goal. The easiest is probably the use of ‘ ‘high-squareness isotropic” media, made from cobalt- doped oxide particles 1191, [67]. As already discussed, these materials suffer from well-known instabilities with regard to stress and temperature. Another approach to a perpendicular particulate medium is the perpendicular physical orientation of conventional acicular particles [loo]-11021. This involves some difficulties in retaining the orientation during the drying of the coating, because the particles must stand on end, and will probably require special techniques if smooth surfaces are to be obtained [ 1031.

The preferred solution is clearly a particle that can be easily oriented in the coating in such a way that its preferred axis of magnetization is perpendicular to the surface. Barium ferrite, in the form of flat platelets with their preferred axes perpendicular to their planes, is an excellent candidate. One means of obtaining the desired perpendicular orientation with barium ferrite particles is to dry the coating while it is in a constant magnetic field, preferably one of about 4000 Oe 11041. Alternating fields have also been employed [IOS] . The plate shape of barium ferrite, besides facilitating perpendicular orientation, also makes possible a stable stacked configuration. This has been seen by electron microscopy and can signifi- cantly influence magnetic properties 11061, 11071.

Good recording results at high densities have been obtained using Co/Ti-substituted barium ferrite, and evidence has been presented that perpendicular orientation is beneficial [ 1081, 11091. Many applications have, however, used coatings having little magnetic particle orientation 11 lo], [ 1 1 11, or a substantial longitudinal orientation [ 1121-[l l s ] . As discussed in the Introduction, properties such as switching-field distribution and surface smoothness may be more important to the success of a high-density recording medium than the direction of magnetic orientation.

The physical and magnetic properties of barium ferrite (understood here to be substituted, usually with CO and Ti) are generally very desirable. Its chemical stability appears to be satisfactory. Particles are available in very small sizes, for example a diameter of 0.05 pm with a plate thickness about one-sixth as large; see Fig. 6. The switching-field distribution is exceptionally narrow. This distinguishing characteristic of barium ferrite may be due to the large magnetocrystalline anisotropy 11 161, or may result from a cooperative behavior of particles that have formed a stack. It is seen most strikingly in longitudinally oriented coatings [ 1 161, but is also present in those having little or no orientation [ 1 171. Another unusual aspect of at least some barium ferrites is a ratio of the anisotropy field to the coercivity that is substantially higher than the ratio seen for acicular particles [ 1 181, 11 191. (The anisotropy field is the strength of the field, applied perpendicular to the preferred axis of a particle, that is just sufficient

Fig. 6. Small particles of barium ferrite. typical of those available commercially in a broad range of coercivities. Note that the scale is ditferent from that used in Figs. 2-5.

to rotate the magnetization vector into collinearity with the field direction.) This property is responsible for the very high resistance of barium ferrite to demagnetization by a field applied perpendicular to the direction of initial magnetization [ 1 171, 11 181. Barium ferrite media also appear to be characterized by unusually strong interparticle magnetic interactions [119]; the effects of these on recording performance are not well understood at this time.

Barium ferrite, like the other hexagonal ferrites, has uniaxial magnetic anisotropy (a single preferred axis of magnetization). This anisotropy is the sum of two large terms, having opposite signs: the crystalline term is predominant and dictates the preferred axis, perpendicular to the plate, but the shape anisotropy has the opposite effect and if sufficiently strong would make this axis an “unfavored” one for magnetization. This conflict of anisotropies, each with its own temperature coefficient, gives the coercivity of barium ferrite a more complicated temperature dependence than the simple monotonic declines seen in acicular particles [ 1201-[ 1221. Depending upon their composition and aspect ratio (diameter to thickness), barium ferrite particles have coercivity temperature coefficients ranging from relatively insignificant negative values to +0.3 percent/’C. The use of metal substitutions, as well as shape control, can be used to bring the coefficient to a desired value, such as zero 11 231.

In the past few years, barium ferrite particles have undergone intensive development. This effort has been very successful; they are now available in a variety of shapes and sizes, and with a range of coercivity values and coercivity temperature coefficients. They have, however, been rather slow to find commercial application. A high-density flexible disk system has been developed


using barium ferrite [ l IO] and is sold commercially. An- other practical application is in tape for contact duplication of digital audio tapes (R-DAT) [ 1 1 11, [ 1241, [ 1251. Barium ferrite appears to have unique qualifications for this application, in which a high-coercivity master tape (containing iron metal particles) is pressed against a lower coercivity copy tape in the presence of an alternating field. This field, rather than high temperature as in the thermomagnetic process mentioned earlier, causes the copy tape to retain a magnetic image of the master tape. Barium ferrite appears to be the only particulate material capable of providing the required output signal strength with coercivities sufficiently low for the process to work.

The principal deficiency of barium ferrite, which makes more difficult its acceptance in advanced applications such as R-DAT and 8-mm video tape, is its relatively low magnetization intensity. Currently available barium ferrite particles have a typical magnetization of about 300 emu/cm3, somewhat less than that of most acicular oxides and substantially less than that of metal particles. On the other hand, relatively high cost (currently comparable with that of the best grades of metal particles) prevents their consideration as a replacement for acicular oxides. The cost of barium ferrite may well decrease, however, if demand for large quantities were to occur.

Current development goals for barium ferrite include the control of the coercivity temperature coefficient and the enhancement of the magnetization intensity; substit- uents such as Sn and Zn have been found useful. Unlike other particulate materials, barium ferrite may well have reached the point where further reduction of particle size is not advantageous [ 111. It is possible, in fact, that very small particle size accounts to a large degree for the good recording results found for this material. Barium ferrite will continue to benefit from material development, and also from applications development, aimed at finding uses for its unusual properties.

SUMMARY

A variety of particulate materials exists for use in recording media, seemingly enough to satisfy almost any application. Still. the search for improved particles goes on, motivated by the demand for ever-increasing information densities and lower media costs. Competition from thin-film magnetic media, as well as from other technologies such as optical and magnetooptical recording, pro- vides still more incentive to advance further. This paper has attempted to briefly describe the leading materials and their directions of development. A trend common to most of the materials is particle size reduction. Any discussion of magnetic media must recognize two facts: that magnetic recording is crucially dependent on close proximity (if not actual contact) between head and medium, and that magnetic domain structure imposes a discrete nature upon the recording process. The first of these implies that sur-

face quality, which in part depends upon the grain structure of the material, will be one of the limiting factors to high-density recording. The second implies that the grain structure of the material will determine the noise properties and, ultimately, the achievable transition density of the medium [ I 11. The effects of surface roughness on the loss due to head-medium spacing have been analyzed and measured [ 121, [ 131, [ 1261, [ 1271. The effects of particle size on noise can be predicted theoretically [IO], and effects of particle size on magnetic transition width [128] and on high-density output [ 1091 have been observed. Al- though the possibility of magnetic transitions occurring within particles is difficult to exclude completely, theoretical work has indicated that a magnetic reversal introduced near one end of a typical particle will tend to prop- agate throughout the entirety [129]. There are therefore a number of motives for reducing the size of particles for recording application; these must be balanced against the need for adequate magnetic stability. A practical lower limit for particle volume can be estimated [ I I ] ; it is, for a given coercivity, inversely related to the particle’s magnetization intensity. This limit is found to be 2 to 3 X

cm3 for high-coercivity oxides and 0.6 to 0.9 X cm3 for iron metal. Some currently used cobalt-

modified acicular oxide particles and acicular metal particles have particle volumes of about 5 X cm-3; this is within an order of magnitude of both estimated lim- its, with the metallic particles having a larger margin of stability.

Currently used acicular particles of both metal and oxide appear to have remaining potential for size reduction; some available barium ferrite platelets may already be near the practical lower limit of size [ 1 I]. All three of these materials have undergone substantial development during recent years. Barium ferrite has been the subject of a large amount of published literature and has found some practical applications [ l IO], [ 1 1 I]. Iron-based metallic particles have also attracted significant research interest and are now used commercially in video and digital audio tapes 1741, [ 1301. They have also been chosen for the most recent generation of digital video types [ 13 11. The already well-established cobalt-modified acicular iron oxide particles have at the same time continued to evolve toward greater capacity for high-density recording. Their increased coercivity values and reduced particle volumes have enabled them to be used in an improved video cassette system (S-VHS) 11321, in digital video recorders [ 1331, and in advanced data-recording cartridges. The overall picture, then, is one of cobalt-modified oxides continuing to advance but with some of the most demanding applications currently being taken over by metal and with barium ferrite attracting interest but having a somewhat undefined role at this time. Pure iron oxides (without cobalt) remain the material of choice in media used at relatively low recording densities (including many flexible and rigid disks), and chromium dioxide has a minor but

4386

significant position as a competitor of cobalt-modified iron oxide.

This review paper has focused on the particulate magnetic materials used in recording media. Although a de- tailed discussion of the organic media components (dis- persants or surfactants, binder resins, lubricants) is beyond the chosen scope, the importance of these must be stressed. The success of a tape or disk in actual use depends critically on its tribological properties. The medium must have adequate durability and must not cause an unacceptable rate of head wear; friction, both static and dynamic, must be sufficiently low. These characteristics depend on the mechanical properties (shape, hard- ness) of the magnetic particles, on the organic components used, and on the interaction between the particles and their organic environment. This interaction determines the physical integrity of the resulting coating and is sensitive to the surface properties of the particles. The particle/organic interface also determines the extent (and cost) of the milling process needed for dispersion and the final smoothness and uniformity of the coating. The importance of surface properties increases as particles are made smaller, because of the increase in surface-to-volume ratio. A common measure of this ratio is the so-called specific surface area, which has a value of around 30 m2/g for many standard products but can exceed 50 m2/g in newly available particles for advanced applications. Pro- prietary processes for making particle surfaces more compatible with organic components of the coating are of vital importance in the competition to make the successful particles of the future.

Finally, it must be emphasized that the coating is required to function as one component of a recording medium and, more broadly, as part of an information storage and retrieval system. This means that nonmagnetic properties such as light absorption, for optical sensing of tape or disk position, and electrical conductivity, to minimize charge accumulation, must be considered by the designer of a practical medium. The opacity and conductivity, as well as the mechanical properties, of a coating must in some cases be augmented by the addition of nonmagnetic particles. The primary needs of the system, however, are for adequate values of both the read-back signal amplitude and the ratio of the signal to noise. These requirements and the frequency bandwidths to which they apply are the major factors governing the choice of magnetic material. The system’s tolerance for defects in the signal due to inhomogeneities in the coating is also important. The continuing evolution of electronic components, and that of procedures for the encoding, detection, and error- correction of information, will have a large influence on the required properties of future magnetic media.

ACKNOWLEDGMENT

The author wishes to thank numerous colleagues at 3M for helpful information and discussion, assistance in pre-

[EEE TRANSACTIONS ON MAGNETICS. VOL. 25. NO. 6. NOVEMBER 1989

paring this manuscript, and electron microscopy. These include J. F. Carroll, R. M. Erkkila, D. C. Lowery, J. T. McKinney, C. A. Newman, K. H. Olsen, J. S. Roden, and R. S. Sapieszko.

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SHARROCK: PARTICULATE MAGNETIC RECORDlNG MEDIA 4389

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Michael P. Sharrock was born in Columbus, MS, on January 25, 1945. He received the B.A. degree in physics from the College of St. Thomas, St. Paul, MN, in 1967 and the M.S. and Ph.D. degrees in physics from the University of Illinois, Urbana, in 1969 and 1973, respectively.

From 1973 to 1977 he was a Research Fellow at the University of Penn- sylvania, Philadelphia. From 1977 to 1979 he was a faculty member in the Physics Department of Gustavus Adolphus College, St. Peter, MN. Since 1979, he has worked in the areas of magnetic materials and magnetic recording at 3M, St. Paul, MN. His research interests include the effects of cobalt in oxide particles, the magnetic behavior of barium ferrite, the recording properties of particulate media, and time-dependent magnetic effects.

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Particulate magnetic recording media: a review

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