METHOD OF MANUFACTURE OF IMPROVED PARTICULATE RECORDING MEDIA AND MEDIA RESULTING THEREFROM

Abstract: A magnetic recording medium is fabricated by the electrodeposition of acicular magnetic particles onto an electrically conductive substrate from a mixture that is continuously dispersed and circulated through the electrodeposition vessel. Electrodeposition is effected at a critical electric field strength so that the particles are aligned in solution, and deposited perpendicular to the substrate surface such that the medium is highly densified. By use of anisotropic particles the resultant medium is adaptable for perpendicular recording, and by use of isotropic particles the resultant medium will support both longitudinal and perpendicular recording. The present invention also teaches electrodeposition of media whose particulate packing density varies in accordance with an areally predetermined pattern. The pattern is incised in one electrode of the deposition apparatus, and the medium as deposited on the other electrode mirrors the pattern as a varying particulate packing density. The packing density varies with the electric field intensity, and the electric field varies due to the unequal interelectrode distances arising from the incisions in the electrode. The medium is then d.c. magnetized, and the density variation provides a magnetically reproducible signal in accordance with the pattern.

Description: METHOD OF MANUFACTURE OF IMPROVED PARTICULATE RECORDING MEDIA AND MEDIA RESULTING THEREFROM BACKGROUND OF THE INVENTION Field of the Invention This invention relates to magnetic recording media utilizing particulate magnetic material, and in particular to media having increased concentrations of magnetic particles. The invention, as well as the prior art, will be described with reference to the figures of which: Figs. 1, 2, 3, and 4 are illustrations of geometrical concepts useful in understanding both the prior art and the invention, Figs. 5 and 6 are illustrations of the electrodeposition of magnetic coating according to the prior art, Fig. 7 is a graph useful in understanding the principle of the invention, Figs. 8a and 8b are illustrations of particle alignment useful in understanding the invention, Fig. 9 is a block diagram of electrodeposition apparatus according to one embodiment of the invention, Fig. 10 is a perspective drawing of a pattern incised in an anode in accordance with a second embodiment of the invention, Fig. 11 is a drawing showing an expanded view of the pattern of Fig. 10, and Fig. 12 is an illustration of a portion of electrodeposition apparatus in accordance with the second embodiment of the invention. The continuing trend in digital magnetic recording is towards increased data density recorded on the magnetic- medium. This requirement translates into both the recording of more tracks per inch and the recording of shorter signal. avelengths, resulting in higher areal data packing densities. Additionally, in modern digital recording, it is necessary that the media accommodate the capability to overwrite previously recorded data; that is, new data recorded over old data must effectively erase the previously recorded data. Overwrite is a complex phenomenon, but it is known in the art that the thinner the magnetic medium the more effective the overwriting of previously recorded long wavelengths by newly recorded short wavelengths. The data recording signals, based on the particular digital encoding method employed, generally have frequency components extending over several octaves. The overwrite (i.e. erase) capability must extend, therefore, over the entire frequency spectrum of the recorded signal. Inadequate erasure of the long wavelength signal results in a remanent long wavelength magnetization of the medium which interacts with the newly recorded signals causing unwanted pulse crowding and peak shift. A solution to this problem has been the reduction of the physical thickness of the recording medium with the goal of restricting the medium thickness to depths that can be adequately overwritten by a short wavelength signal. A price is paid, however, for this solution to the problem. The signal amplitude read from the medium during playback is proportional to the magnetization of the medium, which in turn is proportional to the total volume of magnetized material. By restricting the thickness of the magnetic coating the volume of magnetizable material is reduced, and the signal amplitude is degraded. A variety of techniques have been employed in the prior art to address the volume of magnetizable material problem. In the production of media, the most common fabrication technique involves the coating of particulate magnetic material in suspension with a binder and solvent onto a substrate material. The solvent is driven off during a drying process and the medium is then compacted or "calendered" to increase the density of the magnetic layer. The final density attained is, to a large degree, dependent upon two factors: First, because the coating process involves the flow of the particle-binder-solvent mixture onto the substrate, rheological parameters controlling the dynamics of this flow establish the minimal amount of solvent needed for stable flow. When the solvent is driven off during drying, voids remain; the more solvent utilized, the greater the proportion of voids. While calendering reduces the effects of the voids, the calendering does not compact the material so as to completely eliminate the effects of the voids. Second, the geometric form of the particle itself sets a limit on the density which may be attained even if the compacting were 100 percent efficient in removing the voids. For example, considering a dispersion of identical spherical particles, the maximum packing fraction [the total volume of the spheres divided by the total volume of a cube enclosing them, expressed either as a decimal or percentage] has a theoretical maximum of 0.72. No amount of calendering can compact a medium consisting of identical spherical particles to a greater density. Fig. 1 shows the hexagonal close-packed structure of spherical particles, and the inevitable typical interstices 10, 12, 14 which give rise to the above mentioned packing fraction limitation of 0.72. Generally, magnetic recording particles are not spherical but are needle-like, or acicular, in shape. Such particles typically have aspect ratios, i.e. the ratio of the length of the major axis to that of a minor axis of the particle, ranging from two to seven. For simplicity, it is advantageous to consider acicular particles of ideal cross section, i.e. uniform cross sections which are either rectangles or circles for the entire length of the particle. Considering the ideal case of acicular particles having a rectangular cross section and aligned as shown in Fig. 2, the resultant structure has a packing fraction of 1.0. If particles of cylindrical cross section are randomly arranged in a coating, large interstices 13, 15 [*Fig. 3] will inevitably occur with attendantly reduced packing fraction. On the other hand, if such particles were perfectly aligned as shown in Fig. 4, they would have a packing fraction of 0.91 due to the typical interstices 16, 18, 20 arising from the circular cross sections. While actual particles approximate the aforementioned shapes, in reality, magnetic particles tend to be cigar-shaped and of irregular cross section. In practice, considerably lower packing fractions, of the order of 0.4, are realized in prior art fabrication of coated magnetic tapes and disks; the reasons being the presence of voids, the fact the particles are actually cigar-shaped, and the problem of not being able to completely align the particles. In addition to the previously described web coating process, it is also known in the prior art to produce magnetic coatings by electrodeposition. This technique is an application of the method utilizing the phenomenon of "electrophoresis" which is exploited in the broad field of electropainting. Electropainting has been applied to a wide variety of painting requirements, from coating the insides of metal cans to painting automobile bodies. [Yeates, R. L. "Electropainting" Teddington, Great Britain: Robert Draper, Ltd., 1966]. In Japanese Patent Publication Number 25321/1977 entitled "Electrodeposition Paint for Magnetic Recording", a mixture of magnetic particles in a water soluble acrylic polycarbonate resin emulsified uniformly in water is described for use in the electrodeposition of a magnetic coating. The Japanese technique is illustrated in Fig. 5. A vessel 42 contains a mixture 43 of solvent, binder and magnetic particles. A cathode 44 and an anode 46 are immersed in the mixture 43, and a d.c. voltage source 48 is connected between the cathode 44 and the anode 46. Each particle tends to acquire an electric charge due to the interface between the particle and the binder-solvent solution. A diffuse layer of charge then surrounds the charged particle, so that the combination of charged particle and the diffuse layer is electrically neutral. The particle moves under the electric force due to the field between the cathode and anode, and the diffuse layer tends to move with the particle to which it is attached but, at the same time, the diffuse layer is electrically attracted to the other electrode. As the particle moves, new charge builds in front of the particle in the direction of motion, and the diffuse layer dissipates in the opposite direction. [Bockris, J.M., and Reddy, A.K.N. "Modern Electrochemistry", New York: Plenum Press, 1974, Vol. 2, p. 833]. In the previously referenced Japanese Patent Publication, the electric field magnitude is indicated to be approximately 17 volts/cm. : a field su ficient to move the magnetic particles to the electrode where they deposit as a magnetic coating 41 [Fig. 6]. SUMMARY OF THE INVENTION In the electrodeposition of acicular particles to form a magnetic coating, rather than use of an electric field of just sufficient strength to deposit the particles on the substrate, the invention teaches the use of a field strength of sufficient magnitude to also align the particles with the field. * It has been discovered that in increasing the field strength, a point is reached which is dramatically characterized by a rapid and significant increase of density of the electrodeposited medium. This field strength is defined as the "critical field strength", and the resultant medium, characteristic of deposition at this field strength, has a volumetric packing fraction greater than 0.5, and typically one on the order of 0.9. Expanding on this point, it is hypothesized that at the critical field strength, the electric field not only drives the particles to the electrode on which they deposit, but also rotates the particles so that their major axes are in the direction of the electric field. This rotation occurs because the field exerts a torque on the charged particles proportional to the angle between the major axes and the field direction. The field, which is akin to that associated with a parallel plate capacitor, is essentially uniform between the electrodes. Because the directions of the electric field lines are always perpendicular to the conductive electrode surface, the particles are deposited on the electrode, which is also the medium substrate, with the particles' major axes perpendicular to the electrode and aligned parallel to each other. The graph of Fig. 7 illustrates the discovered dependence of packing fraction for acicular particles as a function of electric field strength. It will be appreciated that there is a broad span of lower value field strengths where the field is of insufficient magnitude to completely align the particles against the couple exerted by the viscose medium, and at these lower field strengths the particles arrive at the cathode with somewhat random orientations. As previously hypothesized, when the particles are completely aligned with the field the increase in coating density occurs; if the particles are only partially aligned " they will still randomly pile up on the cathode [Fig. 8a]. In the present implementation of the invention, the graph of Fig. 7 indicates that it is not until the field reaches the critical strength of approximately 325 volts/cm (76) that complete particle alignment [Fig. 8b], with an attendant increased packing fraction, abruptly occurs. The alignment of the major axes in the coating provides, as previously explained, a highly densified magnetic layer. By control of the deposition time a thin but dense magnetic layer with desirable overwrite characteristics for digital recording is obtained. In the electrodeposited coating obtained by following the teachings of the invention, the major axes are aligned perpendicular to the substrate. As is known in the art, the direction of preferred magnetization for an anisotropic magnetic particle is generally along the particle major axis, and it will be appreciated that such an electrodeposited layer using anisotropic particles is advantageously suitable for perpendicular recording wherein the recording field is perpendicular to the plane of the medium. Perpendicular recording is currently of great interest as it is particularly suitable for recording of short wavelength signals. The teaching of the invention may be applied to the fabrication of a magnetic coating useful not only for perpendicular recording but also suitable for longitudinal recording as well; the resultant longitudinal recording occurring even though the major axes of the acicular particles are perpendicular to the direction of the longitudinal recording field lines. To implement such a coating, cobalt doped acicular particles, of the type described in French Patent No. 2,129,841 and French Patent No. 2,199,155, are deposited on a substrate in accordance with the teachings of the present invention. Such doped acicular particles have the property of supporting magnetization not only along their major axes but along their minor axes as well, and therefore will support longitudinal recording as well as perpendicular recording. A second embodiment of the invention provides magnetic media incorporating integral, unalterable magnetic patterns. Such patterns usefully serve, for example, as servo information "bursts" for positioning the magnetic heads of a disk drive utilizing the medium. Such patterns are implemented by deposition of media having a controlled areal packing density which replicates an etched or machined pattern incised into the surface of the anode used in the deposition; the incised pattern being, for the present example, in the con iguration of the servo burst. By magnetizing the resultant electrodeposited medium, signals may be played back from the patterned medium whose amplitudes are proportional to the packing density variations, thereby providing servo information for system control. In implementing patterned deposition of a medium, the electrodes are disposed so the distance between anode and cathode, has two values arising from the pattern detail incised in the anode surface. The distance dl is the distance of the grooves of the incised pattern of the anode from the cathode, and the distance d2 is the distance of the lands from the cathode. In view of the relation E=V/d, where E is the electric field strength, V is the voltage between anode and cathode, and d is the distance between anode and cathode, essentially two corresponding field values El=V/dl and E2=V/d2 are established. These field intensities are calculated so that El is equal to the critical field intensity, and E2 is less than the critical field intensity. At the positions on the cathode where the field has the value El the particles are deposited with the greater packing density, and at the positions where the field has the value E2 the particles are deposited with the lower packing density, in accordance with the electrodeposition characteristic shown in the plot of Fig. 7. In this manner, the packing density in the deposited medium traces out the incised pattern of the anode. The remaining portions of the anode surface are at the distance dl from the cathode so those areas of the medium deposit at the higher packing density and are suitable for data recording. The resultant medium is then subjected to a d.c. magnetic field, and the variation in packing density gives rise to magnetized areas of essentially two magnetic field strengths. These field strength variations may be read by means of standard magnetic reproducing techniques. While the electrodeposited pattern is uneraseable or unalterable because it is integral with the recording medium,, the non-patterned__ portion of the electrodeposited medium surface may be recorded, reproduced, or erased by any of the magnetic recording and reproducing methods known in the art. DESCRIPTION OF THE INVENTION The practice of the basic invention may be further understood by reference to Fig. 9. A motor 50 drives a "sand mill" 52 of a type well known in the art, i.e. the "Mini Motor Mill" manufactured by Eiger Machinery Corp., Chicago, Illinois. Small glassbeads are employed in the continuously operating sand mill 52 to finely disperse a mixture 53 of solvent, binder, magnetic particles and additive which, for example, are employed in the following percentages by weight: Cyclohexanone as solvent 80.51 Magnetic particles 16.1 Polyurethane as binder 2.3 Surfactant 1.1 The surfactant is a complex phosphate ester free acid of the type manufactured by GAF Corp., Wayne, N.J., under the trade name "Gafac R.E. 610". This surfactant is an anionic surfactant which not.only enhances the amount of charge present on the magnetic particle, but also controls the sign of the charge. In the present embodiment the particles are charged positively. A cationic surfactant would change the sign of the charge, with the result of depositing the magnetic coating not on the cathode but on the anode. The polyurethane binder is available under the trade name "Nippolan" from Nippon Urethane Co. Ltd. , Japan. The mixture is pumped from the sand mill 52 by means of a pump 54 into a vessel 56 in which the electrodeposition takes -place. Within- the. ve&sel 56, a disk anode 58 and an equi-area disk cathode 60 (the latter serving as the substrate for the magnetic coating), are mounted on a nonconductive standoff 62 with a nonconductive spacer 64 separating the anode 58 and the cathode 60. The spacer 64 provides a separation of 1.33 cm and is located at the central area of the cathode 60; an area which requires no magnetic coating because it is not addressed during recording. The mixture flow is continuous, and the mixture 53 returns to the sand mill 52 by means of a return line 66. A d.c. voltage source 68 of approximately 432 volts is connected between the anode 58 and cathode 60 to effect the electrodeposition at a field intensity of approximately 325 volts/cm. The current flow in the mixture is derived primarily from the ionic double layer previously described, and the mixture has a high specific resistance in the range of from 10 9 to 10 ohm-cm. As a result, the current is limited by the resistance of the mixture; the mixture being substantially higher resistance than the coating being deposited, and deposition continues at constant current even as the coating builds up on the cathode. The deposition time is approximately 5 seconds, providing a coating of approximately 10 microns thick. The coated cathode 60 is then rinsed, oven dried to drive off any adherent solvent, and the resultant surface is the magnetic medium configured as a disk. ADDITIONAL EXAMPLES FOR THE PRACTICE OF THE INVENTION 1. Solvent: tetrahydrofuran, (THF) , a readily available organic solvent. Binder: DeSolite 2764-70-130 acrylate, available from De Soto Corp., Des Plaines, Illinois. Surfactant: Emcol CC55, quarternary ammonium acetate, available from Witco Chemical Co., New York, N.Y. 2. Solvent: Acetone, a readily available organic solvent. Binder: Nippolan, polyurethane, available from Nippon Urethane Industry Co. Ltd., Japan. Surfactant: Emcol CC55, quarternary ammonium acetate, available from Witco Chemical Co., New York, N.Y. 3. Solvent: Methylisobutylketone (MIBK), a readily available organic solvent. Binder: DeSolite 2764-70-130 acrylate, available from De Soto Corp., Des Plaines, Illinois, Surfactant: Gafac R.E. 610, phosphate ester free acid, available from the GAF Corp., Wayne, N.J. 4. Solvent: Methylethylketone (MEK) , a readily available organic solvent. Binder: Nippolan, polyurethane, available from Nippon Urethane Industry Co. Ltd., Japan. Surfactant: Gafac R.E. 610, phosphate ester free acid, available from GAF Corp., Wayne, N.J, Ratios for the respective above indicated mixtures may be selected within a wide variety of individual combinations, each of which is associated with a respective critical voltage. In regard to the second embodiment of the invention, the fabrication of patterned media may be understood by first considering the structure of the anode as illustrated in Fig. 10. Assume that the pattern to be replicated is comprised of servo bursts: typical patterns 32, 34 corresponding to servo bursts are showrr-±ncised in the anode 58'. (In the drawings different but related elements are identified with the same reference characters, albeit that corresponding elements in the various drawings are distinguished by the use of primes.) Fig. 11 shows the nature of the incised burst patterns 32, 34 in greater detail. As previously described, the magnetized servo bursts on the medium provide played back control signals, and these servo bursts are not recorded but are derived by laying down regions of cyclically varying packing density in the electro- deposited medium. Referring to Fig. 11, the grooves 92, 94, 96, 90, 98, 100 incised in the anode 58' and the lands 93, 95, 97, 99, 101, 103 provide the mech¬ anism by which the variation in density is achieved in a manner to be later described. It will be appreciated that the grooves 92, 94, 96, 90, 98, 100 are depressions with respect to the principal surface of the anode 58', and the pitch of the grooves is selected to correspond to the frequencies of the servo bursts. For example, grooves 92, 94, 96, 90 related to servo burst pattern 34, have twice the pitch of the grooves 98, 100 related to the companion servo burst pattern 32. The images of these grooves and associated lands in the packing density of the medium will provide servo signal frequencies read by a disk head, during disk rotation, of 2f and f, where 1/f is the interval between the times of occurrence of the signals corresponding to grooves 98 and 100. Referring to Fig. 12, portions of the anode 58" and cathode 60" are shown disposed for deposition of magnetic media by means of the apparatus of Fig. 9. A composition of the mixture 66* of particles, solvent, binder and surfactant for the electro¬ deposition of the medium has been previously described. The principal surface-78 -of. the anode 58" is essentially parallel to the cathode 60". The - perpendicular distance from the cathode 60" to the principal surface 78 of the anode 58" and to the lands 93• , 95 « , 97', 99', 101', 103 « of the pattern that are coplanar with the principal surface 78 of the anode is such that the resultant electric field at the surface of the cathode 60" is equal to the critical field. The perpendicular distance from the surface of the cathode 60" to the grooves 92*, 94*, 96', 90•, 98', 100• of the anode 58" is such that the resultant electric field at the surface of the cathode 60" is less than the critical field. An applied voltage between cathode and anode of approximately 432 volts when the distance from the principal surface of the anode to the cathode is 1.33 cm provides the critical field of 325 volts/cm. Utilizing grooves in the pattern of the anode of at least .3 cm in depth provides a field sufficiently smaller than the critical field value for deposition of the groove images at the lower packing density. Under these conditions a 10 micron thick coating mirroring the pattern incised on the anode is deposited on the cathode in approximately 5 seconds. After removal from the electrodeposition mixture, the medium is rinsed and dried. The medium is then magnetically saturated by means of a d.c. field derived either from a constant current solenoid or a fixed magnet. The pattern of varying packing density is thereby converted to a pattern of varying magnetization which is readable by conventional magnetic reproducing means.

Claims: CLAIMS 1. A method to fabricate a magnetic medium by the electrodeposition of acicular magnetic particles onto a substrate having a conductive surface, said method comprising: a. dispersing said particles in a liquid to provide a dispersion (53) wherein said particles acquire charge at the interface between said liquid and said partides, _ b. immersing electrodes" (58, 60) in said dispersion, wherein at (60) least one of said electrodes is said substrate for the electro¬ deposition thereon of said particles, and c. generating a d.c. electric field at said surface of said substrate by applying a d.c. voltage (68) across said electrodes (58, 60), said field having an intensity substantially equal to the critical field intensity (76) for aligning said particles in the direction of said field, said field cooperative with said electrodes (58, 60) and said dispersion (53) for the deposition of said aligned particles to form said magnetic recording medium on at least one of said electrodes (60) which is said substrate. 2. An improved apparatus for the electro¬ deposition of magnetic particles on a substrate to form a magnetic recording medium, said apparatus of the type wherein a d.c. electric field is established between electrodes (58, 60) immersed in a liquid containing said magnetic particles (53), whereby said particles are electrodeposited onto one of said electrodes (60) to form said magnetic recording medium, said improved apparatus comprising: a. said d.c. electric field having an intensity equal to a critical electric field intensity (76) whereby said magnetic particles are aligned perpendicular to the plane of said magnetic recording medium, and b. said liquid and magnetic particles constitute a dispersion of acicular magnetic particles in a mixture of solvent, binder and surfactant (53) having a specific resistance substantially in the range of 10 to 10 ohm-cm, with the electrical resistance of said dispersion being substantially greater than the electrical resistance of said deposited medium during said electrodeposition, whereby said electrodeposition occurs at essentially constant current. 3. A magnetic recording medium comprising: a. a substrate (60) having at least one electrically conductive bounding surface, and b. at least one layer of acicular magnetic particles disposed relative to said surface wherein the volumetric packing density of said layer is areally controlled in a predetermined manner. 4. The medium of Claim 3 wherein said volumetric packing density is essentially constant. 5. The medium of Claim 3 wherein said volumetric packing density has either a first value or a second value; said first value being greater than 0.5, and said second value being less than 0.5. 6. A method of fabricating a magnetic medium by means of an electrodeposition apparatus utilizing particulate magnetic material and characterized by a critical field intensity (76), wherein the packing density of said magnetic medium varies in accordance with a predetermined pattern (32, 34), said method comprising: a. replicating said pattern (32, 34) by means of grooves (90', 92', 94', 96', 98', 100') and lands (93', 95', 97', 99', 101') in a first electrode (58' •) of said electrodeposition apparatus, b. positioning a second electrode (60'') in said electrodeposition apparatus in opposition to said first electrode (58'') and essentially parallel to said first electrode (58''), wherein the electric field between said grooves (90', 92', 94', 96', 98', 100') of said first electrode (58* « ) and s'aid -second e-lectrode (60'') has a first magnitude less than said critical electric field intensity (76) of said electrodeposition apparatus, and wherein the electric field between said lands (93', 95', 97', 99 « , 101') of said first electrode (58'*) and said second electrode (60*') has a second magnitude essentially equal to said critical electric field intensity (76) of said electrodeposition apparatus, and c. electrodepositing said medium on said second electrode (60*') under control of • said electric field having said first and second magnitudes, whereby said pattern is imaged as a corresponding variation of said packing density of said medium.

Inventors: BYE MATTHEW ROY, JAGANNATHAN RAMESH, BYE, MATTHEW, ROY, JAGANNATHAN, RAMESH
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1987
Main classification: G11B5
IPC codes: G11B5