WO1998019300A1

WO1998019300A1 - Particulate magnetic medium utilizing keeper technology and methods of manufacture

Info

Publication number: WO1998019300A1
Application number: PCT/US1997/016004
Authority: WO
Inventors: David H. Davies; Thomas M. Coughlin; Beverley R. Gooch
Original assignee: Ampex Corporation
Priority date: 1996-10-31
Filing date: 1997-09-10
Publication date: 1998-05-07
Also published as: US5843565A; AU4339597A

Abstract

The flexible, particulate magnetic storage medium includes a substrate and a magnetic storage layer of hard magnetic particles held within a binder. The flexible, particulate magnetic storage medium also includes a soft magnetic keeper layer. The soft magnetic keeper layer may be established by coating the individual hard magnetic particles of the magnetic storage layer with a layer of soft magnetic material to establish a 'keeper layer' around each particle. A break layer of non-magnetic material may be provided between the hard and soft layers. The individual particles are coated by methods such as in situ precipitation. The soft magnetic layer may also be established by providing a separate layer of soft magnetic material within a binder of cross-linkable resins. The resulting flexible, particulate magnetic storage medium is a laminate principally comprising a substrate, a first layer of hard magnetic particles within a binder, and a second layer of soft magnetic particles also within a binder. The laminate medium may be produced through extrusion of binder particle mixes through multi aperture slot die extensions. The flexible, particulate magnetic media may be used in tape or flexible disc formats.

Description

PARTICULATE MAGNETIC MEDIUM UTILIZING KEEPER TECHNOLOGY AND METHODS OF MANUFACTURE

CROSS REFERENCE TO RELATED APPLICATIONS

This application contains subject matter related to the following commonly assigned,

co-pending U.S. patent application: Serial Number 08/674,768 filed June 28, 1996,

designated attorney docket number 112008-23/AMP-3860, entitled "Magnetic Storage and

Reproducing System With a Low Permeability Keeper and a Self-Biased Magnetoresi stive

Reproduce Head".

TECHNICAL FIELD

The present invention relates to magnetic recording and reproducing systems, and in particular to a magnetic tape recording and reproducing system having a keepered particulate magnetic storage medium.

BACKGROUND OF THE INVENTION

In conventional wideband, high density magnetic signal processing, magnetic flux

transferred to or from a flexible particulate magnetic tape storage medium permeates a

magnetic core of a magnetic transducer (i.e., a head). During reproduction operation modes

this flux produces an induced output voltage which, after suitable amplification, is a

reproduced representation of the magnetic flux from the media that permeates the core and is

suitable for use by a utilization device. During record operation modes, the permeating flux

results from current applied to the transducer coil winding, and the flux fringes from a

physical gap provided in the core for recording a representative signal in the magnetic storage

medium. One problem with prior art magnetic tape and flexible disc storage systems is that

various losses occur during signal transfers between the flexible storage medium and the

transducer. One of the more significant losses, called "spacing loss", results from the

physical spacing between the flexible medium and the transducer. Spacing loss is particularly

deleterious during reproduction operations where the effects of such loss are more significant.

Prior efforts to reduce spacing loss primarily involved reducing the physical spacing by

placing the transducer as close to the magnetic storage medium surface as operating

conditions permitted. Such positioning, however, is accompanied by an increase in the

likelihood of collisions between the transducer and the medium, particularly in devices in

which the transducer is normally supported above and out of contact with the storage medium

surface, i.e., the transducer "flies" relative to the storage medium. On the other hand, if the

transducer is in physical contact with the medium, damaging wear occurs due to the contact.

However, it should be noted that if contact heads are used, the head is still separated from the

particulates of the storage medium by several factors. Along with the physical space between

the head and the medium, there are several material phenomenon which contribute to an

effective spacing which is larger than the physical spacing. Several of those phenomenon

shall now be discussed.

During fabrication the tape or flexible disc medium is converted from a "green" state

to a more fully cured and calendered form which is smoother and more durable in terms of

head to tape wear. As known, the calendering process involves the compaction of the particle

matrix in the cross linked binder. This compaction is performed at high temperature and pressure, and results in a resin surface where the magnetic particles are somewhat buried

under the surface. In essence this creates a non-magnetic layer between the head gap and the

medium. Measurements have shown this layer to be as much as 1-2 micro inches in thickness

depending on the magnetic medium chemical formulation, calendering conditions, etc.

A second material phenomenon which undesirably increases the effective spacing is

the microscopically rough, non-planar surface of the flexible particulate magnetic medium.

This roughness originates from the particulate nature of the film, the difficulty in dispersion

of the particles in the binder and the asperities derived from the base film. The transducer

will therefore tend to ride on the tops of the asperities and this in effect creates an additional

separation of, on average, one (1) microinch.

A third material phenomenon is the friction build up at high temperatures that result

as the flexible medium and head pass each other at the relative high speeds associated with

modern storage systems. If head wear is minimal, then over time the degradation products of

the particulate binder of the medium, and the head materials, will bond into a resilient film

referred to as a "headfilm". This film is often thought to reach 0.5 microinches and is known

to cause poor read/write performance typical of increased spacing loss.

Another factor which contributes to the effective spacing is the use of lubricants. As

known, lubricants are added to such magnetic medium formulations to minimize friction

build up. However, the lubricants cause additional spacing loss due to the fact that further

separation occurs as the medium ages and lubricant migrates out of the particle matrix. This

adds about another 0.1 micro inch to the spacing. Yet another factor is that since the hard magnetic particles are acicular in shape and

made of iron, they are pyrophoric, and therefore, are coated with a very thin layer of inert

ceramic material. This, of course, effectively further separates the magnetic layer from the

transducer.

These separate factors combine to establish an effective spacing between the magnetic

particles and the transducer that is larger than the actual spacing between the transducer and

the top surface of the medium.

A problem with the large effective spacing between the head and the medium is

decreased output signal strength and increased intersymbol interference.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a flexible, particulate

magnetic storage and reproducing system with an improved storage density through improved

system signal-to-noise ratio and reduced intersymbol interference.

According to the present invention, a magnetic storage system comprises a read/write

transducer and a flexible, particulate magnetic storage medium. The particulate storage

medium includes a flexible substrate and a magnetic storage layer of hard magnetic particles

within a binder. The magnetic storage medium also includes a soft magnetic keeper layer.

The particulate magnetic storage medium can be either a magnetic tape or a flexible disc.

The soft magnetic keeper layer is established by coating the individual hard magnetic

particles of the magnetic storage layer with a layer of soft magnetic material to establish a

"keeper layer" around each particle. The coating of soft magnetic material is applied so the

thickness of the keeper layer is sufficient to establish a permeability which is preferably

V greater than about two (2), where the permeability of air is one. A separation layer of non¬

magnetic material may be provided between the hard and soft layers.

A preferred method for conformally coating each hard magnetic particle with a soft

layer is in situ precipitation. Other suitable techniques for providing the soft layer coating

include spray coating the hard magnetic particles directly and plasma deposition methods.

The soft magnetic keeper layer may also be established by providing a separate layer

of soft magnetic material in particle form within a binder of cross linkable resins. The

resulting flexible particulate magnetic storage medium is a laminate principally comprising a

substrate, a first layer of hard magnetic particles within a binder, and a second layer of soft

magnetic particles also within a binder. This soft magnetic keeper layer may be positioned

above or below the magnetic storage layer.

The keeper layer preferably has a relatively low permeability (e.g., less than 1000).

When operating in an unsaturated state, the relatively soft magnetic material of the keeper

layer acts as a shunt path for flux emanating from recorded transitions on the magnetic

storage layer, thereby producing an image field of the recorded transitions in the relatively

soft magnetic material which has the effect of reducing the demagnetization, and thus

reducing the recorded transition length. This shunt path substantially reduces the flux levels

emanating from the recorded transitions and reaching a transducer head of the system. The

shunt path also increases the stability of the recorded transitions with respect to thermal

demagnetization.

To read data from a recorded transition on the magnetic storage layer, a saturating bias ζ current is applied to windings of the head, creating a bias flux of sufficient strength and

direction so as to saturate a portion of the soft magnetic material proximate that transition.

While saturated or driven close to saturation, this portion of the soft magnetic material can no

longer shunt flux emanating from the recorded transition. This allows substantially all of the

flux from the recorded transition to couple to the head.

Data representative of those recorded transitions can only be reproduced when the

bias flux is applied to saturate the associated portions of the keeper layer and, thereby,

terminate the shunt. The shunting of flux by the keeper also reduces the side fringing fields

which facilitates obtaining higher track density in the recording system.

The keeper layer acts to mitigate the negative effects of the head to tape separation

described above. In effect, with the keeper layer, the head and the medium are in much closer

magnetic contact than they are in physical contact. This is the result of the imaging effects of

the keeper layer, and the concentrating effect of the keeper aperture that channels flux from

the transition being read, thus making it a closer approximation to the flux from a true contact

head. In addition the shielding effect of the keeper prevents adjacent transitions from being

read by the head in the same way that a closer head would accomplish.

These and other objects, features and advantages of the present invention will become

more apparent in light of the following detailed description of preferred embodiments thereof,

as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic cross sectional illustration of a flexible, particulate magnetic y storage and reproducing system in accordance with the present invention;

Fig. 2 is a cross sectional illustration of the magnetic storage layer and the plurality of coated magnetic particles therein;

Fig. 3 is a cross sectional illustration of one of the plurality of coated magnetic

particles illustrated in Fig. 2;

Fig. 4 is a plot of image efficiency versus permeability;

Fig. 5 is a schematic cross sectional illustration of a keepered flexible, particulate

magnetic tape storage medium and a portion of a transducer having a non-zero bias current

applied to a transducer pole winding which saturates a portion of the keeper to form an

aperture region;

Fig. 6 is a schematic cross sectional illustration of an alternative embodiment

flexible, particulate magnetic storage and reproducing system featuring a distinct particulate

keeper layer;

Fig. 7 is a cross sectional illustration of a portion of the flexible, particulate magnetic

storage medium illustrated in Fig. 6;

Fig. 8 is a schematic illustration of the slot die coating technique used to fabricate the

magnetic storage medium illustrated in Fig. 6;

Fig. 9 is a schematic cross sectional illustration of another alternative embodiment

flexible, particulate magnetic storage medium; and

Fig. 10 is a schematic illustration of the slot die coating technique used to fabricate

the alternative embodiment magnetic storage medium illustrated in Fig. 9. DETAILED DESCRIPTION OF THE INVENTION

Referring now to Fig. 1, a flexible, particulate magnetic storage and reproducing

system 20 is illustrated comprising a magnetic transducer 22 which writes data to and reads

data from a flexible particulate storage medium 24 (which can be in a tape or flexible disc

format). The transducer 22 comprises poles 26, 27 which form a gap 28, and wherein an

electrically conductive winding 30 is disposed about one of the poles. Although the

transducer 22 is shown for ease of illustration as a thin film head, one of ordinary skill will

appreciate that other head designs such as ferrite, or a magnetoresistive (MR) head may

also be used.

The flexible, particulate magnetic storage medium 24 includes a flexible substrate

32 and a magnetic storage layer 34. The substrate 32 is a non-magnetic material such as

polyethylene terephthalate (PET). The magnetic storage layer 34 is segmented into a

plurality of record regions 37-40 which define record transitions 41 at their abutting

boundaries. Either digital or analog signals may be recorded in the magnetic storage

medium in a variety of conventional manners known in the art. In the illustrative

embodiment, digital signals are preferably recorded in the magnetic storage layer in

longitudinal fashion, wherein, each record region 37-40 is suitable for storing encoded bits

of data. Referring to Fig. 2, the storage layer 34 includes a plurality of fine acicular shaped

particles 42 of a high coercivity, hard magnetic material, such as iron within a resin binder 43. The binder 43 should disperse the particles 42 and provide adhesive bonding to the substrate 32. The layer 34 is preferably chosen to have a longitudinal anisotropy which provides record magnetization which is predominantly longitudinal (i.e., horizontal) to the paper as oriented in Fig. 1. As shown in Fig. 1, the magnetization polarity of each record region 37-40 is represented by horizontal solid arrows, wherein the arrow direction is indicative of the polarity of the magnetization in each region. Referring again to Fig. 2, according to an aspect of the present invention, each of the magnetic particles 42 in the magnetic storage layer 34 is coated with a layer of soft magnetic material.

Fig. 3 is a cross sectional illustration of an individual hard magnetic particle 42

comprising a fine acicular shaped iron particle 45 coated with a thin layer of non-magnetic decoupling material (e.g. , oxide or ceramic) to establish a magnetic break layer 44. The particle 42 also includes a layer of soft magnetic material 46, which is referred to hereinafter as a "keeper layer". The soft magnetic material of the keeper layer 46 has preferably a relatively low permeability, which in combination with adjacent keepers, images each of the individual magnetic transitions 41 (Fig. 1) and can be saturated by a

small bias flux. However, the soft magnetic material does not saturate when the flux from the particles is the only flux acting on the keeper layer (i.e., when the bias flux is not applied). Suitable keeper layer materials include permalloy, sendust, super sendust, alloys of iron with nitrogen, and alloys of sendust with nickel.

The characteristics of the keeper layer 46 are selected to ensure that in the absence of a bias flux from the winding 30 (Fig. 1), the layer 46 shunts flux from the record regions 37-40 to create a magnetic image in the keeper. Fig. 1 illustrates the case where

the bias current I_bias through the winding 30 is zero. In this situation, the keeper operates

as a shunt, establishing a inverse image of the magnetization in the record regions. For

example, each keepered coating within record region 38 conducts flux (shown as a dotted

line) which forms an inverse image of the remnant magnetization associated with the hard

magnetic particles 42 within the recorded region 38. The quality of the image (and

therefore the effectiveness of the shunt) can be characterized by an image efficiency which

is graphically illustrated in Fig. 4 as a function of the keeper layer permeability. The

image efficiency is about 75 % for a permeability of approximately seven (where

permeability of air is one), and it approaches 100% for permeabilities above one-hundred.

The image efficiency indicates the effectiveness of the keeper layer as a shunt. As the

image efficiency approaches 100%, the more effective the keeper layer is as a shunt, and

therefore, fewer fringing fields emanate from the magnetic storage medium 24. "Low

permeability" includes permeabilities of less than about 1000, and preferably the

permeability of the keeper layer is less than about 100 in unsaturated portions of the

keeper.

Referring again to Fig. 3, the keeper layer has a thickness X (50) which is sufficient to

achieve adequate magnetic permeability, which is preferably greater than two (2) relative to a

standard of air at 1. The typical keeper layer thickness X is preferably greater than about 75

Angstroms, and a practical upper limit on the thickness is set by the desire to limit the

dilution of the hard magnetic packing layer which reduces the total magnetization moment. Such a limit may be about 200 Angstroms.

The magnetic break layer 44 decouples the hard magnetic particle 45 and the keeper

layer 46. The break layer 44 should be greater than about 5 Angstroms in order to provide

adequate decoupling, and is preferably less than about 50 Angstroms to avoid transmission

losses in the imaging of the magnetic components. Suitable materials for the break layer 44

include carbon and oxides such as aluminum oxide, chromium oxide and ceramic oxides. It

should be noted that the fine acicular iron particles 45 used for advanced high density

recording are often already coated with an outer layer to provide oxidation protection, and

this layer may also satisfy the criteria for a suitable break layer.

The coated particles of the flexible, particulate storage medium according to the

present invention may be produced using the method of in situ precipitation using an "oxine"

reagent such as 8-hydroxyquinoline. The "oxine" reagent may be prepared by dissolving two

grams of oxine in 100 ml. of 2N-acetic acid and then adding ammonia solution dropwise until

a turbidity begins to form. The resultant solution is then clarified by adding a small amount

of acetic acid. This solution is stable for long periods, particularly if it is stored in an amber

bottle. An alternative method of preparing the "oxine" reagent is to dissolve two grams of

oxine in 1200 ml. of methyl or ethyl alcohol (note, this reagent cannot be used for the

precipitation of aluminum to be discussed below) or in acetone. This alternative method

produces a reagent which is stable for approximately ten days if protected from light.

To perform the precipitation method, the soft layer metal mixture is dissolved in an

appropriate solute, dilute acid such as hydrochloric is one choice, in appropriate alloy ratios such as 6 wt% Si, 4 wt% Al, 3 wt% Ni and 87 wt % Fe. Next, the oxide coated magnetic

particles are added to the mixture and dispersed uniformly therein. The dispersion should be

maintained as a uniform suspension within the mixture, by for example agitation or an

equivalent. An appropriate organic complexing agent such as "oxine" or alternatively 4-

methylnioxime, or any other agent that forms an organometallic complex with the metal is

then added. The next step is to precipitate the organometallic complex, preferably by

changing the pH of the mixture, and then drying the resulting complex mixture and heating it

in an inert atmosphere to decompose the organometallic complex back into the metal. The

coated magnetic pigment is then redispersed and applied to the flexible substrate as

"magnetic paint".

This method results in a coating that has precipitated preferentially on the magnetic

particle surface and hence is in intimate contact with the magnetic particle. It should be noted

that microdispersed particles of the soft magnetic material may be interspersed around and

between the coated particles 42.

In a preferred method, nickel, aluminum and iron may be dissolved separately, each in

the presence of hard magnetic material, and then combined in selected ratios to create the

desired alloy composition. Specifically, to precipitate iron, 25 ml. of a ferric solution

(containing about 0.03 g. of Fe) is treated with a dilute ammonia solution until a faint

precipitate persists, and the precipitate is then dissolved in a minimum volume of N-

hydrochloric acid. A solution of 3 g. of ammonium acetate in 125 ml. of water is then added,

followed by oxine solution (2% in N-acetic acid) with a constant stirring until an excess is present, 12-15 ml. is required. The precipitate is then digested at 80-90°C for 90 minutes,

filtered, washed with water, and dried to a constant weight at 130-140°C.

To precipitate aluminum, 25-50 mg. of aluminum is added to 1-2 ml. of concentrated

hydrochloric in a volume of 150-200 ml. Then 0.45 g. of ammonium aluminum sulfate is

dissolved in water which contains about 1.0 ml. of concentrate hydrochloric acid. The

solution is then diluted to about 200 ml. Next, 5-6 ml. of oxine reagent (a 10% solution in

20% acetic acid) and 5 g. of urea are added to the solution, and the breaker containing the

solution is covered and heated at 90°C for approximately 90 minutes. The precipitation

process is complete when the supernatant liquid originally a greenish yellow becomes a pale

orange yellow color. After cooling, the precipitate is collected, washed with water, and then

dried at l30-140°C.

To precipitate nickel, a solution of nickelous salt (150 ml. with up to 0.1 g. of Ni)

containing 3-5 g. of sodium acetate and 8-10% of acetic acid, is warmed to 70°C, treated with

an excess 2% alcoholic oxine solution, and then boiled until the precipitate of Ni(C₉H₆ON)₂

becomes granular. Next the precipitate is collected, thoroughly washed with water and dried

at 130-140°C.

The keeper layer 46 can also be used as a protective coating by selection of the

appropriate material (e.g., Supersendust, 6 wt% Si, 4 wt% Al, 3 wt% Ni and 87 wt % Fe.)

As set forth above, the associated method used to make this kind of coating is in situ

precipitation of the soft magnetic material using the conventional magnetic particle surface as

the nucleation site.

Iβ It is contemplated that an alternative coating method may include precipitation of

metal carbonates. This can be achieved for several metals including cobalt, through for

example, the addition of a soluble carbonate such as sodium carbonate to a solution of metal

salt. The resulting metal carbonate is then heated in an appropriate atmosphere to chemically

reduce the coating to the metal. Other alternative coating techniques include spray coating

the particles directly and plasma spray deposition. In general, a number of coating techniques

may be employed to coat the particle with the keeper layer, and in general these techniques

have the following characteristics: (1) the soft layer materials should be in solution, and the

solute should be non-interactive with the particles of the hard magnetic material, i.e., that they

are undissolved, not chemically reactive and do not become agglomerated by the solvent; (2)

the precipitation reaction should be such that the soft magnetic material forms selectively on

the hard magnetic particle surface, although it need not be exclusively on the surface; and (3)

the precipitate should be such that the metal components can be regenerated without

degradation and with a reasonable degree of purity.

Referring to Fig. 5, during reproduction operations, a DC bias current is applied to

the winding 30 to create a bias flux 58 which permeates and saturates the soft magnetic

material which encapsulates the hard magnetic particles in the magnetic storage layer 34

between the poles 26, 27, to establish a saturated aperture region 60. Since the aperture

region 60 is saturated by the bias flux 58, the shunt path established through that portion

of the keeper is substantially terminated. Significantly, as the flexible medium is moved

past the head and a record transition 41 is passed "through" the saturated aperture region 60, flux from the record transition 41 fringes out of the aperture region 60 and induces a

head output voltage indicative of the data represented by the record transition. The

saturated aperture region 60 operates as an opening, through which flux from the magnetic

storage layer 34 is allowed to pass because of the saturated nature of the region 60.

The keeper layer may also be established as a distinct, thin layer of soft magnetic

material that is synthesized in a particulate form, and applied over the surface of the

conventional hard magnetic layer before the combined layers are dried and calendered. It

should be noted that such a keeper layer may be positioned either above or below the hard

magnetic layer. Referring to Fig. 6, an alternative embodiment magnetic storage medium 70

includes a plastic substrate 72 such as polyethylene terephthalate (PET) over which is coated

a mixture of hard magnetic particles embedded in a binder of cross linkable resins to establish

a hard magnetic layer 74. Other ingredients such as plasticizers, lubricants, head cleaning

agents etc. may be added to the layer as known in the art. The hard magnetic particles may be

iron, coated with a protective ceramic coating or iron oxide doped with cobalt. According to

another aspect of the invention, overlaying this protective ceramic coating is a distinct keeper

layer 76 of soft magnetic material that is applied in a manner such that the keeper layer is in

intimate magnetic contact with the hard magnetic layer 74, yet physically separated from it by

a thin break layer 78.

Referring to Fig. 7, the distinct keeper layer 76 is comprised of a plurality of soft

magnetic particles 80 that have a coercivity of less than about 20 Oe in the frequency range of

interest, and a permeability which is greater than about two (2) where the permeability of air

IS is defined as one. The size of each particle 80 is such that the average does not differ greatly

from the average size of particles 82 within the hard magnetic layer 74 (i.e., within one order

of magnitude on either side of the mean). In addition, the saturation magnetic moment (m_s)

of the keeper layer 76 is comparable to the remnant moment (m_r) of the hard layer 74.

Further, the keeper layer 76 should not be commingled with the hard layer 74, and therefore

the break layer 78 is at least about 5 Angstroms thick.

Fig. 8 is a schematic illustration of a multislot aperture die slot coating technique used

to fabricate the flexible particulate magnetic storage medium illustrated in Fig. 6. As shown,

the hard magnetic layer and the keeper layer are simultaneously applied to the substrate 72 by

extrusion through two small slot apertures 84, 86 that are in contact with the moving flexible

web. By control of the rheology and flow properties of the two mixes and selection of the

appropriate aperture sizes 84, 86 and the relative pressures, the keeper layer 76 is overlaid

onto the hard magnetic layer 74, and not commingled. Such a coated web can be slit or

punched into tape or flexible disc format as desired. The break or decouple layer 78 can be

provided by adding a third aperture or slot to the die shown in Fig. 8. Layer 78 would be

extruded between hard layer 74 and keeper layer 76. Alternatively, as shown in Fig. 8, this

decoupling layer may occur 'naturally' through the formation of a "resin rich" surface at the

upper surface of the hard layer 74. A "resin rich" surface is shown in Fig. 2 in the top region

of the magnetic storage layer 34.

flexible, particulate magnetic storage medium 110 comprising several layers of alternating hard and soft materials. Specifically, the medium 110 includes a low frequency recording

layer 114, a first keeper layer 116, a high frequency recording layer 118 and a second keeper

layer 120. Such a structure is advantageous where both high and low frequency information

is written to the storage medium 110. Significantly, the lower frequency information is

recorded deeper into the storage medium 110 onto lower coercivity particles of the low

frequency recording layer 114. The high frequency information would be recorded onto

smaller, higher coercivity hard magnetic particles within the high frequency recording layer.

Each of the keeper layers includes a plurality of soft magnetic particles (not shown) of

relatively low permeability held within a resin binder. As set forth above, break layer is

preferably provided between the record layer and the keeper layer (not shown). Fig. 10 is a

schematic illustration of a multislot aperture die slot coating technique used to fabricate the

flexible, particulate magnetic storage medium illustrated in Fig. 9.

It is contemplated that other layers may be used, including inert non-magnetic layers

that are positioned to provide physical integrity to the entire multilayer combination.

Although the present invention has been shown and described with respect to

preferred embodiments thereof, it should be understood by those skilled in the art that

various other changes, omissions and additions to the form and detail thereof may be made

therein without departing from the spirit and scope of the invention.

What is claimed is:

n

Claims

1. A flexible, particulate magnetic recording medium, comprising:

a substrate; and

a magnetic storage layer disposed on said substrate for storing magnetic

signals, including magnetic particles held within a binder, wherein a plurality of

said magnetic particles are coated with a layer of soft magnetic material.

2. The flexible, particulate magnetic recording medium of claim 1, wherein said soft

magnetic material has a DC permeability of less than about 1000 when said soft

magnetic material is unsaturated.

3. The flexible, particulate magnetic recording medium of claim 2, wherein a non¬

magnetic break layer is located between said magnetic particles and said layer of

soft magnetic material.

4. The flexible, particulate magnetic recording medium of claim 3 further comprising:

a second magnetic storage layer proximate to said magnetic storage layer,

for storing low frequency information, and including magnetic particles held within

a binder, wherein a plurality of said magnetic particles of said second magnetic

storage layer are coated with soft magnetic material.

5. The flexible, particulate magnetic recording medium of claim 4 wherein said i i coating of soft magnetic material around said soft magnetic particles in said first and second magnetic storage layers establishes a keeper layer around each of said soft magnetic particles wherein the permeability of each keeper layer is greater than about two (2).

6. A flexible, particulate magnetic recording medium, comprising: a non-magnetic substrate; a magnetic storage layer, including hard magnetic particles having relatively

high magnetic coercivity for storing information therein as magnetic states, and held within a binder; and a layer of magnetic material in intimate magnetic contact with said magnetic storage layer, and including soft magnetic particles having a relatively low magnetic

permeability and a relatively low magnetic coercivity and whose thickness relative

to that of said magnetic storage layer is such that the density of magnetic flux required to saturate said soft magnetic particles is less than the density of magnetic flux required to alter the magnetic states of said hard magnetic particles in said magnetic storage layer.

7. The flexible, particulate magnetic recording medium of claim 6 further comprising a non-magnetic break layer located between said magnetic storage layer and said layer of magnetic material.

8. The flexible, particulate magnetic recording medium of claim 7, wherein said magnetic storage layer is located over said substrate.

9. The flexible, particulate magnetic recording medium of claim 7 wherein said layer of soft magnetic material is located over said substrate.

10. The flexible, particulate magnetic recording medium of claim 7, wherein said soft magnetic particles have a coercivity on the order of 20 Oersteds.

11. The flexible, particulate magnetic recording of medium of claim 7, wherein said layer of magnetic material has a permeability greater than about two (2)

12. The flexible, particulate magnetic recording medium of claim 7 further comprising:

a second magnetic storage layer including hard magnetic particles for storing information therein as magnetic states, wherein said hard magnetic particles are held

within a binder; and

a second layer of magnetic material, in intimate magnetic contact with said second magnetic storage layer, wherein said second layer of magnetic material comprises soft magnetic particles and whose thickness relative to said second magnetic storage layer is such that the density of magnetic flux required to saturate said soft magnetic particles of said second layer of magnetic material is less than the density of magnetic flux required to alter the magnetic state of said hard magnetic

Ao particles in said second magnetic storage layer.

13. A method of forming a flexible particulate recording medium, comprising the step

of: providing a substrate; depositing a magnetic storage layer onto said substrate, wherein said magnetic storage layer includes hard magnetic particles held within a binder, and a plurality of said magnetic particles are coated with a layer of soft magnetic material.

14. The method of claim 13, wherein said step of depositing includes depositing said magnetic storage layer by die extrusion.

15. The method of claim 14 wherein said hard magnetic particles include fine iron

particles.

l