JP3031472B2 - Magnetic disk cartridge - Google Patents

Description

The present invention relates to a magnetic disk car in which a magnetic disk is rotatably housed in a cartridge case or a flexible cartridge case formed of a hard synthetic resin. The present invention relates to a trig, and more particularly to a configuration of a magnetic layer and a cleaning sheet thereof.

[Prior Art] In recent years, in order to increase the recording density of a magnetic disk, a magnetic layer containing fine particles made of a ferromagnetic metal such as iron, cobalt-nickel, and cobalt-phosphorus has been used in recent years. Since these ferromagnetic metal powders have a lower hardness than magnetic powders made of metal oxides such as γ-Fe ₂ O ₃ , for example, the magnetic layer is significantly worn by sliding contact with a magnetic head.
Therefore, a lubricant such as oleyl oleate is added to the magnetic layer, and a predetermined amount of reinforcing powder such as aluminum oxide is added.

On the other hand, as a cleaning sheet to be adhered to the inner surface of the cartridge case, a nonwoven fabric made of a thermoplastic fiber such as acrylic fiber or polyethylene terephthalate is usually used.

[Problems to be solved by the invention]

As described above, since the thermoplastic fiber is used as the cleaning sheet, the thermoplastic fiber softens and wears due to frictional heat when it is in contact with the rotating magnetic disk for a long period of time. Reinforcing powder such as aluminum oxide present on the surface of the magnetic layer promotes abrasion of the cleaning sheet.

On the other hand, oleyl oleate used as a lubricant, especially at low temperatures (about 0 to 5 ° C.), becomes opaque and becomes semi-solid, and adhesiveness is remarkably exhibited. As a result, the abrasion powder of the above-mentioned cleaning sheet adheres relatively firmly to the disk surface, resulting in dropout.

Also, when a conventional magnetic disk cartridge is mounted on a recording / reproducing apparatus and used, there is a problem in that the ferromagnetic metal powder falls off the magnetic layer due to sliding contact with the magnetic head, and the running property of the magnetic disk is reduced. is there. In particular, a magnetic disk cartridge using a thin and flexible magnetic disk usually requires a high durability of the magnetic disk because the magnetic head clamps both sides of the rotating magnetic disk at a predetermined pressure. The wear resistance of a magnetic disk is a very important issue.

The present inventors have conducted various studies on the problem of reduction in wear resistance of the magnetic recording medium, and have found that the BET surface area of the ferromagnetic metal powder used for the magnetic disk has a large effect.

That is, in the prior art, an α-Fe metal powder having a high coercive force (Hc), that is, an α-Fe metal powder having a large average BET surface area, was used to increase the S / N value of the magnetic disk. For example, to make the coercive force about 1500 (Oe),
Fine particle α-Fe metal powder having a BET surface area of about 50 to 55 m ² / g has been used.

The fact that the average BET surface area of the α-Fe metal powder is large means that the surface area per magnetic particle is small, and therefore, the coating area of the binder to the ferromagnetic metal powder, in other words, Since the bonding area with the binder (bonding force) is small and the affinity with the binder is small, the excellent properties of the binder cannot be sufficiently exhibited. From these facts, it was clarified that the ferromagnetic metal powder was dropped from the magnetic layer while the magnetic head was in sliding contact with the magnetic layer at a predetermined pressure, and the running property of the magnetic disk was deteriorated. .

An object of the present invention is to eliminate the drawbacks of the prior art such as peeling of the magnetic layer, falling off, and generation of deposits, and to provide a highly durable and highly reliable magnet. is there.

[Means for solving the problem]

In order to achieve the above-mentioned object, the present invention provides a magnetic disk containing a ferromagnetic metal powder as a main component and at least a non-magnetic binder and a lubricant on a substrate made of a non-magnetic material, and the magnetic disk can be rotated. And a cartridge case formed of a synthetic resin molded article or sheet having a cleaning sheet made of a nonwoven fabric or the like supported on the inner surface.

And an α-Fe metal powder is used as the ferromagnetic metal powder,
The average BET surface area is regulated in the range of 30 to 48 m ² / g, and the cleaning sheet is composed of three layers, the surface layer and the back layer are rayon fiber single layers, and the intermediate layer is rayon fiber and thermoplastic fiber. Or a mixed layer of the following.

[Action]

According to the present invention, as described above, since the cleaning sheet is composed of three layers, and the surface layer and the back surface layer are made of rayon fiber alone, the cleaning sheet always comes into direct contact with the magnetic layer regardless of the front or back of the cleaning sheet. The rayon fiber layer comes to the side to be treated. Therefore, even if the magnetic disk is in contact with the rotating magnetic disk for a long period of time, the fibers do not soften and wear due to frictional heat as in the related art, and generation of abrasion powder from the cleaning sheet can be suppressed as much as possible.

Further, if both the surface layer and the back surface layer are composed of a single layer of rayon fiber, it is not necessary to distinguish between the front and back of the cleaning sheet when assembling the magnetic disk cartridge, and the assembling workability is good.

Furthermore, since the intermediate layer connecting the front layer and the back layer of the cleaning sheet contains rayon fibers, the intermediate layer is well adapted to the front layer and the back layer, and the three layers are integrated, and the intermediate layer is made of thermoplastic resin. Since the fibers contain the fibers, the cartridge case and the cleaning sheet have good heat fusion.

On the other hand, the ferromagnetic metal powder used has an average BET surface area of 48m
^Since it is an α-Fe metal powder of ² / g or less, the impact resistance of each particle is high due to the large particle size, and the surface area per particle becomes larger than that conventionally used, so that the binder and Has a large binding area and affinity. Therefore, the excellent properties of the binder are sufficiently exhibited, and the fixing strength of the ferromagnetic metal powder is strong, and even if the magnetic head slides under a predetermined load, the ferromagnetic metal powder hardly falls off and exhibits excellent running properties. .

On the other hand, if the average BET surface area of α-Fe is less than 30 m ² / g, the coercive force distribution becomes large and the electromagnetic conversion characteristics such as peak shift deteriorate. Therefore, the average BET surface area of α-Fe is 3
It is necessary to regulate in the range of 0 to 48 m ² / g.

For this reason, the durability of the magnetic disk can be significantly improved, and the reliability can be improved.

(Example of the invention)

1 to 6 are views for explaining a magnetic disk cartridge according to an embodiment of the present invention. FIG. 1 is an exploded perspective view of the disk cartridge, and FIG. 2 shows a cleaning sheet on an upper case. Bottom view showing the attached state,
FIG. 3 is a plan view showing a state where a cleaning sheet is adhered to the lower case, FIG. 4 is an enlarged sectional view showing the vicinity of a magnetic head insertion port showing a state of use of the magnetic disk cartridge,
FIG. 5 is an enlarged sectional view of the vicinity of the elastic piece of the magnetic disk cartridge, and FIG. 6 is an enlarged sectional view of the cleaning sheet.

The magnetic disk cartridge is the cartridge case 1
And a magnetic disk 2 rotatably stored therein,
And a shutter 3 slidably supported by the cartridge case 1.

The cartridge case 1 includes an upper case 1a and a lower case 1b, which are injection-molded with a hard synthetic resin such as an ABS resin.

An opening 4 for inserting a rotary drive shaft is provided substantially at the center of the lower case 1b.
Is formed, and a rectangular head insertion port 5 is provided near it, and the head case 5 is similarly provided in the upper case 1a. In the vicinity of the front surfaces of the upper case 1a and the lower case 1b, as shown in FIG. 1, recesses 6 which are slightly lower to regulate the sliding range of the shutter 3 are formed, respectively. The magnetic head insertion port 5 is opened at the bottom of the apparatus.

As shown in FIG. 2, counterbore-shaped recesses 7 are formed on the inner surface of the upper case 1a and on both left and right sides of the magnetic head insertion opening 5, respectively. The longitudinal width W1 of the recess 7 is substantially equal to the longitudinal width (longitudinal width) W2 of the magnetic head insertion slot 5,
The width W3 of the concave portion 7 is regulated to be at least 0.3 times, preferably 0.5 to 1.5 times the width (width) W4 in the direction orthogonal to the longitudinal direction of the magnetic head insertion slot 5.

A large number of linearly extending projections 8 are provided on the upstream side of the magnetic head insertion slot 5 in the disk rotation direction of the upper case 1a, and two projections 8 on both sides thereof are slightly higher than the projections 8 on the inner side.

An arc-shaped restricting rib 9 for restricting the storage position of the magnetic disk 2 protrudes from the inner surface of the upper case 1a, a part of which passes through the end of the concave portion 7, and the restricting rib 9 is a reinforcing member around the concave portion 7. It is also useful.

A substantially C-shaped cleaning sheet 10 is inserted inside the regulating rib 9. The width W5 of the opening 11 provided at a position corresponding to the magnetic head insertion opening 5 of the cleaning sheet 10 is designed to be slightly larger than the width W4 of the magnetic head insertion opening 5. By placing the cleaning sheet 10 on the inner surface of the upper case 1a,
Is also covered, and the periphery of the cleaning sheet 10 is the upper case 1a.
Ultrasonic welding. As described above, the recess 7 of the upper case 1a
Has a sufficient width, so that the edge of the opening 11 in the cleaning sheet 10 can be ultrasonically welded 12 in the recess 7 as shown in FIG.

As shown in FIGS. 3 and 5, on the inner surface of the lower case 1b and at a position substantially opposed to the protrusion 8 of the upper case 1a, a ridge support portion 13 and a ridge 14 for preventing sticking are provided. I have. As shown in FIGS. 3 and 5, the base 16 of the elastic piece 15 formed by bending the plastic sheet is attached to the inner surface of the lower case 1b and in the vicinity of the support portion 13 by appropriate means such as adhesion or heat fusion. It is fixed. Free end 17 of elastic piece 15
Is supported by the support portion 13 and its inclined state is maintained.

A regulating rib 9 is also provided on the inner surface of the lower case 1b, and a cleaning sheet 10 is disposed inside the regulating rib 9. The width W5 of the opening 11 provided at a position corresponding to the magnetic head insertion opening 5 of the cleaning sheet 10 is designed to be slightly larger than the width W4 of the magnetic head insertion opening 5. By placing the cleaning sheet 10 inside the lower case 1b as shown in FIG. 3, the elastic piece 15 is covered, and the peripheral portion of the cleaning sheet 10 is ultrasonically welded 12 to the lower case 1b.
In the vicinity of the elastic piece 15, the ultrasonic welding 12 is omitted.

When the magnetic disk cartridge is assembled by combining the upper case 1a and the lower case 1b, the cleaning sheet 10 is removed from the lower case 1b by the elastic pieces 15 as shown in FIG.
Is lifted slightly by a projection 8 provided on the upper case 1a, and the magnetic disk 2 is moved upward and downward by the cleaning sheet 10,
It is elastically clamped by 10. And magnetic disk 2
With the rotation of the disk, the disk surface is cleaned by the cleaning sheet 10.

FIG. 4 is a diagram showing a use state of the magnetic disk cartridge, in which 18 is a magnetic head.

As shown in FIG. 6, the cleaning sheet 10
It has a layered structure. That is, a disk-side nonwoven fabric layer 19 facing the magnetic disk 2, a case-side nonwoven fabric layer 20 facing the cartridge case 1, and an intermediate nonwoven fabric layer for connecting the disk-side nonwoven fabric layer 19 and the case-side nonwoven fabric layer 20. It has 21.

The disk-side nonwoven layer 19 and the case-side nonwoven layer
20, both are composed of rayon fiber alone,
There is no distinction of the back. As the rayon fiber, viscose rayon, cuprammonium rayon, acetate rayon and the like are used. Rayon fiber (stable) has a tensile strength of about 2.5-3.1 g / D, an elongation of about 16-22%, and an elongation modulus (at 3% elongation) of about 55-80%.
Specific gravity is about 1.50 to 1.52.

The intermediate nonwoven fabric layer 21 is composed of a mixed fiber layer of rayon fiber and polyamide fiber. The mixing ratio of rayon fiber and polyamide fiber (rayon / polyamide) is about 1 /
It is appropriately selected in the range of 9 to 9/1, and particularly preferably in the range of 3/7 to 7/3.

Polyamide fiber is a fiber using a polycondensate of adipic acid and hexamethylenediamine as a raw material monomer. The tensile strength of polyamide fiber is about 4.5-7.5, elongation is about 25-
The elongation modulus (at 3% elongation) is about 95 to 100%, and the specific gravity is about 1.14.

Although the cleaning sheet 10 can be composed of rayon fibers alone, an intermediate nonwoven fabric layer 21 made of a nonwoven fabric containing thermoplastic fibers is provided between the disk-side nonwoven fabric layer 19 and the case-side nonwoven fabric layer 20 as in the present invention. As a result, the cartridge case 1 can be satisfactorily thermally fused to the cartridge case 1.

As the intermediate nonwoven fabric layer 21, in addition to the mixture of rayon fibers and polyamide fibers as described above, for example, various thermoplastic fibers such as polyethylene terephthalate, polyethylene, polypropylene, polystyrene, polyvinyl chloride, and acrylic resin can be suitably used. . In particular, since rayon fibers are used for the disc-side nonwoven layer 19 and the case-side nonwoven layer 20 as described above, the intermediate nonwoven layer 21 contains rayon fibers in consideration of the compatibility between these two layers 19 and 20. It is preferable to use a mixed fiber layer.

The basis weight of the cleaning sheet 10 is not particularly limited, but is suitably about 20 to 50 g / m ² . Here, the basis weight is a value obtained by measuring the weight of five 50 × 50 cm samples, and taking the average value thereof.

An appropriate thickness of the cleaning sheet 10 is 150 to 300 μm. Specific physical properties of the cleaning sheet 10 according to the embodiment are as follows.

Although the cleaning sheet 10 has a three-layer structure as described above, when the magnetic disk cartridge is actually assembled using this cleaning sheet 10, all of the fibers that are in sliding contact with the magnetic disk 2 may not be rayon fibers. A very small portion of the thermoplastic fibers (polyamide fibers) in the nonwoven fabric layer 21 may come out of the surface of the cleaning sheet 10 and come into contact with the magnetic disk 2 due to the entanglement of the fibers. However, even in such a case, the ratio of the thermoplastic fibers sliding on the magnetic disk 2 is about 10% or less, and substantially 10% of the fibers in contact with the magnetic disk 2.
0% is rayon fiber.

Next, the composition of the magnetic layer will be described. The magnetic layer is composed of ferromagnetic metal powder, binder, lubricant, reinforcing powder, and the like.

As the ferromagnetic metal powder, a metal iron powder substantially consisting of α-Fe is used. Here, the metallic iron powder substantially composed of α-Fe is defined as, for example, Ni, Cr, Mn, Si, in addition to Fe.
This means that a small amount of metal elements such as W, Mo, and Co are dissolved to form a solid solution. Α used in this embodiment
-The physical properties of the Fe metal powder are as shown in Table 2 below.

As the binder, a vinyl chloride-vinyl acetate-vinyl alcohol copolymer, a polyurethane resin, a polyisocyanate compound, or the like is used.

The lubricant is a liquid state at 0 ° C., which is the lower limit of the operating temperature range of a magnetic recording medium such as a magnetic disk cartridge, that is, a fatty acid ester having a melting point of 0 ° C. or less and a viscosity at 20 ° C. of 30 cp or more. Those having the following general structural formula are preferred.

General structural formula R ₁ and R ₂ in the formula are each a linear or branched saturated alkyl group having 1 to 15 carbon atoms.

R ₃ is a linear or branched saturated or unsaturated alkyl group having 17 carbon atoms.

However, among the fatty acid esters represented by this general structural formula, 2-ethylhexyl oleate is not applied because of a large loss on heating.

Specific examples of the fatty acid ester include, for example, 2-heptylundecyl oleate, 2-octyldodecyl oleate, 2-methylhexadecyl stearate, 2-hexyl decyl stearate, 2-octyl decyl stearate, and 2-ethylhexa. There are decyl oleate, 2-hexyl decyl oleate and the like. Among these fatty acid esters, R ₁ in the above general structural formula particularly has 7 to 8 carbon atoms and R ₂
Has a linear or branched alkyl group having 9 to 10 carbon atoms,
For example, 2-heptylundecyl oleate (R ₁ has 7 carbon atoms and R ₂ has 9 carbon atoms) or 2-octyldodecyl oleate (R ₁ has 8 carbon atoms and R ₂ has 10 carbon atoms) Is preferred. As the fatty acid ester, one kind may be used, or two or more kinds may be used in combination. If necessary, it can be used in combination with another lubricant.

The lubricant (fatty acid ester) is added to the magnetic paint in advance to form a magnetic layer, or dissolved in an appropriate organic solvent such as toluene or n-hexane, and applied to the magnetic layer to form a magnetic layer. An appropriate amount of lubricant can be added to the magnetic layer by the method of impregnating the magnetic layer.

In any case, the addition rate of the lubricant is suitably 5 to 25% by weight based on the ferromagnetic metal powder. If the addition ratio is less than the lower limit, the effect of the lubricant will not be exhibited, and the durability of the magnetic recording medium will not be sufficiently improved. On the other hand, if it is more than the upper limit, the magnetic layer surface becomes sticky and dust easily adheres, or the magnetic recording medium sticks to the flat surface of the magnetic head, so that the starting torque increases and rotation unevenness occurs.

Further, as the reinforcing powder, for example, aluminum oxide, chromium, silicon carbide, silicon nitride, or the like is used. In this embodiment, an example using aluminum oxide is shown. The particle size of the reinforcing powder is suitably from 0.4 to 0.5 μm.

The content of the reinforcing powder is regulated in the range of 5 to 40% by weight. If the content of the reinforcing powder in the magnetic layer is less than 5% by weight, the durability of the magnetic disk is not sufficient because relatively soft metal magnetic powder is used. On the other hand, when the content of the reinforcing powder in the magnetic layer is 5% by weight or more, preferably 10% by weight or more, the durability of the magnetic disk is remarkably improved.

However, when the content of the reinforcing powder is increased, the content of other components such as the ferromagnetic metal powder, the binder, and the lubricant is necessarily limited, and desired characteristics cannot be obtained. Therefore, the upper limit of the content of the reinforcing powder must be kept at 40% by weight, and the preferable content of the reinforcing powder is 10 to 30% by weight.

Table 3 shows the preferred composition ratio of the magnetic paint applied on the base film.

After sufficiently kneading and dispersing this composition, it is applied to both sides of a 75 μm-thick polyethylene terephthalate base film so as to have a thickness of 2.5 μm to form a magnetic layer. Next, this was punched out into a disk having a diameter of 86 mm to obtain a magnetic disk.

The magnetic properties of this magnetic disk are as shown in Table 4 below.

Next, the relationship between the average BET surface area of the ferromagnetic metal powder (α-Fe) used and the wear resistance will be described.

Table 5 below shows that magnetic disk cartridges were assembled using α-Fe metal powders having various average BET surface areas, mounted on a normal recording / reproducing apparatus, and rotated at 360 rpm.
Rotate at the speed of. Then, while repeating the temperature cycle from 0 ° C to 50 ° C, the surface of the magnetic disk is
With a load of g, the sliding surface was brought into sliding contact with a magnetic head having a substantially square shape, and the number of rotations (the number of passes) until running scratches were generated on the magnetic disk surface (confirmed with the naked eye) was counted and evaluated as durability. The composition of the magnetic layer of each sample was the same as that in Table 3.

As is clear from this table, a magnetic disk using an α-Fe metal powder having an average BET surface area of 50 to 55 m ² / g as in the prior art, in the severe running test as described above, until a running scratch occurs. The number of passes is small, and the durability is not sufficient. On the other hand, as in the present invention, the α-Fe metal powder having an average BET surface area of 48 m ² / g or less has a very strong affinity and binding force between the ferromagnetic metal powder and the binder. Even in a severe running test, a sufficient number of passes (4 million times or more) can be obtained for practical use, and it can be seen that the wear resistance of the magnetic disk is excellent.

As described above, the average BET surface area of the ferromagnetic metal powder is 48 m ² / g
By setting the content below, the binding force with the binder can be significantly increased. However, if the average BET surface area is less than 30 m ² / g, the medium noise increases and the C / N value decreases, which is not preferable. Therefore, it is more preferable that the average BET surface area of the ferromagnetic metal powder be controlled in the range of 30 to 48 m ² / g.

Next, specific examples of the lubricant used for the magnetic layer will be described. Specific examples of 2-heptylundecyl oleate (HpUO) and 2-octyl dodecyl oleate (OD
oO) and the comparative 2-ethylhexyl oleate (EH
O), oleyl oleate (OO) molecular structural formula, oxygen ratio, melting point, surface tension and loss on heating
FIG. 8 shows the viscosity characteristics at each temperature in FIG.

Next, Table 6 shows the results of a study on the relationship between the average BET surface area of the α-Fe metal powder in the magnetic layer and the material of the cleaning sheet.

The constituent fibers in the table indicate constituent fibers of the surface layer of the cleaning sheet that comes into contact with the magnetic disk.
PET indicates polyethylene terephthalate, and blending 1 indicates blending of 50% rayon and 50% polyethylene terephthalate. Blending 2 shows a blending of 50% rayon and 50% acrylic. Mixed 3 is made of 50% rayon and 50% polyamide
% Blending is shown.

Accelerated durability is the durability when tested under the conditions of a head load of 40 g and a peripheral speed of 30 m / sec.

The error occurrence indicates whether or not an error has occurred during a 24-hour seek.

As is evident from this table, in each of the comparative examples, an error occurs, and the durability cannot be sufficiently satisfied. In comparison with this, the embodiment of the present invention has not only no error but also excellent durability.

Next, the relationship between the surface roughness of the magnetic disk and the wear of the cleaning sheet will be described.

In order to minimize the wear of the cleaning sheet, it is necessary to consider the surface roughness of the magnetic disk.
Surface roughness (Ra) like a conventional magnetic disk
When the surface is relatively coarse, such as 0.020 to 0.025 μm, the cleaning sheet is inevitably worn.

On the other hand, it has been found that the wear of the cleaning sheet is reduced as much as possible over a long period of time by setting the surface roughness Ra of the magnetic disk to 0.01 μm or less, preferably 0.005 μm or less.

The surface roughness Ra of the magnetic layer was measured with a stylus type surface roughness meter under the conditions of a stylus diameter of 2 μm, a stylus load of 25 mg, a cut-off of 0.08 mm and a scanning speed of 0.03 mm / sec.

The surface roughness of the magnetic layer can be maintained at a desired value by changing the surface treatment conditions (for example, the temperature and pressure of the calendar roll in the calendering apparatus and the number of rolls).

As described in the above embodiment, when the magnetic layer contains a binder made of urethane resin, even if abrasion powder is generated from non-thermoplastic fibers such as rayon fiber, the affinity with the abrasion powder is obtained. As a result, the amount of wear powder adhering to the magnetic disk is extremely small, and therefore, it is possible to help prevent the occurrence of errors.

〔The invention's effect〕

Since the present invention is configured as described above, a highly reliable magnetic disk cartridge with excellent durability can be provided.

[Brief description of the drawings]

1 to 6 are views for explaining a magnetic disk cartridge according to an embodiment of the present invention. FIG. 1 is an exploded perspective view of the magnetic disk cartridge, and FIG. FIG. 3 is a bottom view showing a state where a cleaning sheet is attached, FIG. 3 is a plan view showing a state where a cleaning sheet is attached to a lower case, and FIG. 4 is a magnetic head insertion showing a use state of the magnetic disk cartridge. FIG. 5 is an enlarged cross-sectional view near the elastic piece of the magnetic disk cartridge, and FIG. 6 is an enlarged cross-sectional view of the cleaning sheet. FIG. 7 is a diagram showing a molecular structural formula and various characteristics of each lubricant, and FIG. 8 is a diagram showing a viscosity characteristic of each lubricant. 1… cartridge case, 1a …… upper case, 1b …… lower case, 2 …… magnetic disk, 10 …… cleaning sheet, 19 …… disposed side nonwoven fabric, 20 …… case side nonwoven fabric, 21 …… intermediate nonwoven fabric .

Continuation of the front page (56) References JP-A-64-84425 (JP, A) JP-A-58-159231 (JP, A) JP-A-60-171631 (JP, A) JP-A-61-154112 (JP) JP-A-1-144213 (JP, A) JP-A-63-13121 (JP, A) JP-A-3-95720 (JP, A) JP-A-3-84725 (JP, A) 2-68714 (JP, A) JP-A-3-62316 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G11B 5/706

Claims (1)

(57) [Claims] 1. A magnetic disk having a magnetic layer mainly containing ferromagnetic metal powder and containing at least a binder and a lubricant on a base made of a non-magnetic material, and rotatably storing the magnetic disk. A magnetic disk cartridge having a cartridge case supporting a cleaning sheet on an inner surface thereof, wherein the ferromagnetic metal powder is made of α-Fe, the average BET surface area is regulated to a range of 30 to 48 m ² / g, and the cleaning is performed. A magnetic disk cartridge comprising: a sheet comprising three layers; a surface layer and a back layer each comprising a single layer of rayon fibers; and an intermediate layer comprising a mixed layer of rayon fibers and thermoplastic fibers.