JPH0395719A - Magnetic disk cartridge - Google Patents

Abstract

PURPOSE:To prevent the peeling and dislodging of magnetic layers and the generation of deposits and to improve durability and reliability by consisting ferromagnetic metallic powder of alpha-Fe, the average BET whose surface area is <=48m<2>/g and constituting the disk-side surface layer to be brought into direct contact with the magnetic layer of nonthermoplastic fibers. CONSTITUTION:The alpha-Fe metallic powder is used as the ferromagnetic metallic powder and the average BET surface area thereof is <=48m<2>/g. Almost all of at least, the disk side surface layer 19 of a cleaning sheet 10 brought into direct contact with the magnetic layer is constituted of the nonthermoplastic fibers, such as, for example, rayon fibers. The softening and wear of the fibers by friction heat are, therefore, obviated in spite of the long-term contact with a rotating magnetic disk and the generation of the worn powder from the cleaning sheet is suppressed. On the other hand, the ferromagnetic metallic powder to be used is the alpha-Fe metallic powder having a relatively large grain size and, therefore, the adhesive area to a binder and the tacky adhesive force are large and the excellent characteristics of the binder are sufficiently exhibited. Thus, the durability of the magnetic disk is greatly improved and the reliability is improved.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a magnetic disk cartridge in which a magnetic disk is rotatably housed in a cartridge case made of hard plastic or resin or a flexible cartridge case. In particular, it concerns the structure of its magnetic layer and cleaning sheet 1-.

[Prior Art] In order to increase the recording density of magnetic disks, in recent years, magnetic layers containing fine particles of ferromagnetic metals such as iron, cobal-1-nickel, and cobal-1-1-phosphorus have been used. Stay safe. These ferromagnetic metal powders are, for example, γ-F
This is because it has lower hardness than magnetic powder made of metal acids and arsenics such as e x O j. Significant wear of the magnetic layer due to sliding contact with the magnetic head. For this reason, a lubricant such as oleyl oleate is added to the magnetic layer. A predetermined amount of reinforcing powder such as aluminum oxide is added.

On the other hand, a non-woven sheet made of thermoplastic fibers such as acrylic fibers and polyethylene terephthalate is normally used as the non-woven sheet to be attached to the inner surface of the car/rig case.

[Problem to be solved by the invention]

As mentioned above, since thermoplastic fibers are used as the cleaning sheet, if they are in contact with a rotating magnetic disk for a long period of time, the thermoplastic fibers will become soft and worn due to frictional heat. In addition, l{11 strong powder such as aluminum oxide existing on the surface of the magnetic layer accelerates the wear of the cleaning sheet 1-. When used as a lubricant, Oleyluo L Note becomes cloudy and semi-solid, especially at low temperatures (about 0 to 5°C). Adhesion becomes noticeable.

As a result, the abrasion powder from the cleaning sheet mentioned above adheres relatively firmly to the disk surface, resulting in a Trop-out.Furthermore, when a conventional magnetic disk cartridge is installed in a recording device, the magnetic head There is a problem in that ferromagnetic metal powder falls off the magnetic layer due to sliding contact with the magnetic disk, reducing the running performance of the magnetic disk. Particularly thin and flexible fJ. Magnetic disk cartridges that use 6J&K disks usually require the durability of the magnetic disk to be as high as 1α since the magnetic head holds both sides of the rotating magnetic disk with a predetermined pressure. Abrasion resistance is a very important issue.

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

That is, in the conventional magnetic disk, in order to increase the S/N value of the magnetic disk, α-Fe metal powder having a high coercive force (He), that is, α-Fe metal powder having a large average BET surface area, has been used. For example, the coercive force is 1500 (Oe)? In order to obtain the iS degree, fine particulate α-Fe metal powder with an average BET surface area of about 50 to 55 m'/g was used.

The large average BET surface area of the α-Fe metal powder means that the surface area per magnetic particle is small, and therefore the area covered by the binder on the ferromagnetic metal powder, in other words, Since the bonding area (bonding force) with the binder is small and the affinity with the binder is also small, the excellent properties of the binder 5 cannot be fully exhibited. Based on these findings, it was revealed that while the magnetic head is in sliding contact with the magnetic layer at a predetermined pressure, ferromagnetic metal powder falls off from the magnetic layer, resulting in poor running performance of the magnetic disk. did.

The purpose of the present invention is to eliminate the drawbacks of the conventional technology, such as peeling and falling off of the magnetic layer, and the generation of deposits, and thereby provide highly durable and reliable magnetism. .

[Means to solve the problem]

In order to achieve the above-mentioned object, the present invention. A magnetic disk mainly composed of ferromagnetic metal powder and containing at least a non-magnetic binder and a lubricant is rotatably housed on a base made of a non-magnetic material, and a cleaning cloth made of non-woven fabric or the like is placed on the inner surface. The object of the present invention is a magnetic disk cartridge having an edge case made of a hollow body or a sheet of synthetic resin supporting a magnetic disk.9 And as the ferromagnetic metal powder, α
-Using Fe metal powder, its average BET surface area is 48m"
/g or less, and almost all of the disk-side surface layer in direct contact with at least the magnetic layer of the cleaning sheet 1- is comprised of non-thermoplastic m fibers such as rayon fibers. It is something.

[Effect]

As mentioned above, in the present invention, almost all of the disk surface layer that comes into direct contact with the magnetic layer of the cleaning sheet is
For example, it is made of non-thermoplastic fiber such as rayon fiber.

Therefore, even if it is in contact with a rotating magnetic disk for a long period of time, the fibers will soften and wear out due to frictional heat, unlike conventional products. This makes it possible to suppress the generation of abrasion powder from the cleaning sheet as much as possible. on the other hand. The ferromagnetic metal powder used is α-Fe with relatively large particles 11 and an average B E 71' surface area of 48 m2/g or less.
Since it is a metal powder, it has great adhesion and binding strength with the binder, and can fully demonstrate the excellent properties of the binder.

Because of this, the durability of magnetic disks has been greatly improved. Is it possible to improve reliability? [Embodiment of the Invention] Figures 1 to 6 are for explaining a magnetic disk cartridge according to an embodiment of the present invention, and Figure 1 is an exploded perspective view of the disk cartridge, and Figure 21-! 1 is a bottom view showing the cleaning sheet attached to the upper case, FIG. 3 is a plan view showing the cleaning sheet attached to the lower case, and FIG. 4 is the state in which this magnetic disk cartridge is used. An enlarged sectional view of the vicinity of the magnetic head insertion port showing the fifth [! An enlarged cross-sectional view of the elastic piece of Hoko's magnetic disk cartridge. Figure 6 shows cleaning sheet 1-
4 is an enlarged cross-sectional view of the magnetic disk cartridge.
It mainly consists of a magnetic disk 2 which is rotatably housed therein, and a shirt starter 3 which is slidably supported by a rigid case l to the car 1.

The cartridge case 1 is composed of an upper case la and a lower case Lb, which are injected or molded from a hard resin such as Al3S resin.

Approximately in the center of the lower case 1b is an opening 4 for inserting the rotational drive shaft.
A rectangular head insertion opening 5 is provided near the head insertion opening 5, and a head insertion opening 5 is also provided in the upper case la. Near the front surfaces of the upper case la and the lower case lb, as shown in FIG. The magnetic head insertion opening 5 is provided at an intermediate position of the recess 6.

As shown in FIG. 3, counterbore-shaped recesses 7, 7 are formed on the inner surface of the upper case la on both the left and right sides of the magnetic head inlet 5, respectively. The vertical width Wl of the recess 7 is approximately equal to the longitudinal width W2 of the magnetic head insertion port 5, and the width W3 of the recess 7 in the direction perpendicular to the longitudinal direction of the magnetic head insertion port 5 ( Width) is restricted to 0.3 times or more of W4, preferably in the range of 0.5 to 1.5 times.

A large number of linearly extending protrusions 8 are provided on the upstream side of the magnetic head insertion slot 5 in the disk rotation direction in the upper case la. Two of them on both sides should be slightly higher than the protrusion 8 on the inside.

An arcuate regulating rib 9 for regulating storage and placement of the magnetic disk 2 is protruded from the inner surface of the upper case 1a, and a part of the rib 9 passes through the end of the recess 7 to reinforce the area around the regulating rib 9 or the fourth part 7. Stand up for yourself physically.

A substantially C-shaped cleaning sheet 10 is inserted inside the regulating rib 9. The width W5 of the opening 1l provided at the opening tfft corresponding to the magnetic head entrance 5 of the cleaning sheet IO is designed to be slightly larger than the width W4 of the magnetic head insertion opening 5. Cleaning sheet IO
By placing the cleaning sheets 1 to 10 on the inner surface of the upper case la, the four parts 7 and the protrusions 8 are also covered.
The surrounding part of the upper case 1a is ultrasonically welded to the upper case 1a.As mentioned above, the recess 7 of the upper case 1a has a sufficient width. As shown in FIG. 4, the edge of the opening 1l in the cleaning sheet 10 can be ultrasonically welded l2 within the recess 7.

As shown in FIGS. 3 and 5, a protruding support part l3 and a protruding protrusion l4 for preventing sticking are provided on the inner surface of the lower case 1b and at a position substantially facing the protrusion 8 of the upper case 1a. There is. As shown in FIGS. 3 and 5, the base l of the elastic piece 15 is formed by bending a plastic sheet.
6 is fixed to the inner surface of the lower case lb in the vicinity of the support portion 13 by suitable means such as bamboo grafting or heat fusion. The free end 17 of the elastic piece l5 is supported by the support portion 13 to maintain its inclined state.

A regulating rib 9 is also protruded from the inside of the lower case lb, and a cleaning sheet 1O is arranged inside the regulating rib 9. The width W5 of the opening 11 provided in the cleaning sheet IO at a position corresponding to the magnetic head volatilization port 5 is designed to be slightly larger than the lateral width W4 of the magnetic head insertion port 5. Third
As shown in the figure, insert the cleaning sheet 10 into the lower case lb.
By placing it inside the elastic piece 15, the elastic piece 15 is covered. Although the peripheral portion of the cleaning sheet 10 is ultrasonically welded 12 to the lower case tb, the ultrasonic weld 12 is omitted near the elastic piece l5.

A magnetic tray cartridge is assembled by combining the upper case la and the lower case tb. If given Q. As shown in FIG. 5, the cleaning sheet 1O is partially lifted from the T case lb by the elastic piece 15, and is lifted or slightly pushed down by the protrusion 8 provided on the upper case 1a, and the magnetic disk 2 is on top, cleaning sheets 1 to 10 on g. Be held elastically by Lo. As the magnetic disk 2 rotates, the cleaning sheet 10 cleans the surface of the disk.

FIG. 4 is a diagram showing the state in which the magnetic disk cartridge is used, and 18 in the figure is a magnetic head.

The cleaning sheet 1-10 has a three-layer structure as shown in FIG. That is, the disk side nonwoven layer 19 facing the magnetic disk 2, the case side nonwoven flf layer 20 facing the cartridge case 1, and the disk side nonwoven fabric layer 19 and the case side nonwoven fabric layer 20 are connected. The intermediate non-woven NR layer 2l is provided for this purpose.

The disc-side nonwoven fabric layer 19 and the case-side nonwoven fabric layer 20 are both made of rayon fiber alone. table. It is made so that there is no distinction between the backs. Viscose rayon is a rayon fiber. Copper ammonia rayon, acetate rayon, etc. are used. The tensile strength of rayon fiber (stable) is approximately 2.5 to 3.1 g/D
．． Elongation rate is approximately 16-22%, elongation elastic modulus (at 3% elongation)
is about 55-80%, and the specific gravity is about 1.50-1.52.

The intermediate nonwoven fabric layer 2l is composed of a mixed fiber layer of rayon fibers and polyamide fibers. The mixing ratio of rayon fiber and boria amide fiber (rayon/boria amide) is approximately 1
Appropriately selected in the range of 79 to 9/1, especially 3/1
A range of 7 to 7/3 is suitable.

Boryamide fiber is a fiber whose raw material monomer is an insect condensate of adipic acid and hexamethylene diamine. The tensile strength of the polyamide fiber is approximately 4.5 to 7.5, the elongation rate is approximately 25 to 60%, and the elongation modulus (at 3% elongation) is approximately 95 to 1.
00%, and the specific gravity is approximately 1.14.

The cleaning sheet IO can be made of RAYON fiber alone, but as in this embodiment, it is made of a nonwoven fabric containing thermoplastic fibers between the disc side nonwoven fabric layer 19 and the case side nonwoven fabric layer 20. By providing the nonwoven fabric layer 2l between ψ, it is possible to achieve good thermal fusion with the cartridge case l.

The intermediate nonwoven fabric layer 21 is made of, for example, polyethylene terephthalate 2 polyethylene, polypropylene, polystyrene, etc., in addition to the mixture of rayon fiber and polyamide fiber as described above. Various thermoplastic fibers such as polyvinyl chloride and acrylic resin are available☆Tij! It can be used for In particular, since rayon fiber is used for the disc side nonwoven fabric layer 19 and the case side nonwoven fabric layer 20 as described above, both these layers 19.20
In consideration of stains, it is preferable that the intermediate nonwoven fabric layer 2l is composed of a mixed fiber layer containing rayon fibers.

The basis weight of this cleaning sheet 10 is not particularly limited, but is suitably about 20 to 50 g/rn'. The basis weight here is 50 x 50 cm.
This is the value obtained by determining the amount of insects in each of the five samples and taking the average value. The thickness of the cleaning sheet 10 is 150 to 300μ
m is appropriate. Cleaning sheet IO according to the embodiment
The specific physical properties of are as follows.

As mentioned above, the cleaning sheet 10 has a 3M structure, but when actually assembling a magnetic disk cartridge using this cleaning sheet 10, all of the fibers that come into sliding contact with the magnetic disk 2 may not be rayon fibers.
Thermoplastic fibers (bolyamide fibers m>
A portion of the rod may come out to the surface side of the cleaning sheet 10 and come into contact with the magnetic disk 2 due to the entanglement of the fibers. However, in such a case, even if the thermoplastic M&fiber is in sliding contact with the magnetic disk 2, the ratio of sliding contact with the magnetic disk 2 is about 10.
% or less, and virtually 100% of the fibers in contact with the magnetic disk 2 are Rayon #R fibers.

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

As the ferromagnetic metal powder, a metal iron powder consisting essentially of α-Fe is used. Here, we defined metallic iron powder consisting essentially of α-Fe. In addition to Fe, for example, N t,
Cr, Mn, Si. W. This means that it also includes solid solutions in which a small amount of metal elements such as Mo and CO are dissolved. The physical properties of the α-Fe metal powder used in this example are as follows:
It is as shown in Table 2 below.

Rz and R2 in Table 2 are each a linear or branched saturated alkyl group having 1 to 15 carbon atoms.

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

As the binder, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, polyurethane resin, polyisocyanate compound, etc. are used.

The lubricant is in a liquid state at 0° C., which is the lower limit of the operating temperature range of magnetic recording media such as magnetic disk cartridges. That is, a fatty acid ester having a melting point of 0° C. or less and a viscosity of 30 cp or more at 20° C. and having the following general structural formula is preferable.

General structural formula RX O \ 11 C H C H 2 0 C R 3 R1 Specific examples of this fatty acid ester include 2-heptyl undecyl oleate, 2-octyl todecyl oleate, and 2-methylhexadecyl stearate. rate. 2-hexyldodecyl stearate-1-. Examples include 2-octyldecyl stearate, 2-ethylhexadecyl oleate, and 2-hexyldodecyl oleate. Among these fatty acid esters, especially Rl in the general structural formula has 7 to 7 carbon atoms.
8, R2 has a straight chain or a 111δ alkyl group having 9 to 10 carbon atoms, for example, 2-heptyluntecylolate (Rl has 7 carbon atoms and R.! has 9 carbon atoms).
Alternatively, 2-octyl dodecyl oleate (R You can. It is also possible to use it in combination with other lubricants if necessary.

The lubricant (fatty acid ester) can be added to the magnetic paint in advance to form the magnetic layer, or it can be dissolved in a suitable organic solvent such as toluene or n-hexane and spread over the magnetic layer. by impregnating it into the magnetic layer. A lubricant in an amount of S can be added to the magnetic layer.

In either method, the appropriate addition rate of the lubricant is 5 to 25% by weight based on the ferromagnetic metal powder. If the addition rate is less than the lower limit, the lubricant will not be effective and the durability of the magnetic recording medium will not be sufficiently improved. On the other hand, if the amount exceeds the upper limit, the surface of the magnetic layer becomes sticky and dirt tends to adhere to it, and the magnetic recording medium sticks to the plane of the magnetic head, increasing the starting torque and causing uneven rotation.

In addition, as Ura strong powder, for example, aluminum oxide, chlorine 11
, silicon carbide, silicon nitride, etc., and this example shows an example using hyaluminum oxide. The grain size 4ffi of Ura strong flour is suitably 0.4 to 0.5 μm.

The content of urako flour is regulated within 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, it means that relatively soft metal magnetic powder is used.2.The durability of the magnetic disk will not be sufficient. On the other hand, if the content of Ura Tsuyoshi in the magnetic layer is more than 5%, preferably more than 10%, the durability of the magnetic disk will be significantly compromised. However, as the content of urako powder increases, the content of other components such as ferromagnetic metal powder, binder, and lubricant is inevitably limited, making it impossible to obtain the desired properties. Therefore, it is necessary to keep the upper limit of the content of reinforcing powder at 40% by weight, and the preferable content of Ura strong flour is 10 to 30% by weight.
It is.

The following Table 3 shows suitable composition ratios of the magnetic paint to be applied on the base film.

Table 3 A magnetic disk was punched out into a disk shape of 86 mm.

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

After sufficiently cross-dispersing this #lll product, the thickness was 75 μm.
A base consisting of polyenalene terephthalate of m, r
Coat both sides of the lume so that the thickness b is 2.5 μm. Shape the magnetic layer while intoxicated. Next, the relationship between the average BET surface area and wear resistance of the ferromagnetic metal powder (α-Fe) used will be explained. The following Table 5 shows α-F with various average BET surface areas.
e. Assemble a magnetic disk cartridge using metal powder. This is installed in a normal recording/reproducing device, and the magnetic disk is rotated at a speed of 36 Orpm.

Then, while repeating temperature cycles from 0°C to 50°C, a magnetic head with a nearly rectangular sliding contact is brought into sliding contact with the surface of the magnetic disk with a load of 20 g.
Durability was evaluated by counting the number of rotations (number of passes) until running scratches appeared on the surface of the R-screen (confirmed with the naked eye). The composition of the magnetic layer of each sample was the same as that in Table 3.

Table 5 As is clear from this table, the average B E
A magnetic disk using a-Fe gold Mt powder with a surface area of 50 to 55 m2/FC was subjected to the severe running test 1 as described above.
-, the number of buses taken before running scratches appears is small, and it cannot be said that the durability is sufficient. On the other hand, when the α-Fe metal powder has an average B E 'L' surface area of 48 m2/g or less, as in the present invention, the affinity and binding force between the ferromagnetic metal powder and the binder are always strong. The results show that even in the severe running tests 1 to 1, a sufficient number of buses (over 4 million times) for practical use was obtained, indicating that the magnetic disk has excellent wear resistance. As mentioned above, the average BET area of ferromagnetic metal powder is 48m.
2/g or less, the binding force with the binder can be greatly increased, or if the average BET surface area is 30 m
When it becomes less than 2/g. Media noise increases. C/N
This is not preferable because the value decreases. Therefore, it is preferable to limit the average BET surface area of the ferromagnetic metal powder to a range of 30 to 48 m''/g.

Next, we will explain specific examples of lubricants used in the magnetic layer. Specific examples of 2-heptyl undecyl oleate lpUo) used in the present invention. 2-octyltotesyloleh (
Figure 7 shows the molecular structure, oxygen ratio, melting point, surface tension, and heat loss properties of 2-ethynolehesinooleate (EHO) and oleyl oleate 1 to (oO) in the first comparison row. The viscosity characteristics at each temperature are shown in Figure 8.

Next, Table 6 below 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.

In addition, 4R or fiber in the table indicates the constituent fiber of the surface layer of the cleaning sheet that comes into contact with the magnetic tape.

I) ET is polyethylene terephthalate 1~, mixed paper 1
is 50% rayon and polyethylene terephthalate 1-5
It shows 0% mixed paper. Mixed paper 2 indicates a mixed paper of 50% rayon and 50% acrylic. Mixed extract 3 is. It shows a mixed paper of 50% rayon and 50'l- of boria mite.

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

For error occurrence, indicate whether an error occurred during 24-hour seek.

Table 6 As is clear from this table, errors occurred in all of the comparative examples, and the durability was not fully satisfied. In comparison, the embodiment of the present invention has
Not only does it not cause errors, but it also has excellent durability.Next, we will explain the relationship between the surface roughness of the magnetic disk and the wear on the cleaning sheet 1.

In order to minimize the wear of the cleaning sheet, it is necessary to consider the surface roughness of the magnetic disk. When the surface roughness (Ra) of a conventional magnetic disk is relatively rough, for example, 0.020 to 0.025/J m, the cleaning sheet is inevitably prone to wear.

This is k. Set the surface roughness Ra of the magnetic tape to 0.0.
1 μm or less, preferably 0. It has been found that by setting the cleaning sheet to 0.005 μm or less, wear of the cleaning sheet can be minimized over a long period of time.

Muo, the surface roughness Ra of the magnetic layer is stylus fE2μm,
Stylus cart 2 5 m g. The surface roughness was determined using a stylus type surface roughness meter under the conditions of a cut-off of 0.08 mm and a scanning speed of 0.03 mm/sec.

Furthermore, the surface roughness of the magnetic layer can be maintained at a desired level by changing the surface treatment conditions (for example, the temperature, pressure, and number of rolls of the calender rolls in the calendering processing equipment). .

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 and rayon fibers, the Because of its affinity for wear particles, there is extremely little adhesion of wear particles to the magnetic disk, thus helping to prevent errors from occurring.

In the example f, 11゛ thermoplastic fibers are
3> fiber was used, but the present invention is not limited to this, and the same effect can be obtained even if other cellulose fibers such as cotton fibers are used.

〔Effect of the invention〕

The present invention has the structure described above. It is possible to provide a highly durable and highly reliable magnetic data stacker.

[Brief explanation of drawings]

1 to 6 are for explaining the magnetic disk force-1-ridge according to the embodiment of the present invention.
An exploded perspective view of the magnetic disk force-1. Figure 2 is a bottom view showing the cleaning sheet attached to the upper case. Figure 3 shows how cleaning sheet 1- is installed in the lower case.
FIG. 4 is an enlarged cross-sectional view of the vicinity of the magnetic head insertion slot showing the state in which this magnetic disk cartridge is used, and FIG.
FIG. 6 is an enlarged sectional view of the vicinity of the elastic piece of this magnetic disk cartridge. FIG. 6 is an enlarged sectional view of the cleaning sheet 1-. Figure 7 is a diagram showing the molecular structural formula and various characteristics of each lubricant. Figure 8 is. This is a diagram of the viscosity characteristics of each lubricant. 1...Force-1-rigid case, 1a...Upper case. lb...Lower case, 2...Magnetic tape, l○...Cleaning sheet, l9...Dice side non-woven fabric, 20...Case side non-woven fabric, 21...・Intermediate non-woven intoxication. Figure 2 Tomi 3 Figure 4 Figure 6 Figure 20 2I

Claims (12)

[Claims] (1) A magnetic disk provided with a magnetic layer mainly composed of ferromagnetic metal powder and containing at least a non-magnetic binder and lubricant on a base made of a non-magnetic material, and the magnetic disk rotatably housed. In addition, in the magnetic disk cartridge equipped with a cartridge case supporting a cleaning sheet on the inner surface, the ferromagnetic metal powder is made of α-Fe, and the average BET is
A magnetic disk having a surface area of 48 m^2/g or less, and at least almost all of the disk-side surface layer of the cleaning sheet that is in direct contact with the magnetic layer is composed of non-thermoplastic fibers. cartridge. (2) The magnetic disk cartridge according to claim (1), wherein the lower limit of the BET surface area is 30 m^2/g. (3) A magnetic disk cartridge according to claim (1), characterized in that the lubricant is a fatty acid ester having a melting point of 0° C. or lower and a viscosity at 20° C. of 30 centipoise or higher. (4) A magnetic disk cartridge according to claim (3), wherein the fatty acid ester has the following general structural formula. General structural formula ▲ Numerical formulas, chemical formulas, tables, etc. are available ▼ In the formula, R_1 and R_2 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. (5) The magnetic disk according to claim (4), wherein R_1 in the general structural formula has 7 to 8 carbon atoms, and R_2 is a linear or branched alkyl group having 9 to 10 carbon atoms. cartridge. (6) In claim (4), the fatty acid ester is 2-octyldodecyl yarate, 2-heptyl undecyl oleate, 2-ethylhexadecyl stearate, 2-hexyldodecyl stearate, 2-octyldecyl A magnetic disk cartridge comprising at least one fatty acid ester selected from the group of stearate, 2-ethylhexadecyl oleate, 2-hexyldodecyl oleate, and 2-octyldecyl oleate. (7) In any one of claims (1) to (6), the addition rate of the lubricant to the ferromagnetic metal powder is regulated within a range of 5 to 25% by weight. Features a magnetic disk cartridge. (8) A magnetic disk cartridge according to claim (1), wherein the non-thermoplastic fiber is a rayon fiber. (9) A magnetic disk cartridge according to claim (1), wherein the magnetic layer contains a binder made of urethane resin. (10) In claim (1), the cleaning sheet is composed of three layers, the front layer and the back layer being a single layer of rayon fiber, and the middle layer being a mixed layer of rayon fiber and thermoplastic fiber. A magnetic disk cartridge featuring: (11) A magnetic disk cartridge according to claim (1), wherein the surface roughness (Ra) of the magnetic layer is regulated to 0.01 μm or less. (12) A magnetic disk cartridge according to claim (1), wherein the magnetic layer contains reinforcing powder.