Magnetic recording media for audio use, video use, computer use (disks and memory tapes), and the like, generally include a nonmagnetic support and a magnetic layer provided thereon. The magnetic layer includes ferromagnetic particles dispersed in a binder.
In recent years, the practical recording mode for these magnetic recording media has shifted from conventional analogue recording to digital recording in which there is less chance that the recorded information will deteriorate upon dubbing.
Magnetic recording media for digital recording must be capable of high density recording because more signals are recorded in digital recording than in analogue recording, and because VTRs and magnetic tapes must have high picture and sound quality, but at the same time be capable of being miniaturized in order to save space.
In order to attain high density recording, it is necessary that magnetization transition in a medium occur along a recording track at distances which are as short as possible. Shorter wavelength signals are therefore used. Accordingly, it is necessary to reduce the size of ferromagnetic particles used in the medium, increase the packing density of the ferromagnetic particles in the medium and increase the smoothness of the surfaces of the medium to an ultrahigh degree. At the same time, the speed of writing on magnetic tapes and in this regard the speed of reading magnetic tapes should be as high as possible and increased cylinder revolutions per unit time, magnetic tape running speed, etc. are desired.
Reduced cylinder head diameter is required in order to miniaturize VTRs and magnetic recording media. Reduced recording track width of magnetic tapes is required in order to improve recording density to area. Reduced thickness is required in order to improve volumetric density. For a tape as thin as about 11 .mu.m, it is extremely difficult to make the tape run in a stable manner at a speed as high as 20 m/sec or higher, based upon relative speed of the tape and a small-diameter head revolving at a rate as high as about 5,400 to 9,000 rpm. There is a tendency for the Rf output to be decreased and output fluctuation to become greater as the magnetic tapes become thinner.
In a D3 system [normalized in SMPTE (Society of Motion Picture and Television Engineers)] used as a digital VTR, where the magnetic tape/magnetic head relative speed is as high as 20 m or higher and the head core width is as large as 100 .mu.m or more, spacing between the magnetic head and the magnetic tape is increased and head contact by the magnetic tape becomes so poor that a reduced output occurs.
Carbon black and nonmagnetic particles having a Mohs' hardness of 8 or higher, which are called abrasives, are conventionally used in magnetic tapes in order to achieve certain running and antistatic properties and head-cleaning properties. However, head abrasion due to the presence of such particles tends to become more severe when the magnetic tape/head relative speed increases.
In order to control such head abrasion, the abrading properties of magnetic tape are controlled so as to be within a certain range. Also, a magnetic head having an increased head core/tape contact area is used. The increased contact area is achieved by enlarging the width of the part of the head which a magnetic tape slides on (i.e., the width of the magnetic head core), in a direction perpendicular to the head travel direction.
However, increased magnetic head area tends to decrease Rf output and enhance output fluctuation. This is due to uneven contact between the magnetic tape and the head resulting from, e.g., the attraction of the magnetic tape toward the aperture into which the head chip of cylinder head has been fitted. The uneven contact between the magnetic tape and the head is caused by spacing between the magnetic tape and the head which results in loss during recording and reproduction further causing a decrease in output and output fluctuation.
These problems are severe, particularly in video tapes used in VTRs for broadcasting, such as the D3 or D2 systems [normalized in SMPTE (Society of Motion Picture and Television Engineers)], in which digital signals can be recorded and reproduced.
The loss (Ls) caused by reproduction can be expressed by the following equation: EQU Ls=54.6(d [.mu.m]/.lambda.[.mu.m]) (dB) (1)
wherein d [.mu.m] is a spacing between the magnetic tape and the head and .lambda.[.mu.m] is a recording wavelength. The spacing between the magnetic tape and the head is determined by the pressure of air flowing between the magnetic tape and the head and by the stiffness of the magnetic tape. Problems such as those described above are caused by the fact that the magnetic tape has relatively low stiffness, mainly in the transverse-direction.
The stiffness (M) of a magnetic tape can be expressed by the following equation: EQU M=Ebd.sup.3 /12 (2)
wherein E is the Young's modulus of the base in the transverse-direction, b is the width of the magnetic tape, and d is the thickness of the magnetic tape. It will be appreciated from equation (2) that in order to improve the stiffness of magnetic tape in a transverse-direction, the transverse-direction Young's modulus of the base should be increased, and the total thickness of the magnetic tape should be increased. Examples of magnetic tape having a base with an increased transverse-direction Young's modulus are disclosed in JP-A-50-46303 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-54-34206, JP-A-62-234233, JP-A-63-197643, JP-A-63-212549, JP-A-2-20924, JP-A-4-49515, JP-A-4-69813, and U.S. Pat. Nos. 4,804,736, 4,833,019 and 5,196,265 (corresponding to JP-A-4-146518), the disclosures of which are herein incorporated by reference. In these applications and patents, poly(ethylene terephthalate) (strengthened PET), poly(ethylene 2,6-naphthalate) (PEN), aromatic polyamides (aramids), and composite polyesters, having an increased transverse-direction Young's modulus which is higher than ordinary are used. However, sufficient magnetic tape stiffness cannot be maintained by increasing the Young's modulus in the transverse-direction as discussed above alone if the thickness of the tape is reduced because the stiffness of a magnetic tape is proportional to the third power of the tape thickness. In order to obtain a magnetic tape which is suitable for running on a head having a track width of 20 .mu.m and a sliding surface width of about 80 .mu.m, as measured along a direction perpendicular to the scan direction, a base thickness of at least 9 .mu.m is necessary for poly(ethylene terephthalate) (PET) films. The thickness of the base becomes significant when the sliding surface width of the head, as measured along a direction perpendicular to the scan direction, is increased. If the width of the sliding surface as measured along a direction perpendicular to the scan direction is increased to about 130 .mu.m, even a PEN or aramid base having an increased transverse-direction Young's modulus must have a thickness of at least 9 .mu.m.
Another method for increasing stiffness of magnetic tape is to form a layer of a nonmagnetic metal, such as aluminum, on a PET base by vapor deposition or the like as described in JP-A-54-74706. However, this method is not practical because it is costly.
Increasing packing density of ferromagnetic powders on a magnetic tape by metal-metal calendering during the calendering treatment of the tape, as described in JP-B-60-35243, is known as a method for obtaining a high-output magnetic recording medium. (The term "JP-B" as used herein means an "examined Japanese patent publication".) However, the hardness of the magnetic layer is increased and cushioning properties of the magnetic tape are reduced during such calendering because the surface void content of the magnetic layer is reduced. This reduces the head contacting ability of the magnetic tape. Thus, sufficiently high output has not been obtained.
In summary, the prior art method, in response to a trend toward digital systems have sought to increase the amount of information recorded. The response to a trend toward higher-density recording and higher output, the prior art has sought to improve volumetric density of magnetic recording media (hereinafter also referred to simply as "media"). However, higher loading densities of magnetic materials, reducing the thickness of magnetic tapes, and increasing head-medium relative speed, has caused a reduction of head contact and an increase of magnetic head abrasion. Increasing head core width and the reduction of cushioning properties of magnetic layers has caused further deterioration of head contacting properties and output.
The techniques described above have failed to sufficiently improve output so as to compensate for the decrease of output caused by uneven head contact by magnetic tapes due to a reduction in thickness of the tapes, an increase in density of ferromagnetic particles on the tapes, and an increase in width of the sliding surfaces of heads.