GB2042370A - Magnetic recording medium - Google Patents

Abstract

A magnetic recording medium comprises a first magnetic layer containing magnetite or a cobalt- doped magnetite which is coated on a non-magnetic substrate; and a second magnetic layer coating a cobalt- doped gamma -Fe2O3 which is coated on said first magnetic layer. The magnetic recording medium has a high recording sensitivity, a high dynamic range, less output fluctuation and especially superior sensitivity in low frequency band and superior maximum output level without a deterioration of transfer characteristics.

Description

SPECIFICATION Magnetic recording medium The present invention relates to a magnetic recording medium which has excellent high frequency characteristics.

The choice of a magnetic recording medium having superior high frequency characteristics is important in the manufacture of an audio cassette tape. A magnetic powder having high coercive force is needed for this purpose. The maximum coercive force obtained using conventional ferromagnetic iron oxide is about 400 Oe. A magnetic tape prepared by using chromium dioxide or a cobalt-doped magnetic powder having coercive force has superior characteristics in the high frequency band. Therefore, a refreshable equalizer can be used at 70 sec. Moreover, a magnetic tape having a broad dynamic range and is advantageous in view of S/N. The cassette tape prepared by using chromium dioxide or a cobalt-doped magnetic powder has a narrow dynamic range in comparison with an open type magnetic tape. Therefore tone quality in recording and reproducing is not satisfactory.The conventional two layer coated tape has excellent characteristics in low frequency bands as a result of using a magnetic powder having a low coercive force in the first layer and also has excellent characteristics in high frequency bands as a result of using a magnetic powder having high coercive force in the second layer. Therefore, the dynamic range is broad. However, distortion is caused in middle frequency bands, because the difference between the coercive forces in the first layer and in the second layer is too large. There is therefore a "middle slump" phenomenon. The main components of music are usually in this middle frequency band a reproduction of an original musical sound is not faithful, but a distorted sound results.

When the second layer is coated on the first layer, the second layer is usually thinner, the unevenness of the first layer affects the unevenness of the second layer and thus fluctuation of output is noticeably increased. In order to minimize these characteristics, it is necessary to use a special bias-equalizer. In some decks, such a bias-equalizer is provided, but an optimum biasequalizer is provided in only quite a small number of decks.

It is an object of the present invention to provide a magnetic recording medium which has a high recording sensitivity, a high dynamic range, low output fluctuation and in particular superior sensitivity in low frequency band superior maximum output level (MOL) without any deterioration of transfer characteristics.

The present invention provides a magnetic recording medium which comprises a first magnetic layer, containing magnetite or a cobalt-doped magnetite, which is coated on a nonmagnetic substrate; and a second magnetic layer, containing a cobalt-doped y-Fe203, which is coated on said first magnetic layer, preferably as a magnetic powder.

The second magnetic layer preferably has a coercive force of 590 to 800 Oe, a residual magnetic flux density of higher than 1200 gauss and a thickness of 1.5 to 3.0 #.

The inventors have proposed a two layer magnetic recording medium which has higher recording sensitivity, superior dynamic range and less output fluctuation in comparison with the conventional chromium tape or the conventional cobalt-doped tape, and which can be used by a normal reproducing equalizer having 70 ysec. without using a special bias-equalizer and can be used by a chromium bias having superior S/N than that of the normal bias.

The proposed magnetic recording medium has the first layer containing y-Fe203 or cobaltdoped Fe203 and optionally y-Fe304, which generally has a coercive force 100 to 200 Oe higher than the coercive force of the conventional two layer tape, and the second layer containing a cobalt-doped magnetic powder, and thus has a coercive force and residual magnetic flux density to give excellent frequency characteristics so that it can be used on the conventional chromium position deck with less distortion in the middle frequency band and less output fluctuation.

The proposed magnetic medium will be described in detail with reference to the accompanying drawings wherein: Figures 1 to 4 show characteristics of a magnetic recording medium of a type previously proposed by the inventors; Figure 5 shows characteristics of the magnetic recording medium of the prior proposal and the conventional magnetic recording medium; Figure 6 shows electromagnetic characteristics of one embodiment of the magnetic recording medium of the present invention and the reference; and Figure 7 shows characteristics of the magnetic recording medium of the present invention.

Fig. 1 shows output fluctuations at a frequency of 8 KHz in a medium having the first layer with a thickness of 5.0it and the second layer having a different thickness. The components and the ratios of the magnetic powder and the binder etc. used for the preparation are substantially the same as those of Example 1 described below. As is shown in Fig. 1, when the thickness of the second layer is more than 1 .5y, the output fluctuation is less than 0.5 dB and is no problem in practical use. When the second layer is too thin, an uneven surface is formed on the second layer because of the uneven surface of the first layer caused by swelling of the first layer, whereby the output fluctuation is too large.Therefore the effect of the uneven surface of the first layer is reduced by increasing the thickness of the second layer beyond a certain thickness whereby the output fluctuation can be reduced.

It has been difficult to increase the thickness of the second layer in the conventional two layer magnetic recording tape, because the coercive force of the second layer is much higher than that of the first layer. When the second layer is too thick, distortion occurs in the middle frequency band.

In accordance with the present invention, coercive force of the first layer and the second layer can be controlled to increase the residual magnetic flux density (By) of the first layer so that the thickness of the second layer can be increased.

Fig. 2 shows characteristics of response (dB) against frequency (Hz) of samples having a second layer with a coercive force of 710 Oe and a first layer of a different coercive force: (Curve 1: a coercive force of 320 Oe; Curve 2; a coercive force of 360 Oe and Curve 3: a coercive force of 400 Oe) which were measured by using a deck of Nakamichi 1000#.

As is shown in Fig. 2, when the coercive force of the first layer is lower, the low frequency sensitivity is higher, but distortion occurs in the middle frequency band (middle slump) whereas when the coercive force of the first layer is higher, the low frequency characteristic is inferior and unbalanced.

The inventors have attempted to overcome these disadvantages and have found that the coercive force of the first layer is preferably in a range of 400 to 560 Oe.

It has been found that the distribution of the coercive force is broadened by mixing magnetic powders having different coercive forces so that distortion in the middle frequency band can be relatively decreased. It has been found that distortion in the middle frequency band can be decreased by decreasing the difference between the coercive force of the first layer and the coercive force of the second layer.

The coercive force of the second layer should preferably be higher than 590 Oe so as to give satisfactory characteristics in high frequency band, but is preferably less than 800 Oe to avoid difficulties in erasing at high coercive force.

The residual magnetic flux density BY should preferably be higher than 1200 gauss. When it is lower, the characteristics in the high frequency band are inferior even though the coercive force may be high enough. If the coercive force of the first layer is higher than a certain limit, distortion in the middle frequency band is prevented. Thus, in order to give good characteristics in the low frequency band with high coercive force, it is necessary to have a high residual magnetic flux density B7 and accordingly, it is necessary to increase the packing density of the magnetic powder.

Fig. 3 shows the relationship of the saturated residual magnetic flux density BY (gauss) to the coercive force (Oe) of the first layer. The saturated residual magnetic flux density BY preferably satisfies the inequality: y > 1 .60X + 556 (gauss) wherein X represents the coercive force of the first layer.

When the residual magnetic flux density (B) is less than the value defined above, the characteristics are inferior because of the sensitivity and distortion in the middle frequency band.

It is understood from Fig. 3, that B7 can be 1200 gauss or more when the coercive force is 400 Oe; and BY should preferably be higher than 1350 gauss when the coercive force is 500 Oe. When the packing density is increased to give a value of BY higher than 1800 gauss, the coated layer is too hard. In order to vary B7 in a constant coercive force, it is controlled by including a suitable amount of a dispersing agent.

Fig. 4 shows response as reproducing output at the frequency of 333 Hz in the case of the first layer having various thickness (y) and the second layer having a thickness of 2p.

It is understood from Fig. 4, that when the thickness of the first layer is lower than 3.01l, the characteristics are remarkably inferior. However when the thickness of the first layer is more than 6.0y distortion occurs in the middle frequency band. Therefore, the thickness of the first layer should be in a range of 3.0 to 6.0y. On the other hand, the thickness of the second layer should be more than 1.5cm in view of the output fluctuation. However, when this layer is too thick, distortion occurs in the middle frequency band. Therefore, the thickness of the second layer should preferably be less than 3.0# and more preferably in a range of 1.5 to 2.5#.

In the magnetic recording medium of the present invention, coercive forces, residual magnetic flux densities B7 and the thicknesses of the coated layers of the first and second layers are preferably ranges, whereby the magnetic recording medium can be used by a conventional cassette deck equipped with a chromium position and has superior characteristics in comparison with the conventional chromium tape, the conventional cobalt doped tape or the conventional two layer tape.

The present invention will be further illustrated by certain examples and references which are provided for purposes of illustration only and are not intended to be limiting the present invention.

EXAMPLE 1: Magnetic powder: 7-Fe2O3 100 wt. parts Vinyl chloride-vinylidene chloride copolymer 5 wt. parts Urethan prepolymer 15 wt. parts Dispersing agent: lecithin 2 wt. parts Methyl ethyl ketone 80 wt. parts Methyl isobutyl ketone 80 wt. parts Toluene 80 wt. parts A mixture of these components was kneaded in a ball mill for 36 hours and 3 wt. parts of a curing agent of isocyanate (Colonate L: manufactured by Nippon Polyurethane K.K.) was admixed to prepare a magnetic composition for the first layer. This was coated on a polyester film to form a layer having a thickness of 4.0y. The magnetic powder was orientated and the surface was dried and processed and heated at 60 C for 48 hours to crosslink the polyurethane resin. This is the first layer.

Magnetic powder: Cobalt doped y-Fe203 100 wt. parts Vinyl chloride-vinylidene chloride copolymer 10 wt. parts Urethane prepolymer 20 wt. parts Dispersing agent: lecithin 1 wt. part Methyl ethyl ketone 100 wt. parts Methyl isobutyl ketone 100 wt. parts Toluene 100 wt. parts A mixture of these components was kneaded in a ball mill for 48 hours and 3 wt. parts of a curing agent of isocyanate (Colonate L. manufactured by Nippon Polyurethane K.K.) was admixed. The mixture was coated on the first layer to form a layer having a thickness of 2y. The magnetic powder was orientated and the surface was dried and processed and heated at 60"C for 48 hours to crosslink the polyurethane resin. A magnetic recording tape having two layers was obtained.The resulting tape had a coercive force of 570 Oe, By of 1280 gauss and a total thickness of 5.5y. The first layer had a coercive force of 430 Oe and By of 1300 gauss.

EXAMPLE 2: Magnetic powder: Cobalt doped "-Fe2O3 100 wt. parts Vinyl chloride-vinylidene chloride copolymer 5 wt. parts Urethane prepolymer 15 wt. parts Dispersing agent: lecithin 3 wt. parts Solvent: phosphoric acid ester 2 wt. parts Methyl ethyl ketone 80 wt. parts Methyl isobutyl ketone 80 wt. parts Toluene 80 wt. parts In accordance with the process of Example 1 except that a mixture of these components was used as the mixture for the first layer, the first and second layers were coated and heat-treated to obtain a magnetic recording tape having two layers. The resulting tape had a coercive force of 610 Oe and By of 1400 gauss. The first layer had a coercive force of 530 Oe and By of 1500 gauss.

Fig. 5 shows frequency characteristics of the magnetic recording tape (Curve 5) and the conventional cobalt doped single layer tape (Curve 4).

As it is clearly understood, the magnetic recording tape of the present invention had superior characteristics in all frequency bands and had broad dynamic range.

EXAMPLE 3: In accordance with the process of Example 1 except using 50 wt. parts of y-Fe2O3 and 50 wt.

parts of cobalt doped y-Fe2O3 as the magnetic powder of the first layer, a magnetic recording tape having two layers was prepared. The magnetic recording tape had a coercive force of 580 Oe and By of 1350 gauss. The first layer had a coercive force of 490 Oe and By of 1400 gauss.

In a conventional preparation of a magnetic recording tape having two layers, y-Fe2O3 has been usually used as the magnetic powder of the first layer. Magnetite can be considered as the magnetic powder. However, magnetite has not been usually used because of disadvantages of inferior transfer characteristics though it had high coercive force and accordingly, had superior sensitivity in low frequency band and superior maximum output level (MOL).

The inventors have studied these magnetic recording media and have found that the transfer effect is mainly depending upon the characteristics of the second layer in-the magnetic recording tape having two layers and is not substantially affected by the first layer.

In usual, a magnetization is resulted in deeper part of a coated layer by lower frequency so that the transfer characteristic is deteriorated by lower frequency. When the transfer characteristic is measured by a signal having about 1 KHz, the signal for transfer is recorded through the base surface to the next coated layer and accordingly, it is remarkably weaker than the input signal. Therefore, in the magnetic recording tape having two layers, only the second layer (about 2.0# is affected by the signal having low frequency whereby it is enough to use a magnetic powder having excellent transfer characteristics only in the second layer.

In the magnetic recording medium having two layers of the present invention, magnetite which had inferior transfer characteristic and has not been substantially used, is used in the first layer (which is not affected) whereby a magnetic recording medium having high magnetization as and excellent sensitivity in low frequency band and excellent maximum output level without a deterioration of the transfer characteristic.

REFERENCE: First layer: Magnetic powder: y-Fe203 100 wt. parts Vinyl chloride-vinyl acetate copolymer 10 wt. parts Urethane prepolymer 10 wt. parts Dispersing agent: lecithin 3 wt. parts Methyl ethyl ketone 80 wt. parts Methyl isobutyl ketone 80 wt. parts Toluene 80 wt. parts A mixture of these components was kneaded in a ball mill for 48 hours and 3 wt. parts of a curing agent of isocyanate (Colonate L manufactured by Nippon Polyurethane K.K.) was admixed. The mixture was coated on a polyester film to form a layer having a thickness of 40#.

The magnetic powder was orientated and the surface was dried and processed and heated for 48 hours to crosslink the urethane resin.

Second layer: Magnetic powder: cobalt doped y-Fe203 100 wt. parts Vinyl chloride-vinyl acetate copolymer 15 wt. parts Urethane prepolymer 5 wt. parts Dispersing agent: lecithin 2 wt. parts Methyl ethyl ketone 100 wt. parts Methyl isobutyl ketone 100 wt. parts Toluene 100 wt. parts A mixture of these components was kneaded in a ball mill for 48 hours. The mixture was coated on the first layer to form a layer having a thickness of 2.5jut. The magnetic powder was orientated and the surface was dried and processed.

EXAMPLE 4: Magnetic powder: Fe3O4 100 wt. parts Vinyl chloride-vinyl acetate copolymer 10 wt. parts Urethane prepolymer 10 wt. parts Dispersing agent: lecithin 3 wt. parts Methyl ethyl ketone 80 wt. parts Methyl isobutyl ketone 80 wt. parts Toluene 80 wt. parts In accordance with the process of Reference except that a mixture of these components was used as the mixture for the first layer, the first and second layers were formed.

EXAMPLE 5: Magnetic powder: Co doped Fe3O4 100 wt. parts Vinyl chloride-vinyl acetate copolymer 10 wt. parts Urethane prepolymer 10 wt. parts Dispersing agent: phosphoric ester 3 wt. parts Methyl ethyl ketone 80 wt. parts Methyl isobutyl ketone 80 wt. parts Toluene 80 wt. parts In accordance with the process of Reference except that a mixture of these components was used as the mixture for the first layer, the first and second layers were formed.

Electromagnetic characteristics and transfer characteristics of Reference and Example 4 are shown in Table 1.

Table 1 Reference Example 4 333 Hz +3.1 +4.1 Electromagnetic 8 Hz + 3.5 +4.1 characteristics (dB) 12.5 Hz + 4.3 + 4.5 MOL at 333 Hz + 2.5 + 3.5 Transfer characteristic (dB) 56 56 As it is clearly understood, the magnetic recording tape of the present invention had superior electromagnetic characteristics without deteriorating the transfer characteristics.

Fig. 6 shows a graph of the electromagnetic characteristics shown in Table 1.

Fig. 7 shows transfer chcaracteristics of samples using magnetite as the magnetic powder in which a thickness of the first layer was constant and a thickness of the second layer was varied.

In the condition of the measurement, each signal having 1 KHz was recorded by an input of 10 dB and each sample was kept 30 C for 24 hours and the measurement was carried out.

Claims (6)

1. A magnetic recording medium which comprises a first magnetic layer, containing magnetite or a cobalt-doped magnetite, which is coated on a non-magnetic substrate; and a second magnetic layer, containing a cobalt-doped y-Fe203, which is coated on said first magnetic layer.

2. A magnetic recording medium according to claim 1 wherein said second magnetic layer has a coercive force of 590 to 800 Oe, a residual magnetic flux density of at least 1200 gauss.

3. A magnetic recording medium according to claim 1 or claim 2 wherein said second magnetic layer has a thickness of 1.5 to 3.0p.

4. A magnetic recording medium according to any preceding claim wherein said first layer contains magnetite in the form of y-Fe203 or a cobalt-doped magnetite and has a thickness of 3.0 to 6.01l.

5. A magnetic recording medium according to claim 1 wherein the residual magnetic flux density y satisfies the inequality y > 1 .60X + 556 (gauss) wherein X represents the coercive force of said first magnetic layer.

6. A magnetic recording medium according to claim 1 substantially as herein described with reference to the Examples.