MAGNETIC RECORDING MEDIUM

Abstract: A magnetic recording medium comprising a non-­ magnetic base (1) having provided thereon a magnetizable layer of two-layered structure composed of a first magnetiz­ able layer (4) and a second magnetizable layer (5) each based on iron oxide magnetic powder. The two magnetiz­ able layers (4) and (5) can reduce bias noise while maintain­ ing regeneration output at a high level over the whole range by properly selecting specific surface area (measured by the BET method) of each magnetic powder, coercive force, residual magnetic flux, and thickness of the layer.

Description: DESCRIPTION TITLE OF THE INVENTION MAGNETIC RECORDING MEDIUM TECHNICAL FIELD This invention relates to a magnetic recording medium and particularly to a magnetic recording medium having a magnetic layer formed of a double-layer. BACKGROUND ART A prior art magnetic recording medium, for example, a magnetic tape with magnetic powder, such as, y-Fe2O3 coated on the surface thereof (a specific surface area of the magnetic powder is about 20 m2/g) has large coercive force and residual magnetic flux density, achieves a high reproduced output over the whole frequency band range but has a defect that a bias noise is also increased. DISCLOSURE OF INVENTION The present inventor found out from various experiments and researches that, in a magnetic recording medium, if an iron oxide magnetic powder having a large specific surface area was distributed in the surface of its magnetic layer, a bias noise was reduced and if an iron oxide-based magnetic powder having a small specific surface area was distributed in the inside of the magnetic layer (that is, the under layer below the middle layer in the thickness direction), a reproduced output was increased over the whole frequency band range. Therefore, on the basis of the results of such research, this invention is to provide a magnetic recording medium the magnetic layer of which is formed as a double layer structure made of an iron oxide-based magnetic powder and in which specific surface areas of the respective magnetic powders of both the upper and under magnetic layers, coercive force Hc, residual magnetic flux density Br, thicknesses of the layers and so on are respectively selected so as to reduce the bias noise while keeping the high reproduced output over the whole frequency band range. This invention can provide an excellent magnetic recording medium as compared with a prior art single-layer magnetic recording medium (for example, a metal tape and so on). BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is an enlarged cross-sectional view of a prior art magnetic recording medium; Fig. 2 is an enlarged cross-sectional view of a magnetic recording medium according to this invention; Figs. 3A and 3B are respectively tables for indicating characteristics of respective examples of a single-layer magnetic recording medium, and; Figs. 4A and 4B are respectively tables for indicating characteristics of respective examples of a doublelayer magnetic recording medium according to this invention. BEST MODE FOR CARRYING OUT THE INVENTION The magnetic recording medium according to this invention will hereinafter be described in detail with reference to examples employing y-Fe2O3 magnetic powder as an iron oxide-based magnetic powder. In the description the specific surface area of the magnetic powder is a specific surface area according to the BET adsorption method. Examples 1. y-Fe2O3 magnetic powder having specific surface area of 20 m2/g and coercive force of 320 Oe 6 400 parts by weight thermoplastic polyurethane resin (ESTAN 5702 (trade name), manufactured by B.F. GOODRICH CORP.) ... 50 parts by weight vinyl-chloride-vinyl acetate copolymer (VAGH (trade name), manufactured by U.C.C. CORP.) ¯. 50 parts by weight lecithin ... 4 parts by weight methyl ethyl ketone ¯¯¯ 700 parts by weight cyclohexanone ... 50 parts by weight The afore-described compositions are mixed together and dispersed in a ball mill for 24 hours. After that, 20 parts by weight of polyisocyanate (DESMODUR L-75, manufactured by BAYER A.G.) is added thereto and then dispersed by high speed shear mixing for two hours to thereby produce magnetic paint. This is specified as magnetic paint A. 2. y-Fe2O3 magnetic powder having specific surface area of 20 m2/g and coercive force of 320 Oe ... 400 parts by weight thermoplastic polyurethane resin ... 33.3 parts by weight vinyl-chloride-vinyl acetate copolymer ¯-- 33.3 parts by weight lecithin ¯-* 4 parts by weight methyl ethyl ketone ¯¯¯ 650 parts by weight cyclohexanone ... 50 parts by weight The aforesaid compositions are mixed together and then treated by the same process as in the magnetic paint A of the first example to thereby produce magnetic paint. This is specified as magnetic paint B. 3. y-Fe2O3 magnetic powder having specific surface area of 20 m2/g and coercive force of 320 Oe ... 400 parts by weight thermoplastic polyurethane resin ¯¯¯ 25 parts by weight vinyl-chloride-vinyl acetate copolymer ... 25 parts by weight lecithin ... 4 parts by weight methyl ethyl ketone ... 625 parts by weight cyclohexanone ... 50 parts by weight The aforesaid compositions are mixed together and then treated by the same process as in the magnetic paint A of the first example to thereby produce magnetic paint. This is specified as magnetic paint C. 4. The y-Fe203 magnetic powder employed in the magnetic paint C in the third example is replaced with y-Fe2O3 magnetic powder having the specific surface area 20 m2/g and coercive forces 420 Oe (other compositions are the same) and magnetic paint is obtained by the same. process as in the magnetic paint A of the first example. This is specified as magnetic paint D. 5. The y-Fe203 magnetic powder employed in the magnetic paint C in the third example is replaced with y-Fe2O3 magnetic powders having specific surface area 30 m2/g and coercive forces 320 Oe, 420 Oe and 520 Oe respectively (other compositions are the same) and magnetic paints are obtained by the same process as in the magnetic paint A of the first example. These are specified as magnetic paints E, F and G, respectively. 6. The y-Fe2O3 magnetic powder employed in the magnetic paint A in the first example is replaced with y-Fe2O3 magnetic powders having specific surface area 40 m2/g and coercive forces 320 Oe, 420 Oe and 520 Oe respectively (other compositions are the same) and magnetic paints are obtained by the same process as in the magnetic paint A. These are specified as magnetic paints H, I and J, respectively. 7. The y-Fe2O3 magnetic powder employed in the magnetic paint C in the third example is replaced with Codeposition type y-Fe2O3 magnetic powders having specific surface area 30 m2/g and coercive forces 520 Oe, 720 Oe and 920 Oe respectively (other compositions are the same) and magnetic paints are obtained by the same process as in the magnetic paint A in the first example. These are specified as magnetic paints K, L and M, respectively. 8. The y-Fe2O3 magnetic powder employed in the magnetic paint C in the third example is replaced with Codeposition type y-Fe2O3 magnetic powders having specific surface area 40 m2/g and coercive forces 520 Oe, 720 Oe and 920 Oe respectively (other compositions are the same) and magnetic paints are obtained by the same process as in the magnetic paint A. These are specified as magnetic paints N, o and P, respectively. The characteristics of the aforesaid magnetic paints A to P are indicated on the table below in which P/B represents the ratio of the amount of magnetic powder P in weight to the amount of binder B in weight. EMI6.1 <tb> magnetic <SEP> coercive <SEP> specific <SEP> P/B <tb> paint <SEP> force <SEP> HC <SEP> <SEP> surface <tb> <SEP> of <SEP> magnetic <SEP> area <tb> <SEP> powder <tb> <SEP> A <SEP> 320 <SEP> Oe <SEP> 20 <SEP> m2/g <SEP> 4 <tb> <SEP> B <SEP> B <SEP> <SEP> 320 <SEP> " <SEP> 20 <SEP> " <SEP> 6 <tb> <SEP> C <SEP> 320 <SEP> " <SEP> 20 <SEP> " <SEP> 8 <tb> <SEP> D <SEP> 420 <SEP> " <SEP> 20 <SEP> " <SEP> 8 <tb> <SEP> E <SEP> 320 <SEP> " <SEP> 30 <SEP> " <SEP> 8 <tb> <SEP> F <SEP> 420 <SEP> " <SEP> 30 <SEP> " <SEP> 8 <tb> <SEP> G <SEP> 520 <SEP> " <SEP> 30 <SEP> " <SEP> 8 <tb> <SEP> H <SEP> 320 <SEP> " <SEP> 40 <SEP> " <SEP> 4 <tb> <SEP> I <SEP> 420 <SEP> " <SEP> 40 <SEP> " <SEP> 4 <tb> <SEP> J <SEP> 520 <SEP> " <SEP> 40 <SEP> " <SEP> 4 <tb> <SEP> K <SEP> 520 <SEP> " <SEP> 30 <SEP> n <SEP> <SEP> 8 <tb> EMI7.1 <tb> L <SEP> 720 <SEP> " <SEP> 30 <SEP> " <tb> <SEP> M <SEP> 920 <SEP> " <SEP> 30 <SEP> t <SEP> <SEP> 8 <tb> N <SEP> 520 <SEP> " <SEP> 40 <SEP> n <SEP> <tb> 0 <SEP> 720 <SEP> " <SEP> 40 <SEP> 1 <SEP> <SEP> 8 <tb> P <SEP> 920 <SEP> w <SEP> <SEP> 40 <SEP> " <SEP> 8 <tb> Thus, by the use of the afore-aescribed magnetic paints A to P, as shown in Fig. 1, the magnetic paints are coated on a non-magnetic base (for example, polyethylene telephthalate film having a thickness of 12 pm) 1, which is then subjected to a magnetic orientation treatment. After being dried, it is subjected to a super calender roll treatment to form a magnetic recording medium 3 having a single magnetic layer 2. As shown in Fig. 2, the same magnetic paints A to P are employed to form a first magnetic layer (under layer) 4 in the same process as described above and after this first magnetic layer 4 is hardened enoughly, a second magnetic layer (upper layer) 5 is formed in the same process as described above to construct a double-layer magnetic recording medium 6 according to this invention. Magnetic characteristics (residual magnetic flux density Br and coercive force Hc), reproduced outputs (MOLs), bias noise and so on of the magnetic recording media 3 and 6 formed as described above were measured. The results thereof are indicated on Figs. 3A, 3B and Figs. 4A, 4B, respectively. Figs. 3A and 3B indicate the characteristics of the magnetic recording medium 3 having the single-magnetic layer 2 made of the respective magnetic paints A to P (single-layer examples 1 to 35) and Figs. 4A and 4B indicate the characteristics of the double-layer magnetic recording medium 6 according to the present invention (examples 1 to 34 according to this invention). The magnetic recording medium (magnetic tape) used in the measurements is an audio tape cut with the 1/8inch wide. The measuring method for each characteristic is as follows. 1. The residual magnetic flux density Br is a residual magnetic flux density measured at an external magnetic field of 2000 Oe and the unit thereof is gauss. 2. The coercive force H C is a coercive force measured at an external magnetic field 2000 Oe and the unit thereof is oersted (Oe). 3. The MOL (maximum output level: reproduced output) is measured at a reference frequency (315 Hz) and a high frequency (10 kHz). The former is the output level at which the output signal with the frequency of 315 Hz becomes a distortion of 3 %, while the latter is the maximum saturation output level of a signal with the frequency of 10 kHz. 4. The bias noise is a noise when an auditory sense correction filter A (JIS) is utilized and the unit thereof is decibel (db). 5. The tape speed is 4.8 cm/sec. The MOLs (315 Hz and 10 kHz) and the bias noise are indicated by a relative value (db) which takes the single-layer example 1 in Fig. 3A as a reference (0 db). Figs. 3A, 3B and Figs. 4A, 4B reveal that the double-layer magnetic recording medium 6 according to this invention in which the specific surface area of the magnetic powder forming the second magnetic layer 5 as the upper layer is selected larger than that of the first magnetic layer 4 as the under layer produces a high reproduced output over the whole of low and high frequency band ranges and reduces the bias noise considerably as compared with the prior art single-layer magnetic recording medium 3. The specific surface area of the iron oxide-based magnetic powder forming the first magnetic layer 4 as the under layer is preferably selected about 20 m2/g (18 to 25 m2/g). If it is out of such range, the reproduced output of the low frequency band is lowered. The specific surface area of the iron oxide-based magnetic powder forming the second magnetic layer 5 as the upper layer is selected to be 28 m2/g or above, more preferably in a range from 28 to 45 m 2/g. If it is out of such range, the bias noise can not be reduced. Meanwhile, it is preferred that the coercive force HCl and the residual magnetic flux density Brl of the first magnetic layer 4 are selected in a range from 250 to 400 oersteds and 1600 to 2000 gausses, respectively. If the Br is out of the range of 1600 to 2000 gausses, the output of the low frequency band is lowered. The coercive force HC2 of the second magnetic layer 5 is preferably selected in a range from 250 to 500 oersteds. Particularly when the iron oxide-based magnetic powder is employed, it is desired that the coercive force is selected in a range from 250 to 500 oersteds or when the iron oxide-based magnetic powder having Co-compound or Fe and Co-compound coated thereon is used, it is desired that the coercive force is selected in a range from 300 to 900 oersteds. If it is out of such range, the output in the high frequency band is not increased and the recording bias and the erasing are restricted by a recording and reproducing apparatus. The optimum recording bias is determined in response to the coercive force of the magnetic layer. It is preferable that the relation between the coercive force HCl of the first magnetic layer 4 and the coercive force HC2 of the second magnetic layer 5 satisfies HCl < HC2 Also, it is preferable that the thickness of the first magnetic layer 4 is selected more than 2.0 p but less than 6.0 p. If the thickness thereof is selected less than 2.0 p, the reproduced output in the low frequency band is decreased. If it is selected more than 6.0 p, only the reproduced output in the low frequency band is increased excessively to make the balance between the low and high frequency bands poor. Moreover, the thickness of the second magnetic layer 5 is selected to be preferably in a. range from 0.2 to 2.0 p. If the thickness thereof is selected less than 0.2 p, the bias noise can not be reduced. If it exceeds 2.0 p, the reproduced output in the low frequency band is decreased. As described above, according to this invention, on the basis of the results of the characteristics in Fig. 4, there, is provided the magnetic recording medium 6 having the first and second magnetic layers 4 and 5 formed on the non-magnetic base 1 in turn as shown in Fig. 2 in which the first magnetic layer 4 is formed by coating the iron oxidebased magnetic powder the specific surface area of which is selected in a range from 18 to 25 m2/g according to the BET adsorption method, the coercive force HCl thereof is selected in a range from 250 to 400 oersteds and the residual magnetic flux density Br thereof is selected in a range from 1600 to 2000 gausses and in which the second magnetic layer 5 is formed by coating the iron oxide-based magnetic powder the specific surface area according to the BET adsorption method of which is selected to be 28 m2/g or above, more preferably, in a range from 28 to 45 m2/g and the coercive force HC2 thereof is selected in a range from 250 to 900 oersteds. According to the present invention as described above, it is possible to reduce the bias noise while keeping the high reproduced output over the whole frequency band range.

Claims: CLAIMS 1. A magnetic recording medium having a non-magnetic base, a first magnetic layer formed on said base and a second magnetic layer formed on said first magnetic layer, said magnetic recording medium being characterized in that said first magnetic layer is formed by coating an iron oxide-based magnetic powder the specific surface area according to a BET adsorption method of which is in a range from 18 to 25 m2/g, a coercive force HCl thereof is selected in a range from 250 to 400 oersteds, a residual magnetic flux density Br thereof is selected in a range from 1600 to 2000 gausses, said second magnetic layer is formed by coating an iron oxide-based magnetic powder the specific surface area according to the BET adsorption method of which is 28 m2/g or above, a coercive force HC2 thereof is selected in a range from 250 to 900 oersteds, and the coercive force HCl < the coercive force HC2 is satisfied. 2. A magnetic recording medium according to claim 1, characterized in that said second magnetic layer is formed by coating an iron oxide-based magnetic powder and a coercive force HC2 thereof is selected in a range from 250 to 500 oersteds. 3. A magnetic recording medium according to claim 1, characterized in that said second magnetic layer is formed by coating a Co-compound deposition type magnetic powder or Fe and Co-compound deposition type iron oxide based magnetic powder and a coercive force HC2 thereof is selected in a range from 300 to 900 oersteds. 4. A magnetic recording medium according to claim 1, characterized in that the specific surface area of the magnetic powder forming said second magnetic layer according to the BET adsorption method is selected in a range from 28 to 45 m2/g. 5. A magnetic recording medium according to claim 1, characterized in that a thickness of said first magnetic layer is selected in a range from 2 to 6 p. 6. A magnetic recording medium according to claim 1, characterized in that a thickness of said second magnetic layer is selected in a range from 0.2 to 2 p. 7. A magnetic recording medium according to claim 1, characterized in that said iron oxide-based magnetic powder is y-Fe203.

Inventors: KUBOTA, ISAO SONY CORPORATION
Applicants: SONY CORPORATION
Application year: 1982
Main classification: G11B5
IPC codes: G11B5