GB2299437A - Magnetic recording medium having a high recording density - Google Patents

Description

2299437 Magnetic recording medium having high recording density The

present invention relates to an essentially longitudinally oriented magnetic recording medium which has a very small layer thickness and small nonlinear 5 distortions.

Recently, the recording wavelength has been steadily decreased in order to satisfy the need for increased recording density for magnetic recording media. For example, the recording wavelength for the 8 mm video system is 0.58 pm. This gives rise to the problem of a loss of thickness in signal playback, ie. the playback level does not increase linearly as a function of increasing layer thickness but exhibits a saturation effect. Thus, only a very thin layer is required for is short wavelengths.

In order to meet this requirement, magnetic recording media in which a binder-free ferromagnetic metal layer was applied in a very small thickness by means of a vacuum method were developed in the last 10 years. Although these metal evaporated recording media have a small thickness loss and achieve a very high playback level, the mass production of such tapes still gives rise to considerable difficulties in comparison with magnetic recording media in which the magnetic pigments are dispersed in binders. Moreover, these ME tapes change under the influence of atmospheric oxygen.

However, it has recently been possible to meet the requirement for a small layer thickness also by means of a thin magnetic layer in which the finely divided magnetic particles are dispersed in a polymeric binder and this layer is cast on a non-agnetic substrate. Such application methods are described, for example, in US 2 819 186, German Laid-Open Application DE-A-4,302,516, EP 0 520 155, EP 0 566 100 and the German Applications P 44 43 896 and P 195 04 930.

Magnetic recording with high recording density is -Z- now predominantly carried out by a digital method. This means that, in contrast to the analog video recording, no sinusoidal signals are recorded and instead the information is recorded on the recording medium by switching over the direction of the current of the recording head. The magnetization pattern produced in such a switching process is referred to as the magnetization transition. However, this transition does not occur abruptly but more or less gradually, for example in the form of a Gauss curve. The playback signal of such a magnetization transition is pulse- like because inductive reading, which is typically used in the video system, is based on differentiation. Since the magnetization transition as described above does not occur abruptly, the read pulses is have a certain width, which is usually defined by the PW50 value. This value indicates the distance on the recording medium between the two points at which the actual signal assumes precisely 50% of the maximum value, as shown in the Figure. It is clear that high-density recording requires very small pulse widths.

Since direct measurement of the pulse widths is expensive because a time range measurement (- measurement of the signal as a function of time) is required, a frequency range measurement is therefore frequently relied upon. From the theory of magnetic recording (Ref. B.K. Middleton, Chapter 2 in Handbook of Magnetic Recording, ed. Mee & Daniel, McGraw Hill), it is known that read pulses can be approximated as Lorenz pulses:

e (x) = A 2 1 + ( 3a) A = Amplitude (1) Here, a is the width of the magnetization transition. It is now easy to show that the pulse width is given by:

PW5 0 2 2 a (2).

Prom the theory of magnetic recording, it is also known that the frequency response of a magnetic recording medium is given by:

n A PW50 PW50 2rT E(k) = naA exp(-ka)= 2 -^p(-k 2), where k = -T- (3) X is the recorded wavelength, and the corresponding f requency f is given by f = v/X. E (k)is the Fourier trans f orm of e (x).

If a frequency range measurement is now carried out, the function E(k) can be reconstructed by means of Fourier transf ormation of the function e (x). In a linear system, identical curves E(k) for the frequency response are obtained if the relationship v = fX (f = frequency, v = relative head/tape speed) is used for the conversion. If the two curves do not coincide, nonlinear effects are present. These are undesirable because distortions occur is which are not foreseeable and hence cannot be corrected a priori. It is therefore desirable to keep such distortions as small as possible.

The monograph by J.C. Mallinson, The Foundation of Magnetic Recording, Academic Press 1987, reveals that magnetic recording media under certain circumstances also exhibit nonlinear distortions which can be very troublesome in the case of high- density recording.

Starting from the abovementioned prior art and from the errors encountered in the case of magnetic recording media, it is an object of the present invention to provide a recording medium of the generic type stated at the outset which exhibits only slight nonlinear distortion.

We have found that this object is achieved by a recording medium having the features stated in the defining part of the claim.

We have found that a magnetic recording medium exhibits only small, nonlinear distortions if, in the case of a square-wave recording, the output level oUtT which is calculated after reconstruction of the frequency response from a single pulse measurement at wavelength X = 2 g,, (g,, = electrical head gap width) is greater than the output level out, of a conventional square- wave recording, measured at the same wavelength and in a frequency range of from 100 kHz to 8 MHz. Furthermore, in the case of such a recording medium, the ratio of the thickness of the magnetic layer d to the head gap width gw is less than 0.84.

The invention is illustrated below.

The measured values were obtained using an experimental setup with magnetic recording media whose structure is described below, under the following condi- is tions: Relative head/tape speed: v = 3.17 m/s Write/read head: VS type, measured gap zero point gw = 236 nm The recording current was adjusted so that the level of the fundamental wave of the square-wave signal is a maximum in square-wave recording at a wavelength X = 3 gw = 708 nm. The measurement is carried out using a spectrum analyzer (resolution bandwidth 30 kHz).

The differences, according to the invention, between oUtT and out, are also obtained when the level is set with another wavelength X instead of three times the measured head gap width, for example with twice the wavelength, but in this case the differences between novel recording media and recording media not according to the invention are not as large as under the abovementioned measuring conditions. 1) Time range measurement for calculating oUtT A square-wave signal having a frequency of 96 kHz was recorded on the recording medium. The signal was then read by means of the read head and was scanned with a digitizer. The scanning rate was 5 ns. A single pulse was calculated from the scanned signal, said pulse being obtained by averaging the centered read pulses. A total of 126 pulses were averaged. The averaged single pulse was now subjected to a Fourier transformation. An FFT algorithm was used for this purpose. Examples of such algorithms are to be found in standard works on numerical mathematics. A triangular function (Parzen window) was used as a window function. By converting the peak values into effective values (factor /2), the curve in logarithmic representation (dB scale) can be shifted so that it is directly comparable with a conventional frequency range measurement.

2) Frequency range measurement for determining oUtF A square-wave signal having a variable frequency of from 100 kHz to 8 MHz was recorded using the same current as in 1). The effective value of the fundamental wave of the particular signal was then measured using a spectrum analyzer (resolution bandwidth 30 kHz).

Typical minimum possible wavelengths are about 2 - gw, which in this case corresponds to about 6.5 MHz.

Accordingly, the curves are evaluated at 6.5 MHz.

As is evident from the examples below, it has been found that magnetic recording media whose difference OUtT_oUtF is positive have smaller linear distortions and hence better high-density recording properties.

There are in principle no restrictions to the composition of the magnetic recording medium, preferably consisting of the layer containing the magnetic pigments and a nonmagnetic substrate.

The prior art magnetic pigments, such as iron oxide, Co-doped iron oxides, metal pigments and metal alloys, chromium dioxide and others, may be used, as may the conventional polymeric binders or binder mixtures and the other additives, such as dispersants, nonmagnetic pigments, lubricants, curing agents, wetting agents and solvents.

Suitable components of the magnetic layer and of the nonmagnetic layer are described, for example, in _6 DE-A 4 302 S16.

Known films of polyesters, such as polyethylene terephthalate or polyethylene naphthalate, and polyolefins, cellulose triacetate, polycarbonates, polyamides, polyimides, polyamidoimides, polysulfones, aramids or aromatic polyamides, serve as substrates. The substrate may be subjected beforehand to a corona discharge treatment, a plasma treatment, a slight adhesion treatment, a heat treatment, a dust removal treatment or the like. in order to achieve the object of the invention, the nonmagnetic substrate is one having a center line average surface roughness of in general 0.03 = or less, preferably of 0.02 ym or less, in particular of 0.01 gm or less. It is also desirable for the substrate not only to is have such slight center line average surface roughness but also to have no large protuberances of 1 pm or more. The roughness profile of the surface of the substrate can, if desired, be freely controlled according to the size and amount of the filler to be added to the sub- strate. Examples of suitable fillers are oxides and carbonates of Ca, Si and Ti and fine organic powders of acrylic substances.

The process for the preparation of the magnetic dispersion comprises at least one kneading stage, one dispersing stage and, if required, one mixing stage, which may be provided before and after the preceding stages. Particular stages may each comprise two or more stages. In the preparation of the composition, all starting materials, ie. the ferromagnetic powder, the binder, the carbon black, the abrasive, the antistatic agent, the lubricant and the solvent, may be added to the reactor immediately at the beginning of the process or subsequently in the course of the process. The individual starting materials may be divided into a plurality of portions, which are added to the process in two or more stages. For example, the polyurethane is divided into a plurality of portions and is added in the kneading stage and in the dispersing stage and also the mixing stage for adjusting the viscosity after dispersing.

In order to achieve the object of the present invention, a known conventional technology may also be used as part of the process f or the production of the novel magnetic recording medium. For example, a kneading apparatus having a high kneading f orce, f or example a continuous kneader or a pressure kneader, may be used in the kneading stage in order to obtain a novel magnetic recording medium having a high Br value. If such a continuous kneader or pressure kneader is used, the ferromagnetic powder is kneaded with the total binder, preferably 30% by weight or more. For example, 100 parts is by weight of a ferromagnetic powder are mixed with from 15 to 500 parts by weight of a binder.

After fine filtration through a narrow-mesh filter having a mesh size of not more than 5 pm, the dispersions are applied by means of a conventional coating apparatus at speeds in the usual range of 100-500 m/min, are oriented in the recording direction in a magnetic field and then dried and subjected to calendering and, if required, a further surf ace- smoothing treatment.

Essentially longitudinally oriented is intended to mean the magnetic particles are present oriented essentially in the plane of the layer in the recording direction, but may also be oriented inclined at an angle up to 25 to the plane of the layer.

The coating may be effected by means of a doctor blade coater, a knife coater, a doctor, an extrusion coater, a reverse roll coater or a combination. The two layers can preferably be applied simultaneously by the wet-on-wet method.

The magnetic recording medium thus obtained is then slit longitudinally or punched into the usual width for use and subjected to the conventional electroacoustic tests and the mechanical tests.

Particularly advantageous results are obtained when a very thin magnetic upper layer whose thickness is less than 1 gm is cast on a nonmagnetic substrate whose layer thickness can be 1-8 gm.

The invention is illustrated with reference to practical examples and comparative examples, but without restricting the invention and the apparatus for the production of such a magnetic recording medium to the specific formulation examples.

A magnetic recording medium consisting of a thin magnetic upper layer which was cast on a nonmagnetic r2ubstrate was produced by means of an apparatus as described in more detail in German Application is P 195 04 930. The two layers are based on the following formulations:

a) Composition of the lower layer Vinyl polymer having polar groups Polyurethane having polar groups Ti02 (55 M2/g BET) Lubricant Polyisocyanate Solvents (tetrahydrofuran, dioxane) Parts by weight 85 85 1000 25 30 2209 The viscosity of this lower layer is 50 mPa.s and the flow limit is 18 Pa.

b) Composition of the upper layer Magnetizable metal pigment a-A1203 (particle size = 0.2 pm) Vinyl polymer having polar groups Polyurethane having polar groups Phosphoric eater Lubricant Polyisocyanate Solvents (tetrahydrofuran, dioxane) Parts by weight 1000 70 77 77 10 25 22.5 6170 The viscosity of this upper layer is m.Pa-s and the flow limit is%.5"Pa.

The measurement of the viscosity and of the flow limits was carried out using a Carri-Med CSL Rheometer in the plate-and-cone measuring system at 25C, and the evaluation was effected according to Bingham (descending curve). Table I Example Ec [kA/m] M, [kA/MI S = M,/M, d [gm] 1 (COMP.) 2 (COMP.) 3 (COMP.) 4 6 Fuj i SDC (COMP.) Table 1 shows the magnetic and mechanical data of the magnetic recording media obtained using different magnetic pigments, which are shown in the Table. d in pm is the dry layer thickness of the upper layer, and the mean particle length (in nanometer) of the magnetic particles after volume averaging is shown in the last column. The particle size was measured under the electron microscope at 100,000 times magnification.

Table 2 below shows the results for the difference between OutT [calculated according to (1)] and outF [measured according to (2)], both determined at 6.5 MHz. A magnetic recording medium which has the name Fuji SDC, is available on the market and is intended for Hi-8 recording is also included as a comparison.

144 148 181 187 183 182 133 326 326 363 334 346 336 280 SYD 0.89 0.83 0.9 0.87 0.84 0.83 0.88 1.5 0.2 1.2 0.17 0.14 0.13 0.4 0.27 0.28 0.3 0.31 0.3 0.31 0.3 Mean parti Cle length (=1 so 80 so so 150 Table 2

Difference in output levels Example OUtT - OUtY d/g,, 1 (COMP.) - 1.1 dB 6.36 2 (Comp.) - 0.3 0.85 3 (COMP.) - 0.5 5.08 4 + 1.1 0.72 + 1.7 6 + 1.6 Fuji SDC (Comp.) - 2.5 0.59 0.55 1.69 Although the invention has been described in this specification with regard to magnetic recording media, it will be appreciated that the invention also relates to a method of manufacturing magnetic recording medium and to recorded, e.g. pre-recorded, recording media.

Claims (6)

- 11 CLAIMS

1. A magnetic recording medium which is intended for high-density recording comprising a magnetic layer having magnetizable particles aligned essentially in the longitudinal direction, the magnetic recording medium having small nonlinear distortions, wherein the recording current in the case of square-wave recording at a wavelength corresponding to three times the electrical head gap width is adjusted to the maximum level of the fundamental wave of the square-wave function, and the output level oUtT? which is calculated after reconstruction of the frequency response from a single pulse measurement at the wavelength X = 2g,, (g,,= electrical head gap width), is greater than the output level OUtFJI measured directly at the same wavelength for conventional square-wave recording.

2. A magnetic recording medium as claimed in claim 1, wherein the ratio of the thickness d of the magnetic layer to the measured head gap width g. is less than 0. 84.

3. A method of recording high-density signals on a magnetic recording medium having magnetizable particles present in a magnetic layer which are aligned essentially in the longitudinal direction, the magnetic recording medium having small nonlinear distortions, comprising adjusting a square-wave recording current, at a wavelength corresponding to three times the electrical head gap width, to the maximum level of the fundamental wave of the square-wave function, the output level OUtT#' which is calculated after reconstruction of the frequency response from a single pulse measurement at the wavelength X = 2g,, (g,,= electrical head gap width), being greater than the output level oUtF measured directly at the same wavelength for conventional square-wave recording.

4. A magnetic recording medium carrying signals recorded by the method claimed in claim 3.

12 -

5. A magnetic recording medium substantially as herein described with reference to the accompanying drawing.

6. A method of recording on a magnetic recording medium substantially as herein described with reference to 5 the accompanying drawing.