JPH038015B2 - - Google Patents

Description

[Detailed description of the invention]

Acicular alloy magnetic powders containing iron as a main component are CrO ₂ , γ-Fe 2 O 3 , CrO 2 , γ-Fe ₂ O ₃ ,
Compared to oxide magnetic powder such as Co-coated γ-Fe ₂ O ₃ ,
It has a coercive force and saturation magnetic moment more than 1.5 times,
It is possible to greatly improve the playback output,
It has excellent properties suitable as a magnetic material for high-density recording. Many methods for manufacturing this alloy powder have been published in patents, literature, etc. so far. These methods can be broadly classified into the following four methods. (1) A method of thermally reducing acicular iron oxides such as α-FeOOH. (2) A method in which metal powder is reduced and precipitated from an aqueous metal salt solution using a reducing agent such as a borohydride compound. (3) A method to obtain metal powder by evaporating metal in an inert gas such as Ar. (4) There is a method of electrolytically depositing metal powder on the Hg cathode. However, in either method, the surface of the metal powder obtained is highly chemically active, causing a chemical reaction with the binder during the tape or disk manufacturing process, or even if no chemical reaction occurs, the metal magnetic powder has a large magnetic moment. Because of this, agglomeration of magnetic powders tends to occur, which causes damage to the dispersibility of the magnetic powders and the surface properties of the coated film after application. For these reasons, even though recording media coated with alloy magnetic powder show an improvement of 4 to 10 dB in output C compared to media using oxide magnetic powder, the modulation noise generated during signal recording is (N _n ) also increases almost proportionally, and no significant improvement is seen in the signal-to-noise ratio (C/N). For the purpose of increasing C/N, (1) forming a dense ultra-thin oxide film on the surface of the alloy magnetic powder or modifying the surface of the alloy magnetic powder using a surfactant; A method for improving the dispersibility of magnetic powder in a coating film and the surface properties of the coating film. (2) Measures have been taken to reduce the particle size of magnetic powder. However, the cause of noise is that the surface roughness of the recording medium and the particle size and magnetic properties of the magnetic powder used are closely linked to each other.
C/N cannot essentially be improved by improving only one of them. For example, as is done in the audio field, the S/N is calculated as the output at a certain recording wavelength.
When evaluated as a ratio of AC cancellation noise (N _AC ), S/N improvement is achieved by reducing AC cancellation noise, but as will be explained later, this means that the amount of AC cancellation noise is Since this is influenced more by the particle size than by the medium surface roughness, this can be achieved mainly by making the particle size finer. On the other hand, in the video domain, modulation noise (Nm) that occurs as a sideband near the carrier when recording a carrier signal has a large effect on image quality, and noise evaluation must also include modulation noise. It is more severe compared to . The cause of this modulation noise has traditionally been thought to be agglomerates of magnetic particles in the coating film or surface roughness, and modulation noise has been mainly reduced by improving the tape coating method. was. However, according to the studies conducted by the present inventors, modulation noise is greatly influenced by the particle size of the magnetic powder, in addition to the agglomerates of the magnetic powder and the coating surface roughness, and improvements in surface roughness, etc. alone cannot improve modulation noise. It is difficult to obtain a magnetic recording medium using alloy magnetic powder that has superior C/N characteristics to magnetic recording media using oxide-based magnetic powder, and it is difficult to obtain a magnetic recording medium using alloy magnetic powder that has superior C/N characteristics to magnetic recording media using oxide-based magnetic powder. I saw what needed to be done. An object of the present invention is to provide a means for improving the C/N characteristics of a magnetic recording medium coated with acicular alloy magnetic powder, and to provide a tape coated with Co-coated γ-Fe ₂ O ₃ magnetic powder. The C/N characteristic is 3dB or more.
This specifies the relationship between the particle size of the acicular alloy magnetic powder, the magnetic properties, and the surface roughness of the recording medium, which should be 5 dB or more. The above Co-coated type γ-Fe ₂ O ₃
The standard application tape was T-60E (trade name of Hitachi Maxell, Ltd.) tape for VTRs. The principle of the present invention will be explained below. Total media noise (N) generated from magnetic recording media
consists of two parts as shown in Figure 1. One occurs when performing AC erase.
AC cancellation noise (N _AC ) 21 and modulation noise (N _n ) 22 that occurs when the signal is recorded.
It is a kind. In the following text, the amount of noise refers to the value integrated over the shaded area of the band from 1MHz to 7MHz, as shown in Figure 1. Furthermore, the relationship shown in equation (1) holds between these noise amounts. N=√ _A ² _C + ² (1) The inventors have clarified the cause of these noises as described below. The AC cancellation noise is mainly related to the saturation magnetization amount (σ _s ) of the magnetic powder, its particle size, volume (v), and the saturation magnetic flux density (B _s ) of the medium.
In addition, in the case of alloy magnetic powder produced by the hydrogen reduction method, the presence or absence of sintering also matters. Figure 3 shows the relationship between AC cancellation noise and surface roughness.
It can be seen that the AC erase noise has little correlation with the dispersibility and surface roughness of the magnetic powder in the coating film, and is mainly related to the particle size of the magnetic powder. On the other hand, modulation noise occurs as a sideband near the signal when recording a carrier signal, and its magnitude is proportional to the signal output. And its proportional coefficient is the second
As shown in the figure, the worse the surface roughness and the larger the particle size, the larger the modulation noise becomes. In order to reduce total media noise in this way, especially to reduce modulation noise, it is necessary to improve the tape surface roughness and reduce the particle size.
-60E), the C/N characteristic should be 3 dB or more.
It has been found that the relationship between tape surface roughness and particle size is at a point on the curve shown in FIG. 4 or in a region below it. However, there are lower limits to the appropriate tape surface roughness and particle size as described below, and the magnetic properties of the alloy magnetic powder to effectively exhibit the effects of the present invention must satisfy the following conditions. No. First, the smoother the tape surface, the better the C/N. However, if it is made too smooth and the surface roughness becomes less than 0.02 μm _PP , as shown in Example 3, then the C/N will be improved. In this case, the relative speed between the tape and the head is high, so the friction between the tape and the head increases, resulting in poor strike characteristics.
The tape coating peels off and causes clogging of the head. For this reason, in the case of magnetic tape that comes into direct contact with the head, the surface roughness should be
It is desirable to make it 0.025μm _PP or more. However, this restriction is not necessary in the case of a coated disk in which the head floats above the medium surface without contacting it. Furthermore, regarding the particle size of the acicular alloy magnetic powder, since there is a lower limit to the surface roughness, the length of its major axis must be determined.
It is desirable that the particle size be 0.45 μm or less, but if the particle size is too small, for example, in the case of acicular alloy magnetic powder with a major axis length of less than 0.1 μm, as in Example 4, the surface will be oxidized in the atmosphere, resulting in an oxide film. It reaches the core of the needle-like alloy and can no longer exist as a magnetic substance, or it loses its magnetic characteristics as a metal magnetic powder. The lower limit of the particle size of this acicular alloy magnetic powder was conventionally thought to be around 100 Å, where superparamagnetism begins to appear in the case of granular Fe powder. However, Fe
In the case of alloy powders whose main components are All of the alloy magnetic powder must be oxidized to provide stability in air. Furthermore, as described in Example 4, if the particle size is too small, most of the Fe
Since it becomes an oxide, the magnetic properties will be impaired. Therefore, the lower limit of particle size is determined by the thickness of the oxide film that provides stability. For this reason, the short axis length of the acicular alloy magnetic powder to be used in the present invention is 0.02 μm or more, and the long axis length is approximately
It is desirable that the thickness is 0.1 μm or more. Further, the long axis short axis ratio may be in the range of 5 to 15 so as to maintain uniaxial anisotropy. Further, the magnetic properties of the acicular alloy magnetic powder to be applied to the present invention must satisfy the following conditions. Generally, the coercive force of the acicular alloy magnetic powder is affected by the particle size, and the smaller the particle size, the higher the coercive force tends to be. The long axis length of the acicular alloy magnetic powder in the present invention is in the range of 0.45 to 0.1 μm. ,
The measured magnetic properties should have a squareness ratio of at least 0.49 and a coercive force of 1100 Oe or more.If an acicular alloy magnetic powder with a lower squareness ratio or coercive force is observed under an electron microscope, it will show a needle. In the case of alloy powders with poor shape or produced by hydrogen reduction, a sintering phenomenon is observed. As shown in Example 5, a tape coated with such an alloy magnetic powder has a low output and increases noise, so that the effects of the present invention cannot be effectively exhibited. As described above, it is desirable that the acicular alloy magnetic powder according to the present invention has a coercive force of 1100 Oe and a squareness ratio of 0.49 or more. The alloy magnetic powder to be compared in the present invention is an acicular alloy magnetic powder mainly composed of iron, and includes pure iron metal powder with an iron content of 100% by weight as well as nickel, cobalt, chromium, and tin. , refers to alloys containing additional elements such as aluminum, titanium, boron, and silicon. The present invention will be described in detail below with reference to Examples. Example 1 An alloy magnetic powder with a major axis length of 0.33 μm and a major axis to minor axis ratio of 10 was suspended in toluene at a predetermined temperature, air was blown into this suspension, and aeration treatment was performed to improve the surface of the alloy magnetic powder. A dense oxide film layer was formed on the surface. The magnetic properties of the magnetic powder A are Hc1350Oe, σ _s 126emu/g,
σ _r /σ _s =0.50. Using this magnetic powder, a magnetic paint with polyurethane resin as a binder was prepared, and after coating it on a polyester film to a dry coating thickness of 4 μm, it was calendered to obtain magnetic tapes with different surface roughness. Two types of tapes were obtained which were subjected to different number of roll treatments. These tapes were named 2 and 3, and their properties are shown in Table 1. Example 2 Magnetic powder B having an oxide film layer formed on the surface of alloy magnetic powder having a major axis length of 0.37 μm and a major axis to minor axis ratio of 10 was obtained in the same manner as in Example 1. Its magnetic properties are
Hc1280Oe, σ _s 128emu/g, σ _r /σ _s =0.51. Using the same binder as in Example 1, the magnetic powder was treated with a ball mill for a longer time than in Example 1 to prepare a paint with good dispersibility, and the paint was applied to a tape. Thereafter, three types of tapes 4, 5, and 6 having different surface roughness were obtained in the same manner as in Example 1. First, its characteristics etc.
Shown in the table. Example 3 The same oxide film coating treatment as in Example 1 was performed using three types of alloy magnetic powders having different particle sizes from those in Examples 1 and 2. CDE these alloy magnetic powders
The particle size and magnetic properties were as follows. Powder C: Long axis length 0.45μm, axial ratio 12 Hc = 1250Oe, σ _s = 129emu/g, σ _r /σ _s = 0.53 Powder D: Long axis length 0.30μm, axial ratio 8 Hc = 1270Oe, σ _s = 127emu /g, σ _r /σ _s = 0.52 Powder E; major axis length 0.22 μm, axial ratio 9 Hc = 1400 Oe, σ _s = 124 emu/g, σ _r /σ _s = 0.50. A magnetic powder coating was prepared from these alloy magnetic powders using the same binder and dispersion conditions as in Example 2, and was applied to a tape. Thereafter, in the same manner as in Example 1, two types of tapes 7, 8, 9, 10, 11, and
I got 12. Its characteristics are shown in Table 1. The magnetic properties of these 11 types of magnetic tapes obtained in the above examples were measured under the conditions of a maximum magnetic field of 10 kOe, and the surface roughness was measured using a stylus roughness meter (Peltmeter WVR model manufactured by Pelt). Table 1 shows the peak-to-peak values (Rz) when using a 75 μm high-pass filter. The recording and reproducing characteristics of the tapes prepared in Examples 1, 2, and 3 were measured using a modified VHS type household VTR deck strike system. In the measurement, the head output is
The output, AC
Erasing noise and modulation noise were measured. Measurement conditions were head tape relative speed 4.0m/s, recording frequency 4M.
It is Hz. The effective gap length of the head used is 0.4μm.
It is. The results are also shown in Table 1. As a standard tape, Co-coated γ-Fe ₂ O ₃

【table】

[Table] Hitachi Maxell KK VTR tape T- using
60E, the output C/N (marked with **) is the value when the value of the standard tape is set to 0 dB, and the modulation noise to output ratio and AC cancellation noise (marked with *) are the values of the standard tape, which are set to 1. The relative ratios are shown in Table 1. Figure 2 shows the relationship between tape surface roughness and modulation noise to output ratio (N _n /C) for each particle size. The smoother the tape surface and the smaller the particle size, the smaller the N _n /C. area decreases.
That is, it shows that C/N _n is improved.
In addition, in this figure, each curve is removed from the surface roughness of 0, and the extension line passes almost near the origin, indicating that the main cause of modulation noise is surface roughness rather than the dispersion of magnetic particles. It also shows that C/
This shows that smoothing the surface roughness is very important for improving N _n . However, tape 8
Although the surface roughness was very smooth at 0.02 μm _PP , the tape coating peeled off due to the increased frictional force with the head, making measurement impossible. Figure 3 shows the relationship between AC cancellation noise (N _AC ) and surface roughness, and it is clear that AC cancellation noise is hardly affected by surface roughness, and furthermore, the order of its size corresponds to particle size. I know what you're doing. The numbers attached to the measurement points in FIGS. 2 and 3 are the tape numbers of the tapes obtained in each of the above Examples. Example 4 After producing alloy magnetic powder with a major axis length of 0.08 μm and an axial ratio of 5, it was suspended in toluene in the same manner as in Example 1 without being exposed to air, and then subjected to mild oxidation conditions (temperature 50 ℃) to form an oxide film on the powder surface to obtain alloy magnetic powder F. The magnetic properties of this magnetic powder were Hc = 850 Oe, σ _s = 95 emu/g, σ _r /σ _s = 0.43, and it had already lost the magnetic properties of the alloy magnetic powder. Example 5 After producing alloy magnetic powder with a major axis length of 0.27 μm and an axial ratio of 7, it was suspended in toluene in the same manner as in Example 1, and aerated with air to form an oxide film on the powder surface to obtain alloy magnetic powder F. I got it. The magnetic properties of this magnetic powder were Hc = 1030 Oe, σ _r = 127 emu/g, and σ _r /σ _s = 0.48. This alloy magnetic powder has lower coercive force and squareness ratio than alloy magnetic powder E having the same major axis length. When this alloy magnetic powder was observed under an electron microscope, it was found that it had poor acicularity, and branching and sintering between particles were observed. This alloy magnetic powder was mixed with the same binder as in Example 2,
A magnetic powder paint was prepared under the same dispersion conditions to obtain a magnetic tape 13. Examples 1, 2, and 3 show the recording and playback characteristics of this tape.
As a result of measurement under the same conditions as shown in Table 1, tape 11 has the same particle size and surface roughness as shown in Table 1.
Both output and C/N are inferior. To summarize the above examples, an acicular alloy having a major axis length between 0.1 μm and 0.5 μm, an axial ratio of 5 to 15, and a magnetic property with a coercive force of 1100 Oe and a squareness ratio of 0.49 or more. A magnetic tape was made using magnetic powder, and the C/N characteristic of the tape was 3 dB better than the standard tape (manufactured by Hitachi Maxell KK, T-60E for VTR; using Co-coated γ-Fe ₂ O ₃ magnetic powder). Curve 31) or
The relationship between particle size and tape surface roughness when exhibiting excellent characteristics of 5 dB (curve 32) is shown in Figure 4. In FIG. 4, tapes with surface roughness of 0.02 μm _PP or less should be excluded from the present invention from the viewpoint of tape running characteristics and coating peeling, as described above. Additionally, since there is a lower limit to tape surface roughness, acicular alloy magnetic powder with a major axis length exceeding 0.5 μm is difficult to achieve a C/N of 3 dB or more compared to standard tape, and is therefore excluded from the present invention. Should. Similarly, acicular alloy magnetic powder with a major axis length of less than 0.1 μm should be excluded from the present invention due to its stability in the atmosphere as described above. Furthermore, the effect of the present invention was confirmed when the tape surface roughness was 0.08 μm _PP or less.

[Brief explanation of the drawing]

Figure 1 is a graph showing AC erasure noise and modulation noise of magnetic tape, and Figure 2 is a graph showing the relationship between tape surface roughness and modulation noise to output ratio (N _n /C) for acicular alloy magnetic particles with different particle sizes. Figure 3 is a graph showing the relationship between tape surface roughness and AC cancellation noise (N _AC ) for tapes coated with acicular alloy magnetic powder of different particle sizes. Figure 4 shows that the tape coated with acicular alloy magnetic powder is Co-coated γ-
2 is a graph showing the correlation between the particle size of acicular alloy magnetic powder and tape surface roughness when the C/N ratio is 3 dB or more or 5 dB or more compared to a standard tape coated with Fe ₂ O ₃ . 1 to 12 are tape numbers, 21 is AC erasure noise, 22 is modulation noise, 31 is a case where the C/N is 3 dB better than the standard tape, and 32 is a case where the C/N is 5 dB better than the standard tape.

Claims (1)

[Claims] 1. A magnetic recording medium in which acicular alloy magnetic powder mainly composed of iron is coated on a non-magnetic base material, wherein the coercive force of the acicular alloy magnetic powder is at least 1100 Oe or more,
The squareness ratio is 0.49 or more, and when the acicular alloy magnetic powder is applied, the surface roughness (Rz; peak-to-peak value in μm) of the coating film is 0.08μm _PP or less,
0.025μm _PP or more, and the long axis length of the used acicular alloy magnetic powder (l; simple average value in μm)
is a point on the curve expressed by l = 0.070 x Rz ^-05 or smaller, the lower limit is 0.1 μm, the major axis minor axis ratio is in the range of 5 to 15, and recording in the video frequency band. A magnetic recording medium characterized by being capable of. 2 The upper limit of the long axis length of the acicular alloy magnetic powder is l=
The magnetic recording medium according to claim 1, which lies on a curve expressed by 0.062×Rz ^-048 . 3. The magnetic recording medium according to claim 1 or 2, wherein the magnetic recording medium is for video. 4. The magnetic recording medium according to claim 3, wherein the non-magnetic base material is tape-shaped.