JPH10105901A - Method of inspecting magnetic disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a high detection sensitivity by writing a signal at a specific current value corresponding to an output value lower than a saturated output value in a saturation characteristic of a magnetic head and judging a point as defective, from which a signal larger than a specific level is detected. SOLUTION: A current value at rising of a saturation characteristic and a saturated current of 3.5 inch HD are measured at a recording frequency of 500kHz on an outer circumferential part of a disk at 4,500rpm, and 9mA and 35mA are obtained respectively. At beginning of inspection procedures, the HD is DC-degaussed by using a magnetic head with a 2.54μm track width, and a 500kHz signal is written thereon at 9mA; a reproduced signal is checked; and a level 1.5 times as high as an average level is set to be a judgement level. And then, the HD is inspected in same procedures while traveling on a track sequentially, and defects are judged according to the judgement level. The track width used here is 10μm, and a track travel is also made to be 10μm. An inspection time is reduced to a quarter of the conventional method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inspecting a magnetic disk for defects. In particular, the present invention relates to a magnetic disk inspection method capable of inspecting a short defect at high speed.

[0002]

2. Description of the Related Art In a conventional magnetic disk inspection,
By using a magnetic head, signals are actually written and read one track at a time in the same manner as in a practical case.
There is a method of detecting an error. The write current value of the signal used at that time is large enough to record on the disk, and the signal frequency is such that the recording density on the disk is about the same as the recording density when used in a drive for normal practical use Is used.

[0003] In recent years, however, the linear recording density of a magnetic disk has been dramatically increased, and the operating frequency has been accordingly increased. Therefore, in the conventional inspection method of performing recording and reproduction at a frequency at the time of practical use, there is a problem that the inspection machine needs to correspond to a high frequency, so that the inspection machine becomes expensive. Further, in recent years, the track density of the magnetic disk has been increased, and the conventional inspection method of repeating writing and reading one track at a time has a problem that the inspection time is long. In order to avoid this problem, a method of inspecting tracks at regular intervals may be adopted, but in this case, the defect in the skipped track cannot be detected, so there is a roughly long circumferential direction there. If there is a defect, there is a problem that error correction becomes impossible when incorporated in the drive, and data cannot be read.

In addition, if a head having a wider track width than that of a practical machine is used, inspection can be speeded up without skipping a track. However, if a wide head is used, the magnetic head will be defective if the width of the defect is narrow. And the normal part is read, and a superimposed signal of both is output, so that the rate of change of the output change due to the defective part becomes small and its detection becomes difficult, and it becomes difficult to detect a minute defect. was there.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of inspecting a magnetic disk capable of inspecting at a high speed without overlooking even a minute defect. is there.

[0006]

SUMMARY OF THE INVENTION According to the present invention, there is provided a method of controlling a magnetic head to give an output of 10% of a saturated output value in a current-output characteristic (saturation characteristic) after uniformly magnetizing or demagnetizing a disk in advance. This is accomplished by writing a signal with a lower current value and determining that a defect is detected at a location where a signal greater than a predetermined level is detected as a defect with high sensitivity.

That is, according to the present invention, after a magnetic disk is uniformly magnetized, a signal write operation is performed using a magnetic head at a current value giving an output of 10% or less of a saturation output value in the current-output characteristic of the head. Then, reproduction is performed using a magnetic head, and a portion where a signal of a predetermined level or higher is detected is regarded as a defective portion.A method for inspecting a magnetic disk, and after demagnetizing the magnetic disk, The saturation output value of 10 in the current-output characteristic of the magnetic head
%, A signal write operation is performed with a current value giving an output of not more than%, and then reproduction is performed using a magnetic head, and a portion where a signal of a predetermined level or more is detected is determined as a defective portion. To provide.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention resides in a method for inspecting a magnetic disk for defects. The magnetic disk is not particularly limited as long as it writes and / or reads information using magnetism, and is coated with ceramic magnetic fine powder such as γ-Fe ₂ O ₃ or metal fine magnetic powder such as Fe. The present invention can be applied to a magnetic disk formed by the above method or a magnetic disk formed of a metal thin film formed by vapor deposition, sputtering, plating, or the like. Also,
The direction of the magnetic field may be either longitudinal recording or perpendicular recording.

An object of the present invention is to detect a short defect in the circumferential direction, which may cause a detection failure in a general detection method. That is, as described above, in the defect measurement according to the conventional method, the track is skipped for the purpose of shortening the time and the inspection is performed, so that a defect near the circumference may be overlooked. However, if a signal is written with a current value lower than usual by using a magnetic head or the like to perform uniform magnetization by DC erasing or demagnetize by AC erasing, recording is performed only on the defective portion, and this is reproduced. Then, even if a wide magnetic head is used, even a minute defect can be detected with high sensitivity without overlooking it.

The reason can be considered as follows. That is, the defective portion is a portion where the output becomes low during normal recording. It is conceivable that the magnetic layer is thinned due to scratches, or the substrate under the magnetic layer is scratched, and the head is separated from the magnetic layer only at that portion. On the other hand, such a scratch is caused by a partial abnormality at the time of manufacturing or a local external pressure generated at the time of transportation. In such a case, when a concave portion is generated due to the abnormality or the external pressure, the concave portion rises as a recoil at a position in contact with the concave portion. Part occurs.

Therefore, if a write operation is performed with a very small current after erasing the magnetic disk, recording is hardly performed at a normal portion or extremely weak recording is performed. Due to the strong magnetizing action, only the swell portion is selectively and strongly recorded. When this is reproduced, an output appears at a portion where a defective portion is formed, and the position of the defective portion can be detected.

The method of erasing the magnetic disk, that is, the method of uniform magnetization or demagnetization is not particularly limited.
AC erasing or DC erasing can be performed using an external magnetic field such as a permanent magnet or an electromagnet. In the present invention, the term “uniform magnetization” refers to an operation of erasing a recording or signal detectable by a magnetic head by magnetizing a magnetic disk. Generally, it is magnetized in the circumferential direction, radial direction or perpendicular direction of the magnetic disk. Furthermore, it is desirable that the magnitude of the magnetization be constant over the entire surface of the magnetic disk, but fluctuations in which low-frequency pulsations occur during reproduction are allowed.

For erasing by an external magnetic field, for example, the apparatus shown in FIG. 1 can be used. That is, external magnetic field heads 3 and 3 'having a plurality of permanent magnets or magnets 2a to 2e and 2a' to 2e 'are provided at adjacent positions on both sides of the rotating magnetic disk 1. Each magnet element 2a ~
2e, 2a 'to 2e' are magnetized so that the directions of the magnetic fields are alternately reversed as indicated by the arrows, and the magnetic field of one magnet, the left magnet 2a in the figure, is strengthened. The magnetic field of the other magnet element 2e is formed to be weak. Therefore, the distribution of the magnetic field strength is as shown in FIG.
When the magnetic disk 1 runs to the left, it is DC erased and magnetized, and when it runs to the right, it is AC erased and demagnetized.

Further, DC erasing can be performed by a magnetic head, and furthermore, an electromagnet can be disposed on both surfaces of the magnetic disk 1 and the magnetic disk 1 can be magnetized by applying a pulse current. As a method of applying a pulse current to perform DC erasure, the apparatus shown in FIG. 3 can be used. That is, the disk-shaped electromagnets 4 and 4 'are disposed on both sides of the magnetic disk 1 so as to be in contact with each other. The disc-shaped electromagnets 4 and 4 'are formed of coils 5, 5' and yokes 6, 6 ', respectively. When a pulse current is applied to the coils 2, 2', the central yokes 6a, 6a 'and the outer annular yokes 6b, 6b, 6' are formed. 6
A magnetic field is generated between b 'and as a result, the magnetic disk 1 is uniformly magnetized in the radial direction.

A method using a permanent magnet is advantageous for miniaturization of the apparatus, and a method using a pulse magnetic field can magnetize in a short time, which is effective in shortening the inspection time. Since no special device for magnetizing is required, the inspection device is advantageous in cost. The magnetization direction is not particularly limited. However, in the case of a thin film medium formed by sputtering or the like used in recent hard disks, it is preferable to magnetize in the circumferential direction because the noise level is low.

Since it is necessary to sufficiently magnetize the disk, the generated magnetic field is desirably at least 1.5 times the coercive force of the magnetic material of the magnetic disk. recent years,
As the coercive force of the disk is increasing, the required magnetic field is also increasing. If a pulse magnetic field generated when a pulse current is applied to the electromagnet is used, a disk having a large coercive force can be sufficiently magnetized. In this way, the magnetic disk which has been uniformly magnetized or demagnetized performs a signal writing operation with a low current value.

The frequency at the time of low-current recording is preferably lower so that the recording waveforms do not interfere with each other, that is, as a so-called isolated reproduction wave, because the level of the defect detection signal increases. As the current value at the time of low current recording, a current value that gives an output of 10% or less of the saturation output value in the current-output characteristic is used. It is particularly preferable to set the current value at the rise of the current-output curve (so-called saturation curve). This is because if it is lower than this, recording is not sufficiently performed on a defective portion, and if it is higher than this, recording is performed on a portion other than the defective portion.

As a magnetic head used for writing and detecting signals, a practical head (track width 0.5 to 10) is used.
It is preferable to use a head having a wider track width as compared with (μ) in view of shortening the inspection time. When a wide head is used in the conventional method, an output change due to a minute defect is diluted by an output of a normal portion, so that it is difficult to detect the change.

However, according to the present invention, since the magnetic disk is erased and writing is performed with a very small current, almost no signal is output in a normal portion and only an abnormal portion is output. However, it is easy to detect a change in the output of the abnormal portion, and thus it is possible to perform a high-speed inspection using a wide head. Specifically, the track width of the magnetic disk is 2 to 10
A magnetic head having a track width about twice as wide can be used.

Since the predetermined level for detecting a defective portion differs depending on the head to be used, the output of the defective portion is checked in advance using a known defect, and the output is determined in a range larger than the output of the normal portion and smaller than the output of the defective portion. Is set. If the predetermined level is too small, a normal part is also detected as a defect, and if it is too large, a necessary defect is missed. Therefore, it is necessary to select an optimum value to prevent such a defect. Normally, 1.5 to 2 times the output level of the normal part
A value of about twice is preferred.

FIG. 4 shows a defect detection waveform according to the conventional method and a defect detection waveform according to the present invention. When using the conventional method, FIG.
As shown in (B), a large signal output 7a is output in a normal part, and if there is an abnormal part, the signal output 7a is reduced and an abnormal output 7b is generated. However, when the head is wide, the ratio of signal output in a normal portion increases, and it becomes difficult to detect an output change due to a minute defect. However, according to the present invention, as shown in FIG. 4A, a low-level noise output 7a containing almost no output signal is output in a normal portion, and a signal output 7b is output in an abnormal portion. Even if it is used, the detection sensitivity of the abnormal portion is hardly reduced, and a minute defect can be detected.

[0022]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the scope of the invention. (Example 1) The sample used was a 3.5-inch hard disk. First, at the outer peripheral portion of the disk, the rising current value and the saturation current value of the saturation characteristic were measured at a recording frequency of 500 kHz (isolated reproduction wave). They were 9 mA and 35 mA, respectively. The inspection was performed in the following procedure. First, the disk was DC-erased with a magnetic head having a track width of 2.5 μm at 35 mA, a signal with a recording frequency of 500 kHz was written at 9 mA, and the output was inspected. did. Then they moved to the next track and repeated the same. The track width of the used head and the amount of one track movement are 10 μm (the track width of the normally used head is 2.5 μm). The disk rotation speed was 4500 rpm. Defects can be detected in the same manner as in the conventional inspection method, and the time taken is about 1/4, about 7 minutes.

Comparative Example 1 Normally Used Track Width
Using a head having a track width of 5 μm, the error was measured by the following procedure while moving at a 2.5 μm pitch from the inner circumference to the outer circumference of the magnetic disk. 1.
Writing a signal at a constant frequency (0.5 μm). 2. An output that is 65% or less of the average playback output level is detected as an error.
3. Move to next track. It took about 30 minutes. The disk rotation speed was the same as in Example 1.

[0024]

According to the present invention, after the disk is uniformly magnetized or demagnetized in advance, the current is reduced by the magnetic head.
A signal is written with a current value lower than a current value that gives an output of 10% of a saturation output value in an output characteristic (saturation characteristic), and a portion where a signal of a predetermined level or more is detected is determined as a defect, so that a track width is more than usual. A wide head can be used to reduce the inspection time, and even when a head with a normal track width is used, the write signal can be set lower than usual, which results in a cost reduction in the signal processing circuit. Can be reduced.

[Brief description of the drawings]

FIG. 1 is a side view showing an external magnetic field for magnetizing or demagnetizing a magnetic disk.

FIG. 2 is an explanatory diagram showing the distribution of the magnetic field strength of the external magnetic field of FIG.

FIG. 3 is a longitudinal sectional view showing another example of an external magnetic field for magnetizing a magnetic disk.

4A and 4B are diagrams showing the output at the time of inspection, wherein FIG. 4A shows the output according to the method of the present invention, and FIG. 4B shows the output of the conventional method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Magnetic disk 3 External magnetic field head 4 Disk-shaped electromagnet 7a Normal part output 7b Abnormal part output

Claims (10)

[Claims] After uniformly magnetizing a magnetic disk, a signal is written using a magnetic head at a current value that gives an output of 10% or less of a saturation output value in a current-output characteristic of the magnetic head. Playback using a magnetic head,
A method for inspecting a magnetic disk, wherein a portion where a signal of a predetermined level or more is detected is regarded as a defective portion. 2. After degaussing a magnetic disk, a signal is written using a magnetic head at a current value that provides an output of 10% or less of a saturation output value in a current-output characteristic of the magnetic head. Play using
A method for inspecting a magnetic disk, wherein a portion where a signal of a predetermined level or more is detected is regarded as a defective portion. 3. The magnetic disk inspection method according to claim 1, wherein, when the magnetic disk is uniformly magnetized, the magnetic disk is magnetized in a circumferential direction of the magnetic disk by DC erase. 4. When uniformly magnetizing a magnetic disk, an electromagnet is brought into direct contact with both sides of the magnetic disk and a pulse current is applied to the electromagnet to magnetize the magnetic disk in a circumferential direction or a radial direction. 2. The method for inspecting a magnetic disk according to claim 1, wherein: 5. The magnetic recording medium according to claim 1, wherein, when uniformly magnetizing or demagnetizing the magnetic disk, the magnetic disk is magnetized or demagnetized by an external magnetic field generated by a permanent magnet or an electromagnet. How to inspect disks. 6. The magnetic disk inspection method according to claim 1, wherein a rising current value of a saturation characteristic of a magnetic head is used as a signal writing current value. 7. The method according to claim 1, wherein the magnitude of the external magnetic field when uniformly magnetizing the magnetic disk is at least 1.5 times the coercive force of the magnetic disk. Inspection method of a magnetic disk according to the item. 8. The inspection of a magnetic disk according to claim 1, wherein the reproducing magnetic head for detecting a defect has a track width of 2 to 10 times the track width of the magnetic disk. Method. 9. The method according to claim 1, wherein the signal is written at a frequency lower than the frequency at which the reproduction waveform becomes an isolated reproduction wave.
9. The method for inspecting a magnetic disk according to any one of claims 1 to 8. 10. The magnetic disk inspection method according to claim 1, wherein the predetermined level is 1.5 to 2 times the normal portion output.