JPH10208321A - Information recording/reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately record/reproduce information, by detecting the difference between a peak held signal and a reference signal, adjusting the space between a probe and a recording medium, and compensating the detected difference. SOLUTION: A tunnel current detected by a probe electrode is amplified by a current amplifier 310, and is logarithmically compressed by a logarithmic compression circuit 311 and subjected to sample-and-hold by a sample-and-hold circuit 312. When the output signal of the circuit 312 is compared with a reference voltage VB2 by a comparator 313, and an edge approach is detected, first, a threshold value voltage VB3 is set in a prescribed level, and the device enters write-in operation. After a time when control of the gap between the probe electrode and recording medium is stabilized is waited, a sample-and-hold control signal is made a hold state synchronized with a data write-in clock, and a write-in pulse is generated from a pulse generator 321. This data pulse is written in until the probe electrode in-version scans, and when the probe electrode inversion scans, the scanning returns to a read-out scan.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an information recording / reproducing apparatus. More specifically, a scanning tunneling microscope (ST
M).

[0002]

2. Description of the Related Art In recent years, the use of memory materials is at the core of the electronics industry such as computers and related equipment, video disks, digital audio disks, and the like, and the development of such materials has been very active.
The performance required for memory materials varies depending on the application, but in general, high density, large recording capacity, fast response time for recording and reproduction, low power consumption, high productivity, low price, etc. No.

Conventionally, semiconductor memories and magnetic memories using magnetic materials or semiconductors as materials have been mainly used. However, in recent years, with the development of laser technology, inexpensive optical memories using organic thin films such as organic dyes and photopolymers have been developed. And high-density recording media have appeared.

On the other hand, recently, a scanning tunneling microscope (hereinafter abbreviated as STM) capable of directly observing the electronic structure of surface atoms of a conductor has been developed [G. Binnig et al P
hys. Rev .. Lett, 49, 57 (198
2)], high resolution measurement of a real space image can be performed regardless of whether it is single crystal or amorphous, and it has the advantage that it can be observed with low power without damaging the sample by current. Above all, it operates and can be used for various materials, so that a wide range of applications is expected.

[0005] The STM utilizes the fact that a tunnel current flows when a voltage is applied between a metal probe (probe electrode) and a conductive material to approach a distance of about 1 nm. This current is very sensitive to changes in the distance between them. By scanning the probe so as to keep the tunnel current constant, it is possible to read various kinds of information on all electron clouds in the real space.
At this time, the resolution in the in-plane direction is about 0.1 nm.

Therefore, if the principle of STM is applied, it is possible to perform high-density recording / reproducing on the order of atoms (sub-nanometer). For example, JP-A-61-
In the recording / reproducing apparatus disclosed in Japanese Patent No. 80536,
Atomic particles adsorbed on the medium surface are removed by an electron beam or the like, writing is performed, and this data is reproduced by STM. No. 4,575,822.
As disclosed in US Pat. No. 6,064,098, using a tunnel current flowing between the recording medium surface and the probe electrode, injecting and recording an electric charge into a dielectric layer formed on the medium surface, or
A method has been proposed in which recording is performed by physical or magnetic collapse of the medium surface using a laser beam, an electron beam, a particle beam, or the like.

A method has been proposed in which recording / reproduction is performed by STM using a material having a memory effect on the switching characteristics of voltage and current, for example, a thin film layer of a π-electron organic compound or chalcogen compound as a recording layer. [JP-A-63-
161552, JP-A-63-161553]. According to this method, the recording bit size is set to 10n.
Assuming that m, a large capacity recording / reproducing of 10 ¹² bit / cm ² is possible.

[0008]

When recording and reproducing information using a tunnel current, it is necessary to control the distance between the probe electrode and the surface of the recording medium. However, conventionally, no interval control suitable for recording and reproducing information has been proposed.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to provide an information recording / reproducing apparatus having a space between a probe electrode and a recording medium suitable for recording and reproducing information. I have.

An object of the present invention is to provide an information recording / reproducing apparatus which records and / or reproduces information on a recording medium using a probe, in which a signal obtained by peak-holding a signal obtained via the probe is compared with a reference signal. This is achieved by providing means for detecting a difference, and means for adjusting the distance between the probe and the recording medium so as to compensate for the difference detected by the detecting means.

[0011]

Next, the present invention will be described in detail with reference to the drawings.

FIG. 1 shows the main configuration of an information recording / reproducing apparatus according to the present invention. 101 is a substrate of a recording medium, 102 is a lower electrode, 103 is a recording / reproducing layer, 104 is an electrode pattern for forming an edge portion, 121 is a recording / reproducing area, and 120 is an edge portion.

A stage 201 supports a recording medium, and this stage is driven by a linear actuator 206 in the Y-axis direction. Numeral 203 denotes a recording / reproducing head which supports the probe electrode 202 and is position-controlled by actuators 205 and 204 which are driven in the X or Z-axis direction, respectively.

The distance between the surface of the recording medium and the probe electrode is controlled by the tunnel current flowing between the surface of the recording medium and the probe electrode. This is because a tunnel current flowing between the recording medium probe electrodes is detected by the load resistance _RL by the bias voltage V _B , amplified by the amplifier 301, and an appropriate probe height is determined by the probe height detection circuit 302. Adjusts the probe height.

The recording / reproducing area 121 is determined based on the edge 120. That is, the probe electrode 202 scans in the X-axis direction by the X-axis drive control unit 306 and the actuator 205. At this time, the probe electrode is
When approaching zero, the tunnel current increases rapidly between the probe electrode and the electrode at the edge. This rapidly increasing tunnel current is detected by the edge detector 303. The X-axis drive control circuit reverses the scanning direction of the probe according to this detection signal. After a certain period of time has elapsed, the scanning direction of the probe electrode is reversed again, and scanning is performed in the direction toward the edge. At this time, a drive signal is sent to the Y-axis drive control circuit to move the stage by one step in the Y-axis direction.

By repeating the above operation, the probe electrode can always scan the recording / reproducing area along the edge and at a certain angle with respect to the edge.

When recording is performed on a recording medium, a write voltage is applied from the probe electrode through the SW by the data modulator 308 with reference to the time when the probe electrode detects the edge portion 120 and performs reverse scanning in the X-axis direction. . At this time, a series of data pulse trains are recorded until the probe electrode scans in the X-axis direction for a fixed time and starts reverse scanning again in the edge direction.

In this recording operation, the probe electrode is moved to the edge portion 1.
20 is performed every time a 20 is detected, a data string having a certain angle is written to the edge portion, and the Y-axis stage is sequentially sent in synchronization with this, and an area 121 where two-dimensional data is recorded is obtained. It is formed.

At the time of reproduction, the probe electrode is scanned on the basis of the edge portion as in the case of recording. At this time, the azimuth adjustment between the X-axis scanning direction of the probe electrode and the recorded data string may be performed by a so-called wobbling method as disclosed in Japanese Patent Publication No. 54-15727, or by two-dimensional area scanning. May be corrected when demodulating data by a method such as pattern matching.

FIG. 2A shows a state of data recording on the surface of the recording medium. 122 is a data recording bit, 12
Reference numeral 3 (solid arrow) indicates the trajectory of the probe tip when the probe electrode scans away from the edge, and reference numeral 124 (dashed arrow) indicates the trajectory of the probe tip when the probe electrode moves toward the edge.

FIG. 2B shows a case where the edge portion 120 is not a perfect straight line but is disturbed. Even in such a case, since the data sequence is written so as to follow the edge portion, there is no problem in recording and reproduction. That is,
The shape of the edge part has a large tolerance for the finishing accuracy,
It is also possible to use an arbitrary shape such as a stripe shape or a spiral shape.

Further, since the two-dimensional scanning is performed along the edge portion, the probe electrode can be guided to an arbitrary data recording area by using the edge. For example, when a recording medium is first installed in a recording / reproducing apparatus, an error occurs in the positional relationship between the probe electrode and the recording area of the recording medium due to the mechanical accuracy of the attachment. This error is usually 10
About 100 μm. This is much larger than the data recording area. However, by using this edge detection scanning, the data area can be easily found and tracked.

The recording layer of the recording medium used in the present invention may be made of any material that can detect information written in the recording layer by a tunnel current flowing between the probe electrode and the recording layer. it can. For example, in the case of recording by forming irregularities on the surface, HOPG (Highly
-Oriented-Pyrolithic-Graph
h) Cleaved substrate, Si wafer, Au, Ag, Pt, Mo, Cu grown by vacuum evaporation or epitaxial growth
Metal thin film such as, include glass metals such as _{Ph 25 Zr 75, Co 35 Tb} 65. In the case of recording based on the electronic state of the surface, a thin film layer of amorphous Si, a π-electron organic compound, a chalcogen compound, or the like can be given.

The recording medium may have any shape and substrate material. For example, the shape may be a card-shaped or tape-shaped substrate suitable for forming a stripe-shaped edge portion, a disk-shaped substrate suitable for forming a spiral-shaped edge portion, or the like. The material may be a cleavage crystal substrate such as HOPG or mica, or Si. , Sapphire, crystal substrate with polished surface such as MgO, fused quartz, Corning 7059
Watch glass and the like. Further, as a material that can be used also as a substrate material of the tape-shaped medium, polycarbonate, acrylic, PEEK, PET, nylon, and the like can be given.

FIG. 3 shows a first example of the recording medium used in the present invention.
An example of the embodiment will be described. This will be described below with reference to the drawings.

101 is a substrate, 102 is a lower electrode, 103
Is a recording layer, and 104 is an electrode for forming an edge.

The probe electrode is guided from the point a, sequentially scans along the edge of the striped electrode 104, and escapes to the point b. At the point a of the guide portion of the probe electrode, the electrode 104
The edge portion of the recording medium expands in a funnel shape, thereby compensating for a mechanical installation error between the probe electrode and the recording medium. That is, the probe electrode is guided to the recording area by being guided to the funnel-shaped edge.

Next, a method of manufacturing a recording medium will be described with reference to FIGS.
The description will be given with reference to the cross-sectional views of FIG.

As shown in FIG. 6A, a lower electrode layer 102 is formed by epitaxially growing gold on the entire surface of a mica substrate 101 cleaved in the air by a vacuum deposition method. The lower electrode layer was formed by maintaining the substrate temperature at 500 ° C. and at a deposition rate of 1
0 ° / sec, deposition pressure 5 × 10 ⁻⁶ Torr, film thickness 5
The test was performed under the condition of 000 °.

Subsequently, as shown in FIG. 6B, a two-layer cumulative film (thickness: 8 °) of a polyimide monomolecular cumulative film is formed on the lower electrode layer 102 by using the LB method to form a recording layer 103. .

The details of the method for forming the polyimide monomolecular cumulative film will be described below.

The polyamic acid represented by the formula (3) is N, N-
After dissolving in a dimethylacetamide solvent (concentration in terms of monomer: 1 × 10 ⁻³ M), a separately prepared 1 × 10 ⁻³ M solution of N, N-dimethyloctadecylamine in the same solvent was mixed with 1: 2 (V / V) to prepare a polyamic acid octadecylamine salt solution represented by the formula (4).

[0033]

[Outside 1]

The solution was spread on an aqueous phase composed of pure water at a water temperature of 20 ° C., and a monomolecular film was formed on the water surface. After removing the solvent, the surface pressure was increased to 25 mN / m. While keeping the surface pressure constant, the above-mentioned substrate with a base electrode was immersed gently in a direction crossing the water surface at a speed of 5 mm / min.
The film was pulled up gently at m / min to form a two-layer Y-type monomolecular cumulative film. Next, the substrate is heat-treated at 300 ° C. for 10 minutes to imidize the polyamic acid octadecylamine salt (formula (5)).

[0035]

[Outside 2] A two-layer polyimide single molecule cumulative film was obtained.

Then, as shown in FIG. 6C, gold is deposited on the entire surface of the recording layer 103 by a vacuum evaporation method at a thickness of 500.degree., And the unnecessary metal pattern is removed by etching with an aqueous solution of iodine by photolithography. Thus, the edge forming electrode 104 having a line width of 1 μm and a pitch of 2 μm is formed.

According to the above-mentioned recording medium, (1) an edge portion for tracking can be formed by photolithography, so that an inexpensive recording medium suitable for mass production can be provided.

(2) A guide portion for introducing the probe electrode into the recording area of the recording medium can be easily formed, and the allowable range of mechanical accuracy for replacing and mounting the recording medium can be widened. Also becomes easy to design.

FIG. 4 shows a second example of the recording medium used in the present invention.
An example of the embodiment will be described.

Reference numeral 101 denotes a substrate, 102 denotes a lower electrode having an edge portion, and 103 denotes a recording layer. Reference numeral 202 denotes a probe electrode which moves on the surface of the recording layer in a direction perpendicular to the stripe at the edge and accesses an arbitrary track (one stripe at the edge). In each track, the probe electrode scans along an edge portion (in this case, the recording layer on the convex side becomes a recording / reproducing area).

Next, the manufacturing method will be described with reference to the cross-sectional views of the respective manufacturing steps shown in FIGS.

First, as shown in FIG. 7A, an electrode layer 102 is formed by epitaxially growing gold on the entire surface of a mica substrate 101 cleaved in the air by a vacuum deposition method. The electrode layer 102 was formed at a substrate temperature of 500 ° C., a deposition rate of 10 ° / sec, a deposition pressure of 5 × 10 ⁻⁶ Torr, and a film thickness of 5000 °.

Further, as shown in FIG. 7 (b), the electrode layer 102 is immersed in an etching solution comprising an aqueous solution of iodine potash by a usual photolithographic etching method to form a groove having a line width of 1 μm, a pitch of 2 μm, and a depth of 200 °. 120
Is formed as an edge portion.

Subsequently, as shown in FIG.
A two-layer cumulative film (thickness: 8 °) of a polyimide monomolecular cumulative film was formed on the sample No. 2 by using the LB method to form a recording layer 103.

The production of the polyimide single molecule cumulative film is as described in the first embodiment.
This is performed in the same manner as in the manufacturing method of the embodiment.

According to the recording medium of the second embodiment, since the edge portion can be formed on the lower electrode, the recording layer is not exposed to the etching solution in the photolithography etching step for forming the edge. That is, the formation of the recording layer is the last of the manufacturing process, and the recording layer of any kind of material can be applied.

FIGS. 5A and 5B show a third embodiment of the recording medium used in the present invention. FIG. 5A is a plan view,
FIG. 5B is a sectional view taken along the line AA ′ of FIG.

Reference numeral 101 denotes a disk-shaped substrate, 102 denotes a lower electrode layer, and 103 denotes a recording layer. Reference numeral 105 denotes an edge portion formed by ion implantation.

The edge is formed in a spiral shape on a disk-shaped substrate. By guiding the probe electrode while rotating the substrate, recording and reproduction are performed by moving the disk from the outer periphery to the center or from the center to the outer periphery along the spiral edge.

Next, a method of manufacturing this recording medium will be described with reference to FIG.
The description will be made according to the cross-sectional views for each step of (a) to (c).

First, as shown in FIG. 8A, an electrode layer 102 is formed on a mica substrate 101 cleaved in the air by epitaxial growth of gold on the front surface by a vacuum deposition method. The formation of the electrode layer is performed by keeping the substrate temperature at 500 ° C. and the deposition rate at 10 ° /
sec, deposition pressure 5 × 10 ⁻⁶ Torr, film thickness 5000
The test was performed under the conditions of Å.

Further, as shown in FIG. 8B, a silicon (Si) element is applied to the electrode layer 102 by using a focused ion beam apparatus in the direction indicated by the arrow at an acceleration voltage of 80 kV and a dose of 10 ¹⁵ ions / cm ² . By injection, a denatured portion 105 having a width of 0.1 μm and a pitch of 1.1 μm is formed to be an edge portion.

Subsequently, as shown in FIG.
A two-layer cumulative film (thickness: 8 °) of a polyimide single-molecule cumulative film was formed on the layer No. 02 by the LB method to form a recording layer 103.

The polyimide single-molecule cumulative film is manufactured by the same method as the manufacturing method of the first embodiment of the recording medium.

According to the recording medium of the third embodiment, (1) Since an edge portion having a very small occupied area is formed by ion implantation, a non-recordable / reproducible area generated due to the edge formation can be minimized. .

(2) The position control accuracy of this ion implantation is 0.1 to 0.1 μm when the edge formation pitch is 1.1 μm.
It is sufficient that the thickness is about 0.2 μm, and it can be sufficiently performed with equipment using a normal stage. Therefore, the manufacturing cost required for forming the edge can be kept low.

(3) Since the spiral edge portion is used, continuous recording and reproduction of a large amount of data without a switching time for track access or a record gap becomes possible, which is suitable for recording and reproducing of a moving image. Can be used.

[0058]

FIG. 9 shows a specific embodiment of the information recording / reproducing apparatus according to the present invention.

FIG. 10 shows an example of the locus of the tip of the probe electrode on the recording medium. 11 and 12 show waveform examples of the signal lines of the recording / reproducing apparatus of FIG. 9 during reproduction and recording, respectively. This will be described below with reference to the drawings.

Reference numeral 101 denotes a substrate of a recording medium, and reference numeral 201 denotes a stage which can be moved in a range of 10 mm in each of the X, Y, and Z directions to draw a probe electrode into an arbitrary recording area on the recording medium. Reference numeral 210 denotes a cylindrical PZT actuator which scans the probe electrode 202 along a data row on a recording medium, and can move up to 2 μm in each of the X, Y, and Z directions.

The probe electrode 202 is connected to a current amplifier 310. The output of the current amplifier 310 is a logarithmic compression circuit 3
The signal is input to the sample-and-hold circuit 312 via the line 11. The output signal (a) of the sample hold circuit is input to the comparator 313, the peak hold circuit 314, and the high-pass filter 318, respectively.

The output (b) of the peak hold circuit 314 is connected to an error amplifier 315 and a low-pass filter 316. The output of the error amplifier is connected to the ΔZ drive electrode of the cylindrical PZT. On the other hand, the output of the low pass filter 318 is input to the comparator 317 through the attenuator VR _3.

The output (b) of the high-pass filter is connected to the other input of the comparator 317. further,
The output of the comparator 317 is input to the data demodulator of the data modem 323.

The data modulation output of the data modulator / demodulator is connected to the pulse generator 321 and combined with the DC bias voltage V _B1 to be connected to the recording medium electrode.

The tracking control section 322 drives the cylindrical PZT actuator 210 via the up / down counter 319 and the D / A converter 320. The OR of the output of the edge detection comparator 313 and the up control signal from the tracking control unit is connected to the up input (g) of the uptown counter 319. On the other hand, a down input signal (f) of the up / down counter 319 is connected to a down control signal of the tracking control unit. The count output of the up / down counter is converted into an analog voltage by the D / A converter 320 and drives the cylindrical PZT by ΔX. Further, the output of the D / A converter 320 is combined with the wobbling signal from the variable resistor R ₂ and via a resistor R ₁ tracking control unit which is controlled by the scanning azimuth control signal (k), the cylindrical PZT [Delta] Y drive I do.

Assuming a data reproducing operation, the trajectory of the probe electrode 202 on the recording medium is as shown in FIG. When the cylindrical PZT 210 starts data scanning from the probe position 140 initially set by the stage 201, when the edge 120 is detected through the trajectory indicated by 141, the scanning of the probe electrode is inverted and becomes the trajectory of 142.
Further, after scanning for a certain distance, it is inverted again and proceeds in the direction toward the edge 120.

When the above operation is repeated and the probe electrode detects the data bit string 122, the data string is sequentially scanned while adjusting the scanning direction.

The scanning azimuth of the probe electrode is performed by changing the variable resistor R ₂ according to the scanning azimuth control signal and changing the ΔX / ΔY drive ratio of the cylindrical PZT actuator 210. Although not shown in FIG. 10, the detection of the appropriate scanning azimuth is determined by driving the wobbling voltage (k) by ΔY and monitoring the envelope signal (c) at which the tunnel voltage is peak-held.

FIG. 10B is a cross-sectional view showing how the probe electrode scans a data string on the recording medium. The distance between the probe electrode and the surface of the recording medium is controlled by comparing the signal (b) obtained by peak-holding the logarithmically compressed value of the tunnel current with the reference voltage V _B3 by the error amplifier 315, and by controlling the cylindrical PZ.
T is driven by ΔZ.

With this control, the probe electrode scans so as to envelope the convex part of the data string or the part having the highest electronic state.

Next, the operation during reproduction will be described with reference to the timing chart of FIG. 11 showing the state of each signal.

The tunnel current detected by the probe electrode is amplified by the current amplifier of FIG.
11 is logarithmically compressed and sampled and held at 312. This sample-and-hold circuit enters a through state during reproduction, and the output of the logarithmic compressor is output as it is.

The sample-and-hold output (a) is compared with a threshold voltage V _B2 by an edge detection comparator to detect that the probe electrode approaches the edge. However, V _B2 is set higher than the output voltage based on the data string.

The edge detected signal forcibly switches the up / down counter to the up-count operation. Further, after counting up by a certain count value, the countdown is switched again. At this time, the up and down control signals of the up / down counter are shown in (g) and (f) of FIG. 11, respectively. FIG. 11H shows the ΔX drive output.

With this operation, the probe electrode can scan without colliding with the edge of the recording medium.

The sample-and-hold output (a) extracts a high-frequency component, that is, a data information component, by a high-pass filter, and a comparator 317 compares the envelope signal (c) of the data string with a voltage that has been appropriately attenuated and binarized. Obtain data (e). The respective input signals of the comparator at this time are shown in (c) and (d) of FIG. Also 2
The quantified signal is shown in FIG. The envelope signal (c) is obtained by integrating the peak hold output (b) shown in FIG.

Next, the operation at the time of recording will be described with reference to the timing chart of FIG. 12 showing the state of each signal.

Output signal (a) of sample hold 312
Is compared with V _B2 by the comparator 313 to detect the approach of the edge. First, V _B3 is set to a predetermined level, and the writing operation starts. Waiting for the time when the control of the gap between the probe electrode and the recording medium stabilizes, the sample hold control signal (i)
In a hold state in synchronization with the data write clock, and the pulse generator 321 generates a write pulse (j). The writing of the data pulse is performed until the probe electrode performs the reverse scanning, and immediately after the reverse scanning of the probe, the reading scan returns, and the data writing operation is in a standby state until the next edge is detected.

[0079]

According to the information recording / reproducing apparatus of the present invention described above, accurate recording and reproduction of information can be performed by precise interval control.

In particular, in the present invention, the peak hold circuit is used for the gap control circuit between the probe electrode and the recording medium surface, so that the probe electrode has a fast time constant even if the probe electrode has a sharp projection such as an edge. When the probe electrode comes off the projection of the data bit, the probe electrode approaches the surface of the recording medium with a slow time constant. By this operation, useless vertical vibration of the probe electrode can be reduced as much as possible, and the probe electrode can be searched at a very high speed.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an information recording / reproducing apparatus according to the present invention.

FIG. 2 is a diagram illustrating the operation of the present invention.

FIG. 3 is a first embodiment of the recording medium of FIG. 1;

FIG. 4 is a second embodiment of the recording medium of FIG. 1;

FIG. 5A is a third embodiment of the recording medium of FIG. 1; (B) AA 'sectional drawing of FIG.5 (a).

FIG. 6 is a diagram illustrating a method of manufacturing the recording medium of FIG.

FIG. 7 is a view for explaining a method of manufacturing the recording medium of FIG. 4;

FIG. 8 is a view for explaining a method of controlling the recording medium of FIG. 5;

FIG. 9 is a diagram showing a specific configuration of an information recording / reproducing apparatus of the present invention.

FIG. 10 is a diagram showing a locus of a probe electrode of the recording / reproducing apparatus in FIG. 1 on a recording medium surface.

FIG. 11 is a view showing signal waveforms at the time of reproduction of the recording / reproducing apparatus of FIG. 1;

FIG. 12 is a diagram showing signal waveforms at various points in the recording / reproducing apparatus of FIG. 9 during recording.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Ryo Kuroda, Inventor 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Hiroyasu Nose 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inside (72) Inventor Miya ▲ saki ▼ Toshihiko 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (1)

[Claims] An information recording / reproducing apparatus for recording and / or reproducing information on a recording medium using a probe detects a difference between a signal obtained by peak-holding a signal obtained via the probe and a reference signal. Means for adjusting the distance between the probe and the recording medium so as to compensate for the difference detected by the detecting means.