Title: Domain wall displacement type magneto-optical recording medium and recording method
Abstract: There is provided a domain wall displacement type magneto-optical recording medium and a recording method that allow signals recorded with a high density over the limit of resolution of optical systems to be regenerated without increasing complexity of a recording and regenerating apparatus or narrowing an operation margin even if a substrate having address information or the like incorporated therein is used. An information track has portions used for recording and regenerating information in the form of a magneto-optical signal and portions having irregularities to provide address information or servo information formed therein alternately arranged, and directions of magnetization of the third magnetic layer are polarized in one direction above the portions having the irregularities.
Patent Number: 6,980,491 Issued on 12/27/2005 to Shiratori, et al.
| Inventors:
|
Shiratori; Tsutomu (Tokyo, JP);
Arisaka; Katsumi (Kanagawa, JP)
|
| Assignee:
|
Canon Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
419856 |
| Filed:
|
April 22, 2003 |
Foreign Application Priority Data
| Apr 26, 2002[JP] | 2002-126267 |
| Current U.S. Class: |
369/13.42; 369/13.47; 428/694.MM |
| Intern'l Class: |
G11B 011/00 |
| Field of Search: |
369/1342,134.6,134.5,134.4,135.4,135.5,275.4,275.3,134.7,135.2,130.6,130.7,130.8
428/694.ML,694.MM,694.SC
|
References Cited [Referenced By]
U.S. Patent Documents
| 5168482 | Dec., 1992 | Aratani et al.
| |
| 5962154 | Oct., 1999 | Hashimoto et al.
| |
| 6027825 | Feb., 2000 | Shiratori et al.
| |
| 6041024 | Mar., 2000 | Ishii.
| |
| 6150038 | Nov., 2000 | Hirokane et al.
| |
| 6197440 | Mar., 2001 | Shiratori.
| |
| 6249489 | Jun., 2001 | Fujii et al.
| |
| 6345016 | Feb., 2002 | Shiratori.
| |
| 6519211 | Feb., 2003 | Murakami et al.
| |
| 6572957 | Jun., 2003 | Aratani.
| |
| 6633514 | Oct., 2003 | Awano et al.
| |
| 6747919 | Jun., 2004 | Kawaguchi et al.
| |
| 2003/0156502 | Aug., 2003 | Murakami et al.
| |
| 2004/0076110 | Apr., 2004 | Hino et al.
| |
| 2004/0218476 | Nov., 2004 | Ishibashi et al.
| |
| Foreign Patent Documents |
| 3-93058 | Apr., 1991 | JP.
| |
| 6-290496 | Oct., 1994 | JP.
| |
| 9-235885 | Sep., 1997 | JP.
| |
| 2000/-187898 | Jul., 2000 | JP.
| |
Primary Examiner: Dinh; Tan
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper & Scinto
Claims
1. A magneto-optical recording medium having a plurality of information tracks comprising:
a substrate, each of the information tracks having portions used for recording
and reproducing information in the form of a magneto-optical signal and portions
having pre-pits formed therein alternately arranged; and
at least first, second and third magnetic layers laminated on the substrate,
wherein the first magnetic layer has a domain wall coercivity smaller than that
of the third magnetic layer, the second magnetic layer has a Curie temperature
lower than that of the first magnetic layer and the third magnetic layer, and directions
of magnetization of the third magnetic layer are aligned above the portions having
the pre-pits and wherein regions in which the directions of magnetization are aligned
are only areas in the portions having the pre-pits which immediately precede the
portions used for recording and reproducing information in the form of an magneto-optical
signal, and in the remaining areas in the portions having the pre-pits, the direction
of magnetization of the third magnetic layer is polarized according to a predetermined
signal pattern within a signal bandwidth used for recording and reproducing information.
2. A magneto-optical recording medium having a plurality of information tracks comprising:
a substrate, each of the information tracks having portions used for recording
and reproducing information in the form of a magneto-optical signal and portions
having pre-pits formed therein alternately arranged; and
at least first, second and third magnetic layers laminated on the substrate,
wherein the first magnetic layer has a domain wall coercivity smaller than that
of the third magnetic layer, the second magnetic layer has a Curie temperature
lower than that of the first magnetic layer and the third magnetic layer, and directions
of magnetization of the third magnetic layer are aligned above the portions having
the pre-pits and wherein a fourth magnetic layer is interposed between the first
magnetic layer and the second magnetic layer, the fourth magnetic layer having
a domain wall energy density higher than that of the first magnetic layer and a
Curie temperature lower than that of the first magnetic layer and higher than that
of the second magnetic layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording medium for recording
and regeneration of information through the use of a laser beam using a magneto-optical
effect. More specifically, it relates to a magneto-optical recording medium that
uses a magnetic domain wall displacement due to a temperature gradient and can
regenerate signals recorded therein with a high density over the limit of resolution
of optical systems.
2. Related Background Art
As rewritable recording media, various magnetic recording media have been put
into practical use. In particular, magneto-optical recording media, which records
information by forming a magnetic domain in a magnetic thin film using thermal
energy from a semiconductor laser and regenerates the information using a magneto-optical
effect, are promising mass commutative media capable of high-density recording.
In recent years, in accordance with the trend toward digitization of moving picture,
it has been required to increase the recording density of the magnetic recording
media to provide a recording medium with a higher capacity.
In general, the track recording density of optical recording media significantly
depends on a laser wavelength and a numerical aperture NA of an objective lens
in a regenerating optical system. In other words, the laser wavelength λ
and the numerical aperture NA of the objective lens in the regenerating optical
system determine the diameter of the beam waist. Thus, the spatial frequency of
a recording pit that can regenerate a signal is limited to about 2NA/λ. Therefore,
for increasing the recording density of conventional optical disks, it is required
to reduce the laser wavelength of the regenerating optical system or increase the
numerical aperture of the objective lens. However, the laser wavelength is difficult
to reduce in terms of efficiency and heat generation of the element, and if the
numerical aperture of the objective lens is increased, more severe mechanical precision
becomes required because of reduction of the focal depth or the like.
Thus, various super resolution techniques have been developed which are improved
in configuration of the recording medium and method of regenerating information
and provide improved recording density without changing the laser wavelength and
the numerical aperture of the objective lens. For example, in Japanese Patent Laid-Open
No. 3-93058, there has been proposed a signal regenerating method in which a signal
is recorded in a multilayer recording layer comprising a regenerating sub-layer
and a recording sub-layer magnetically coupled to each other, the directions of
magnetization of the regenerating sub-layer are aligned before the regenerating
sub-layer is irradiated with laser light to be heated, and then the signal recorded
in the recording sub-layer is read out by being transferred to a region of the
regenerating sub-layer in which the temperature has been increased. According to
this method, since the signal detection region can be limited within a desired
region in which the temperature is increased, which is smaller than the regenerating
spot, an intersymbol interference during regeneration of information can be reduced,
and a signal of a spatial frequency equal to or higher than 2NA/λ can be
regenerated. However, this method has a disadvantage that, since the available
signal detection region is smaller than the spot of the regenerating laser, the
amplitude of the regenerated signal is reduced, the resulting output power of regeneration
is insufficient. Therefore, the available signal detection region cannot be significantly
smaller than the spot size. Consequently, the recording density cannot be significantly
increased compared to that determined by the diffraction limited of the optical system.
In view of such a problem, in Japanese Patent Application Laid-Open No. 6-290496,
the present inventors have already proposed a magnetic recording medium and regenerating
method in which a magnetic domain wall located at a boundary of a recording mark
is displaced due to a temperature gradient toward a side of higher temperature,
and the domain wall displacement is detected, whereby signals recorded therein
with a high density over the limit of resolution of optical systems can be regenerated
without reducing the amplitude of the regenerated signals.
In method described in Japanese Patent Application Laid-Open No. 6-290496, if
the temperature gradient is to be provided by heating the medium using the regenerating
light beam itself, the peak of the temperature distribution appears in the regenerating
light beam spot. Accordingly, in a region having a temperature that allows the
domain wall to be displaced, a domain wall displacement from a front end of the
region and a domain wall displacement from a rear end of the region are both read
out by the regenerating spot, and a satisfactory regenerated signal cannot be obtained.
Thus, besides the regenerating light beam, it is required to additionally provide
means for providing a desired temperature distribution. Therefore, there is a problem
that the regenerating apparatus becomes more complicated. Furthermore, in Japanese
Patent Application Laid-Open No. 9-235885, there has been proposed a method in
which the domain wall displacement from the rear end is suppressed by applying
a regenerating magnetic field. However, this method has a problem that a margin
of the regenerating magnetic field that provide a satisfactory regenerated signal
is narrow, and the resulting signal waveform is asymmetrical.
Furthermore, in Japanese Patent Application Laid-Open No. 2000-187898,
there has been proposed a method in which the domain wall displacement from the
rear end is suppressed by inserting a magnetic layer having a high domain wall
energy density (control layer) to control a transfer condition in the rear part
of the temperature distribution. This method has completely solved the above-described
problem regarding essential operations. However, it is proved that, if a pit portion
having irregularities for providing address information or servo information is
provided on the substrate, controllability of the transfer condition or the like
is lost due to a variation of deposition of a magnetic film onto the portion or
the like, and thus, the domain wall is easily displaced from the rear end in the
portion. That is, after the regenerating spot passes over the pit portion, the
direction of magnetization of the portion behind the regenerating spot is inverted,
and the regenerated signal largely varies in a DC manner. This phenomenon causes
an edge shift of the regenerated signal and becomes a problem in assuring a margin
for regenerating information or increasing the recording density.
SUMMARY OF THE INVENTION
The present invention is to provide a magneto-optical recording medium and a
recording method that allow signals recorded with a high density over the limit
of resolution of optical systems to be regenerated without making the recording
and regeneration apparatus complicated and narrowing an operation margin. As employed
herein the term "regenerate" and its derivatives as "regenerated", "regenerating"
and "regeneration", means to "reproduce" as in "reproducing information."
A magneto-optical recording medium according to the present invention is a magneto-optical
recording medium having a plurality of information tracks, comprising: a substrate,
each of the information tracks having portions used for recording and regenerating
information in the form of a magneto-optical signal and portions having prepits
(irregularities) formed therein alternately arranged; and at least first, second
and third magnetic layers laminated on the substrate, in which the first magnetic
layer has a domain wall coercivity smaller than that of the third magnetic layer,
the second magnetic layer has a Curie temperature lower than that of the first
and third magnetic layers, and directions of magnetization of the third magnetic
layer are aligned above the portions having the irregularities.
Furthermore, a recording method according to the present invention is
a method of recording information on a magneto-optical recording medium, the magneto-optical
recording medium comprising a substrate having information tracks each having portions
used for recording and regenerating information in the form of a magneto-optical
signal and portions having prepits (irregularities) formed therein alternately
arranged and at least first, second and third magnetic layers laminated on the
substrate, the first magnetic layer having a domain wall coercivity smaller than
that of the third magnetic layer, the second magnetic layer having a Curie temperature
lower than that of the first and third magnetic layers, and directions of magnetization
of the third magnetic layer being aligned above the portions having the irregularities,
the method comprising the steps of:
applying a magnetic field to the portions used for recording and regeneration
while irradiating the same with a light beam with a magnetic head to record information
on the portions; and
applying a DC magnetic field to the portions having the pepits with the
magnetic field while irradiating the portions with the light beam to align the
directions of magnetization of the third magnetic layer above the portions.,
Detailed description will be made later with reference to examples.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B show waveforms of regenerated signals for a sample in Example
1, observed with an oscilloscope, FIG. 1A showing an envelope for a round of track,
and FIG. 1B showing an envelope for one segment obtained by enlarging the envelope
in FIG. 1A along a time base through a 50 kHz HPF, in which the upper waveform
is of a sum signal regenerated from a clock pit, a first wobble pit for servo,
a second wobble pit and an address pit, and the lower waveform is of a differential
signal obtained by regenerating the magneto-optically recorded signal.
FIGS. 2A and 2B show waveforms of regenerated signals for a sample in Comparison
Example 1, observed with the oscilloscope, FIG. 2A showing an envelope for a round
of track, and FIG. 2B showing an envelope for one segment obtained by enlarging
the envelope in FIG. 2A along a time base through a 50 kHz HPF, in which the upper
waveform is of a sum signal regenerated from a clock pit, a first wobble pit for
servo, a second wobble pit and an address pit, and the lower waveform is of a differential
signal obtained by regenerating the magneto-optically recorded signal.
FIG. 3 is a circuit block diagram for implementing the present invention.
FIG. 4 is a signal waveform diagram for illustrating operations of the circuit blocks.
FIG. 5 shows a recorded signal timing circuit.
FIG. 6 is a diagram for illustrating an operation of the recorded signal timing circuit.
FIG. 7 illustrates recording and reproducing of information on the magneto-optical
recording medium of Example 1.
FIG. 8 illustrates recording and reproducing information on the magneto-optical
recording medium of Example 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A magneto-optical recording medium according to the present invention comprises
a substrate having an information track having portions used for recording and
regenerating information in the form of an magneto-optical signal and portions
having prepits (irregularities) for providing address information or servo information
alternately arranged, and at least first, second and third magnetic layers laminated
one on another on the substrate, in which the first magnetic layer has a coercivity
of the domain wall lower than that of the third magnetic layer, the second magnetic
layer has a Curie temperature lower than those of the first and third magnetic
layer, and directions of magnetization of the third magnetic layer are aligned
above the portion having irregularities.
In addition, a recording method according to the present invention is to record
information on the magneto-optical recording medium described above, in which the
magneto-optical recording medium is irradiated with a light beam, and at the same
time, a DC magnetic field is applied thereto from a magnetic head, thereby recording
information thereon with the directions of magnetization of the third magnetic
layer above the portions having the prepits (irregularities) being aligned.
Deposition of a magnetic film onto the portions having the prepits (referred
to as irregularities, hereinafter) is different from that onto the portions used
for recording and regenerating information. The film deposition onto the portions
having the irregularities is affected by the irregularities, and thus, the anisotropy
or film thickness is reduced. Thus, magnetization transfer readily occurs, and
the domain wall is readily displaced. Accordingly, if magnetization of the first
magnetic layer behind the regenerating spot is directed in an astable direction
with respect to the direction of magnetization of the third magnetic layer when
a regenerating spot passes thorough a portion having irregularities, the magnetization
state of the third magnetic layer is transferred to the first magnetic layer, and
the domain wall is displaced so as to expand the magnetization state. As a result,
magnetization of the first magnetic layer behind the regenerating spot is directed
in a stable direction with respect to the direction of magnetization of the third
magnetic layer. To the contrary, if magnetization of the first magnetic layer behind
the regenerating spot is directed in a stable direction with respect to the direction
of magnetization of the third magnetic layer when the regenerating spot passes
thorough the portion having irregularities, no change occurs. Accordingly, above
the following portion used for recording and regenerating information in the form
of a magneto-optical signal, magnetization of the first magnetic layer behind the
regenerating spot is directed in a stable direction with respect to the direction
of magnetization of the third magnetic layer above the portion having irregularities.
Therefore, if only directions of magnetization of the third magnetic layer
is aligned at least in all areas immediately preceding the portions used for recording
and regenerating information in the form of a magneto-optical signal of the portions
having the irregularities directions of magnetization of the first magnetic layer
behind the regenerating spot can be aligned in all the portions used for recording
and regenerating information in the form of a magneto-optical signal.
On a substrate having irregularities for providing address information or the
like, a base layer, a first magnetic layer, a fourth magnetic layer, a second magnetic
layer, a third magnetic layer and a top layer are stacked one on another. The substrate
may be made of polycarbonate, acrylic resin, glass or the like. If recording or
regeneration of information is not conducted through the substrate, the substrate
need not be a translucent material. The base layer and top layer may be made of
a dielectric material, such as SiN, AiN, SiO, ZnS, MgF and TaO. The layers other
than the magnetic layers are not essential. The order of the magnetic layers stacked
may be reversed. A metal layer made of Al, AlTa, AlTi, AlCr, Cu, Pt, Au or the
like may be added to this arrangement to control the thermal characteristics thereof.
In addition, a protective coating made of a polymer resin may be added thereto.
Alternatively, substrates after film deposition may be bonded together.
Each of the layers may be deposited by successive sputtering by means of a magnetron
sputtering apparatus, successive evaporation or the like. In particular, the magnetic
layers are successively deposited while maintaining a vacuum state and thus, are
exchange-coupled to each other.
In the medium described above, each of the magnetic layers may be made of various
magnetic materials including magnetic bubble materials and antiferromagnetic materials,
as well as materials typically used for magnetic recording media or magneto-optical
recording media.
For example, the magnetic layers may be made of an amorphous rare-earth iron
group alloy containing 10 to 40 atomic % of one or more rare-earth metals including
Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho and Er and 90 to 60 atomic % of one or more iron
group elements including Fe, Co and Ni. Furthermore, in order to enhance corrosion
resistance, a small amount of an element such as Cr, Mn, Cu, Ti, Al, Si, Pt or
In may be added to the alloy.
Furthermore, a platinum-group iron-group periodic structure film such
as Pt/Co and Pd/Co, a platinum-group iron-group alloy film, an antiferromagnetic
material such as Co—Ni—O and Fe—Rh based alloys, or a magnetic
garnet may be used.
The saturation magnetization of heavy-rare-earth iron-group amorphous alloys
can be controlled by changing the composition ratio between the rare-earth element
and the iron-group element. A compensation composition can provide a saturation
magnetization of 0 emu/cc at a room temperature.
The Curie temperature also can be controlled by changing the composition ratio.
If the Curie temperature is to be controlled independently of the saturation magnetization,
a method may preferably be used in which some of the Fe atoms, an iron-group element,
are replaced with Co atoms, and the amount of the Fe elements replaced is controlled.
Specifically, if 1 atomic % of Fe is replaced with Co, the Curie temperature is
expected to increase by about 6 degrees C. Thus, according to this relation, the
amount of added Co is adjusted to provide a desired Curie temperature. Alternatively,
a small amount of a nonmagnetic element, such as Cr, Ti and Al, may be added to
reduce the Curie temperature. Alternatively, two or more rare-earth elements may
be used, and the composition ratio therebetween may be controlled to control the
Curie temperature.
Domain wall coercivity and domain wall energy density are mainly controlled
by selecting material element, and furthermore can be controlled by condition of
the base layer or film formation conditions such as sputtering gas pressure or
the like. A material such as Tb and Dy has high domain wall coercivity and domain
wall energy density. A material such as Gd has a low domain wall coercivity and
domain wall energy density. Control can be also conducted by addition of impulities
or the like.
The film thickness can be controlled based on the film deposition speed and film
deposition time.
Recording of a data signal onto the magneto-optical recording medium of
the present invention is conducted by associating the direction of magnetization
of the third magnetic layer with the data signal according to a thermomagnetic
recording method. The thermomagnetic recording method includes a method of modulating
an external magnetic field in response to the data signal by irradiating the recording
medium with laser light having a power that provides the third magnetic layer with
a temperature equal to or higher than the Curie temperature while moving the medium,
and a method of modulating the power of the laser while applying a magnetic field
in a certain direction to the recording medium. In the case of the latter, if the
intensity of the laser light is adjusted in such a manner that a temperature equal
to or higher than the Curie temperature of the third magnetic layer is attained
only in a predetermined area in the light spot, a recording magnetic domain smaller
than the diameter of the light spot can be formed, and thus, a high-density recording
pattern over the limit of the resolution of the optical system can be provided.
The present invention will be described in detail below with reference to specific
examples. However, the present invention is not limited to the following examples
without departing from the spirit thereof.
EXAMPLE 1
In this example, a sample servo method was used and a substrate described below
was prepared. In the substrate, a round of track is divided into 1280 segments,
and each segment includes a leading portion in which a clock pit, a first wobble
pit for servo, a second wobble pit and an Address pit are formed in this order
in the form of irregularities and a trailing portion which is a data area used
for recording and regenerating information in the form of a magneto-optical signal.
In a direct-current magnetron sputtering apparatus, targets of B-doped Si, Gd,
Tb, Fe, Co and Cr were installed, the substrate was fixed to a substrate holder,
and then, the chamber was evacuated with a cryopump until a high vacuum of 1×10
-5
Pa or lower is attained in the chamber. Then, an Ar gas was introduced into
the chamber until a pressure of 0.5 Pa is attained while conducting evacuation,
and the targets were sputtered while rotating the substrate, thereby forming the
respective layers. When forming an SiN layer, an N
2 gas was introduced
in addition to the Ar gas, and the SiN layer was formed by direct-current reactive sputtering.
First, as shown in FIG. 7 the SiN layer, which is the base layer 2,
was deposited on substrate 1 to a thickness of 35 nm. Then a GdFeCoCr layer,
which is the first magnetic layer 3 (domain wall displacement layer), was
deposited to a thickness of 35 nm, a TbFeCoCr layer, which is the fourth magnetic
layer 4 (adjustment layer), was deposited to a thickness of 15 nm, a TbFeCr
layer, which is the second magnetic layer 5 (blocking layer), was deposited
to a thickness of 10 nm, and a TbFeCoCr layer, which is the third magnetic layer
6 (memory layer), was deposited to a thickness of 60 nm. Finally, a SiN
layer serving as a protective layer 7 was deposited to a thickness of 50 nm.
The composition ratio of each of the magnetic layers was controlled by controlling
the ratio of powers applied to the targets of Gd, Tb, Fe, Co and Cr. The composition
ratio of each of the magnetic layers was adjusted to generally approximate to the
compensation composition.
The first magnetic layer comprised a first constituent sub-layer having a Curie
temperature of about 300 degrees C., which is closer to the base layer, and a second
constituent sub-layer having a Curie temperature of about 220 degrees C. The Curie
temperatures of the fourth, second and third magnetic layers were adjusted to be
about 180 degrees C., 160 degrees C. and 320 degrees C., respectively.
At a room temperature, the domain wall coercivity of the first magnetic layer
was about 500 Oe, and the domain wall coercivity of the third magnetic layer was
equal to or higher than 15 kOe. The domain wall energy density of the first magnetic
layer was about 2 erg/cm
2, and the domain wall energy density of the
fourth magnetic layer was about 4 erg/cm
2.
The magnetic film of the medium between the information tracks was locally and
selectively laser-annealed from the side of the film surface using a drive apparatus
having mounted thereon a three-beam optical system with a wavelength of 405 nm
and a NA of 0.85, with a linear velocity of 3 m/sec and an LD power of 6.0 mW.
By this processing, coupling due to exchange interaction along the surface of the
magnetic layer is reduced in areas on both sides of the information track.
After that, an ultraviolet curing resin layer having a thickness of about 15
μm was formed on the film surface, thereby providing a sample of the magneto-optical
recording medium of the present invention.
Then, recording and regeneration of information onto this sample was conducted
using an optical system with a wavelength of 650 nm and an NA of 0.60 and a magneto-optical
disk evaluation apparatus having a magnetic head for magnetic field modulation
recording. The sample was rotated at a linear velocity of 3 m/sec and direct-current-irradiated
with a recording/regenerating laser of a power of 4.5 mW, and the magnetic field
was modulated between intensities of ±200 Oe at a recording frequency 10 MHz.
Thereby, a signal pattern of an upward magnetization region and a downward magnetization
region having a mark length of 0.15 μm was formed in the data area during
a cooling process after heating to or above the Curie temperature of the third
magnetic layer.
In this processing, when the laser passes through the leading portion of each
segment in which the irregularities of the clock pit, the wobble pit and the address
pit are formed, the laser was kept being applied thereto, and the magnetic field
was fixed at -200 Oe. Due to this operation, directions of magnetization of the
third magnetic layer above the portions having the irregularities were aligned
in whole of the recording track, and a magneto-optical recording medium meeting
the requirements of the present invention was provided.
COMPARISON EXAMPLE 1
Recording onto the sample described above was conducted in a similar manner.
However, even when the laser passes through the leading portions having irregularities
of the segments, the magnetic field was continuously modulated, and as a result,
the directions of magnetization of the third magnetic layer above the portions
were not aligned.
Results of observation, with an oscilloscope, of waveforms regenerated with
a regeneration power of 2.5 mW for the samples of Example 1 and Comparison Example
1 are shown in FIGS. 1A, 1B, 2A and 2B, respectively. FIGS.
1A and 2A show an envelope for a round of track, and FIGS. 1B and 2B show an envelope
for one segment obtained by enlarging the envelope in FIGS. 1A and 2A, respectively,
along a time base through a 50 kHz HPF. In FIGS. 1B and 2B, a sum signal regenerated
from the clock pit, the first wobble pit for servo, the second wobble pit and the
address pit in the leading portion of the segment is also shown above a differential
signal obtained by regenerating the magneto-optically recorded signal.
In Example 1, as shown in FIG. 1B, the magneto-optical signals for the leading
portion having irregularities of each segment are aligned in a certain direction
(downwardly, in this case), and the following data area is regenerated with the
directions of magnetization of the first magnetic layer behind the regenerating
spot being aligned for the segments. As a result, a stable envelope is observed
as shown in FIG. 1A without a variation of the envelope among the segments.
To the contrary, in Comparison Example 1, a high-frequency magneto-optical signal
pattern is recorded also for the leading portion having irregularities of each
segment as shown in FIG. 2B. In such a case, directions of magnetization of the
third magnetic layer are not aligned in a part where the anisotropy or thickness
is reduced due to an effect of the irregularities. Accordingly, the signal in the
following data area is regenerated with the directions of magnetization of the
first magnetic layer behind the regenerating spot being varied among the segments.
As a result, the envelope is varied among the segments as shown in FIG. 2A.
Such low-frequency variation of the signal waveform can be apparently suppressed
by means of the HPF. However, in such a case, the directions of magnetization of
the first magnetic layer behind the regenerating spot are actually varied, and
an edge shift of the regenerated signal is caused by an effect of a floating magnetic
field or the like, and the jitter is increased. The jitter was measured for both
cases using the 50 kHz HPF. Then, the jitter was 2.6 nsec for the medium of Example
1, while the jitter was 3.2 nsec for the medium of Comparison Example 1.
EXAMPLE 2
As can be seen from FIG. 1B, the envelope of the signal regenerated through the
50 kHz HPF has a sag due to an effect of the preceding DC signal immediately after
entering the data area. Such a sag is not significant if the signal is detected
according to a differential detection method. However, the sag is not desirable
because it results in an increased apparent signal amplitude, and thus, a dynamic
range becomes needed in the circuit.
In order to suppress the sag described above, for the entire recording tracks,
the directions of magnetization of the third magnetic layer can be aligned only
in parts, immediately preceding the data area, of the portions having the irregularities
for providing the address information and servo information, and a predetermined
signal pattern falling within a signal bandwidth used for recording and regenerating
information can be recorded in the remaining parts of the portions having the irregularities.
This can significantly reduce the length of the parts in which magnetization has
to be DC-directed, and thus the sag can be suppressed.
However, while according to the format of the substrate used in Example
1, the address pit is arranged immediately preceding the data area, the address
pit is difficult to handle since the presence or absence thereof or the position
thereof varies from segment to segment for providing the address information. Thus,
in this example, a substrate was used in which the clock pit, the address pit,
the first wobble pit for servo and the second wobble pit are formed in this order.
In addition, in Example 1, there has been contemplated a case where controllability
of transfer suppression and domain wall displacement prevention is maintained in
some areas in the portions having the irregularities, and the direction of magnetization
of the first magnetic layer behind the regenerating spot is not always transferred
to the third magnetic layer. Thus, the directions of magnetization of the third
magnetic layer were aligned in all the areas where the controllability of transfer
suppression and domain wall displacement prevention might be lost, so that even
if the magnetization state is transferred, the direction of the first magnetic
layer behind the regenerating spot was kept constant. The irregularities may not
significantly affect the deposition of the film depending on depths or shapes thereof,
and thus, the condition of the magnetic film above the portions having the irregularities
becomes critical, resulting in such a local variation.
However, in this example, when magnetization of the first magnetic layer
is inverted above the part of the portion having irregularities for which the direction
of magnetization of the third magnetic layer is not controlled, the data area must
not be entered before inverting again the magnetization of the rearmost part of
the portion having irregularities, for which the direction of magnetization of
the third magnetic layer is controlled. Thus, the irregularities were formed deeper
and steeper than those in Example 1 to ensure that the direction of magnetization
of the first magnetic layer behind the regenerating spot is transferred to the
third magnetic layer at least in the rearmost part of the portion having the irregularities
in which the direction is controlled.
A sample was fabricated using this substrate in the same manner as in Example
1.
This is illustrated in FIG. 8. Each layer 1-7 in FIG. 8 corresponds
to the same layer in FIG. 7 as identified by numerals 1-7. Then,
except for the part having the second wobble pit in each segment, which was polarized
over 0.6 μm in one direction, a signal pattern having a mark length of 0.15
μm was recorded in whole of the track.
Regeneration of information from the sample according to Example 2 was
conducted in the same manner as in Example 1, and the regeneration waveform was
observed with the oscilloscope. As a result, any variation of the envelope among
segments did not occur, and the envelope of the signal regenerated involving the
50 kHz HPF had no sag in the data area.
This example has an advantage that, even if the direction of magnetization of
the first magnetic layer behind the regenerating spot is inverted due to a defect
in the film, such as dust or scratch, an appropriate direction of magnetization
can be recovered in the following segment.
EXAMPLE 3
Now, a method of magnetizing the leading portion of each segment having the
irregularities of the clock pit, the wobble pit and the address pit in one direction
when recording actual user data will be described with reference to FIG. 3.
First, the user data output from a buffer (not shown), which temporarily retains
data, is modulated by a modulation circuit 31 in accordance with RLL (1,
7). RLL (1, 7) is a modulation rule in which the run length of bit "1" is always
1 channel clock and the run length of bit "0" ranges from 1 to 7. To the modulated
data, a SYNC signal or the like is added in an addition circuit 32 in the
next stage. The added signal is needed to provide clock extraction and byte synchronization
during regeneration. Then, the output from the addition circuit 32 is transmitted
to an NRZI circuit 33, which outputs a signal which is inverted only when
it is the bit "1". This output is transmitted to a magnetic head driver, and a
magnetic head (not shown) produces a positive or negative magnetic field depending
on whether the signal is 1 or 0. Furthermore, the output from the NRZI circuit
33 is latched in an F/F circuit 34 at an appropriate timing and returned
to the addition circuit 32.
Here, an operation of each of the blocks shown in FIG. 3 will be described
with reference to the timing chart in FIG. 4, which illustrates output signals
of the blocks. In FIG. 4, signals (a1) and (a2) are the outputs of
the addition circuit 32, signals (b1) and (b2) are the outputs
of the NRZI circuit 33, and signals (c1) and (c2) are the
outputs of the F/F circuit.
Since the user data is essentially random data, whether the number of HIGH
pulses in the user data of each segment is even or odd is not fixed. Thus, in the
case where the number of the HIGH pulses in the user data of a relevant segment
is odd, the value after the NRZI is 1 when the segment is ended as shown as the
signal (b1), even if the value is 0 when the preceding segment is ended.
Thus, in the signal (b1), the value immediately after the user data is latched
at a timing t1. Based on the resulting signal (c1), the addition
circuit adds a value 0 or 1
to the output data at a timing t2. In this case, a value 1 is added. After
the addition, the addition circuit keeps outputting the value "0" during a period
corresponding to the leading portion having irregularities of the following segment.
Therefore, the output of the NRZI circuit is kept at 0 without being changed.
The case where the number of the HIGH pulses is even is as shown as the signals
(a2), (b2) and (c2). In the case whether the number is even,
after the value 0 is added to the output data, the addition circuit keeps outputting
the value "0" during a period corresponding to the leading portion having irregularities
of the following segment.
These timings can be generated by a recorded signal timing circuit counting
channel clocks from the end of the user data.
The channel clock is regenerated by a PLL circuit, not shown. The PLL circuit
serves to keep an interval between clock pits at a certain count value based on
detection signals of the clock pits. FIG. 5 specifically shows the recorded signal
timing circuit. As shown in FIG. 5, the recorded signal timing circuit comprises
a counter 51 that counts channel clocks when recording information and is
reset by a clock pit detection signal, and a circuit 52 for decoding the
count output.
FIG. 6 is a timing chart showing a relation between a sum signal for the leading
portion of each segment and a timing of recording data. The operation of the recorded
signal timing circuit will be described with reference to FIG. 6. In this example,
the clock count between the clock pits is 2000. The counter 51 sequentially
counts clocks from a point of a clock pit detection signal where the count value
is set at 0. The decoder outputs a timing signal of a data start at the HIGH level
only when the count value reaches 100, which is beyond the leading portion having
the irregularities of each segment. This output is transmitted to the modulation
circuit 31 shown in FIG. 3, causing the circuit to start modulation.
The same holds true for the timing of data end. As for the number of pieces of
data in each segment, the data length is set in such a manner that the timing of
data end is reached before the leading portion having the irregularities of the
following segment arrives. With the clock count set value being 1990, the decoder
52 outputs "HIGH" only when the count value output from the counter 51
reaches 1990. This output is provided to the addition circuit 32 as a timing
of the last channel bit after user data modulation. Based on this output, the above-described
timings t1 and t2 are generated, and the above-described operations
are implemented. In this example, the leading portion having the irregularities
of each segment has a size corresponding to 110 clocks, 10 clocks of which preceding
the Clock pit and 100 clocks of which following the Clock pit.
As a result, regardless of the content of the user data, the output level for
the leading pit portion of each segment is kept at 0. Thus, when the signal is
recorded onto the medium with the magnetic head, magnetization is always the same
for the pit portions.
Of course, the present invention is not applied only to the modulation rule and
the signal processing method, such as NRZI, described above. It is essential only
that whether the signal to be output to the magnetic head, that is, the digital
signal output to the magnetic head driver is 1 or 0 is determined, a part of the
output data can be varied in response to the detected value, and thus, the digital
signal is always fixed to either value during the period corresponding to the pit portion.
Besides Examples described above, various applications of the magneto-optical
recording medium of the present invention can be contemplated. The present invention
can be effectively applied not only to the sample servo method, but also to a case
where a substrate having a continuous groove based on a push-pull method is used,
if the information track has features that have an effect on deposition of the
film, such as irregularities for providing the address information. The same holds
true for a case where the information track has a break in a groove, or land or
the like, instead of the pits.
In Examples described above, the fourth magnetic layer serving as a control layer
is inserted. However, the fourth magnetic layer is not necessarily required to
be inserted if the information is recorded in accordance with a modulation method
in which the mark length is limited to about 0.3 μm, for example.
In Examples described above, the first magnetic layer comprises the first and
second sub-layers different in Curie temperature. However, if the composition is
precisely controlled, the first magnetic layer can be composed of a single layer.
In Examples described above, the areas on both sides of the information tracks
are laser-annealed to reduce coupling between the information tracks due to exchange
interaction of the magnetic layer. However, essential operations can be provided
even if this processing is omitted. In addition, if the substrate has a groove,
coupling between the information tracks can be reduced by taking advantage of the
radial variation of film deposition caused by irregularities on the substrate.
Interfaces between the magnetic layers are not necessarily required to
be obvious. The recording medium may have a configuration in which the characteristics
are gradually changed along the thickness.
*
