 |
Description  |
|
|
The present invention relates to magneto-optic data storage and, more
particularly, to magneto-optic compensation point recording and erasing
without external magnetic bias.
BACKGROUND OF THE INVENTION
Thin film, ferrimagnetic materials such as rare earth-transition metal
amorphous alloys of terbium iron cobalt (TbFeCo), gadolinium terbium
cobalt (GdTbCo) and gadolinium terbium iron cobalt (GdTbFeCo) have been
known as high-density, magneto-optic recording media. Magnetic domains on
the order of one micrometer in size can be recorded in the magneto-optic
film. These ferrimagnetic materials have high coercivity at room
temperature and low coercivity at high temperatures. The recording medium,
preferably in a coated disk form, can be magnetized in a particular
direction perpendicular to the surface by heating the disk in the presence
of an external magnetic field, and thin permitting the disk to cool or by
applying a saturating magnetic field. Data can thereafter be stored on the
disk by heating a small spot (preferably by laser energy) in the presence
of an external magnetic field of the desired magnetic polarity. The heated
area is magnetized in the direction of the external magnetic field when
the area cools and returns to the high coercivity state at room
temperature. Data on the disk is "read" by noting the effect on polarized
light reflected off the disk surface. Such systems operating with external
magnetic bias operate by either heating above the compensation temperature
of the medium or by heating above the Curie temperature of the medium.
Systems using a reversible external magnetic field have the advantage of
directly controlling the recorded magnetic state according to the polarity
of the applied external field, but tend to be slow in operation.
Curie point systems operating without external magnetic bias are also
known. See Japanese patent application No. 59/1984-113506 "Method and
Apparatus for Opto-Magnetic Recording, Reading and Erasure", filed Dec.
21, 1982, published June 30, 1984, and identifying M. Okada et al as
inventors. The magneto-optic medium in this system has a relatively low
Curie point, in the range of 80.degree. C. to 180.degree. C. Where an area
is heated above the Curie temperature, the heated area loses its
magnetization and, upon cooling, forms a stable domain of reverse magnetic
polarity at approximately one-half the radius of the area heated above the
Curie point. To erase a previously recorded domain, the previously
recorded domain area is heated above the Curie point and the magnetization
of the heated area disappears. If the erase pulse is small and just
sufficient to heat the area of the prior domain above the Curie point, the
domain of reverse magnetic polarity that tends to form upon cooling is
unstable and therefore collapses during cooling.
With the Curie point system described by Okada et al, recording and erasing
are achieved by a single-beam, direct over-write operation. The laser
pulse used to record a spot domain is greater than the laser pulse
required to erase that domain. Therefore, a large record pulse creates a
domain indicating a "1" state regardless of prior magnetic history and,
likewise, a smaller erase pulse results in the absence of a domain
indicating a "0" state regardless of prior magnetic history. In both
cases, the area heated above the Curie point loses its magnetism thereby
wiping out the prior magnetic history. The existence of a domain, or lack
thereof, when the area cools depends on the size of the area heated above
the Curie point.
SUMMARY OF THE INVENTION
In parent applications Ser. Nos. 033,931 and 837,130, the inventors hereof
describe their discovery of biasless compensation point operation. By
using a ferrimagnetic recording medium with a compensation point between
40.degree. C. and 140.degree. C. and a high Curie point it is possible to
record and erase without using external magnetic bias by heating selected
areas above the compensation point and below the Curie point. If a laser
pulse is applied to heat a large enough area above the compensation point,
the coercivity is lowered and the self-demagnetizing field of the medium
forms a domain of reverse magnetic polarity as the area cools. If a
smaller pulse is thereafter applied to the same area, a domain is created
within the prior domain which expands and draws in the surrounding domain
wall until the walls annihilate one another.
Even though the mechanism for erasing is significantly different from that
used in Curie point erasing, applicants have discovered that with
compensation point operation the laser erase pulses are smaller than the
record pulses. From this discovery it becomes obvious that single-beam
direct overwrite operation is possible in a compensation point system, at
least for single data spot operations.
A danger of domain growth exists in compensation point operations, however,
where data is recorded on multiple passes in which successive "1"s are
recorded without an intervening "0" (erase). The domain or data spot may
grow due to slight misalignment on subsequent passes creating an enlarged
domain that cannot be erased reliably. Also, there is a danger that
successive erase pulses would have a cumulative effect and result in a
recorded domain. Compensation point operation does not heat the area above
the Curie point and therefore does not wipe out the prior magnetic history
each time new data is recorded as is the case with Curie point operation.
Applicants have discovered that if the recording medium is formulated to
provide sufficient domain mobility and sufficient time elapses between
successive record or erase operations, a domain will not grow upon
application of subsequent record pulses, but instead will move, maintain
the same size and realign on the last laser pulse. Also, under these
conditions, erase pulses do not accumulate to form domains.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating a magneto-optic recording disk and the
apparatus to erase and over-write digital data recorded on the disk;
FIGS. 1a-1d are a series of illustrations showing the method of controlling
the laser to achieve direct over-write according to the invention;
FIGS. 2a-2h are graphical representations of net magnetic moments of
several adjacent data bit storage regions, at times before, during and
after two successive changes in stored value; and
FIG. 3 is a diagram illustrating alternative apparatus to read, erase and
over-write digital data recorded on the disk.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
FIG. 1 is a schematic illustration of the compensation point magneto-optic
data storage system for writing and erasing spot domains of reverse
magnetic polarity. The magneto-optic recording medium is on the surface of
a disk 10 which is rotated by a suitable drive motor 12. A laser source 14
provides a polarized beam of laser energy directed toward the disk surface
passing through a half-silvered mirror 16 and a lens 18. Light reflected
from the disk surface passes through lens 18, is reflected off mirror 16,
and then passes through a polarizer 20 to a photo detector 22 which in
turn supplies a signal to an amplifier 24 indicating the amount of light
received. Laser 14 and lens 18 make up a head unit 38 which can be
selectively positioned at different distances from the center of disk 10
corresponding to different recording tracks by means of a track
positioning mechanism 36. Magnetic field producing means 50 is provided
beneath disc 10. This magnetic field producing means is used to initially
orient all the magnetic domains in a known direction, and is not used for
magneto-optical recording and erasing.
Disk 10 includes a magneto-optic recording medium 8 placed on a substrate 7
by sputtering or like processes. The recording surface is overcoated with
an optically-transparent protective layer 9 such as silicon oxide. Other
suitable overcoat materials include silicon nitride, aluminum nitride,
titanium oxide and zirconium oxide. The substrate is made of aluminum,
glass (such as Corning glass types 0211 or 7059) or polycarbonate and
glass substrates coated with photosensitive layers.
The recording layer is fabricated of a ferrimagnetic magneto-optic
material, such as an alloy of gadolinium terbium cobalt (GdTbCo),
gadolinium terbium iron cobalt (GdTbFeCo) and the like, which has a
compensation temperature T.sub.c preferably higher than the highest
expected ambient temperature T.sub.a, but much less than the
crystallization temperature of the alloy (about 250.degree. C.).
Compensation temperatures should be at least a few tens of degrees C.
above the normal room temperature. Experimental work has established that
magnetic domains in ferrimagnetic materials can be erased without magnetic
bias according to the compensation point technique of this invention in
materials with a compensation temperature in the range of 40.degree. C. to
140.degree. C. Erasure, although possible, does not work reliably outside
this range. The preferred range of compensation temperatures is 60.degree.
C. to 100.degree. C. The Curie temperature should be at least 150.degree.
C. and at least 50.degree. C. above the compensation point and preferably
at least 100.degree. C. above the compensation point. In-plane hysteresis,
which may be caused by oxidation, should be avoided.
In general it is known that the compensation temperature of a ferrimagnetic
recording medium is a function of the ratio of the two types of magnetic
materials. In rare earth, transition metal, magneto-optic formulations,
the compensation temperature is a function of the rare earth to transition
metal ratio. If more than one rare earth is included in the formulation,
the compensation temperature is approximately independent of the ratio of
the rare earths. In formulations including iron, such as gadolinium
terbium iron cobalt (GdTbFeCo), the compensation temperature is
approximately independent of the iron/cobalt (Fe/Co) ratio. In
formulations in which cobalt is the transition metal, about 77% cobalt
yields a compensation temperature of 50.degree. C., whereas about 75.8%
cobalt yields a compensation temperature of 100.degree. C.
Good domain mobility is required in the recording medium so that, with
multiple pass recording of a particular spot, the domain will realign and
not grow. Ferrimagnetic alloys including light rare earths such as
gadolinium usually provide good mobility but generally require an
approximately equal proportion of a heavy rare earth like terbium to
increase coercivity to an effective operating level.
A preferred formulation (in atomic %) tested in the laboratory is as
follows:
Gd.sub.13 Tb.sub.13 Fe.sub.59 Co.sub.15 having a compensation temperature
of 90.degree. C., a Curie temperature of 330.degree. and a coercivity
H.sub.c of 2.5 kilo-oersteds at 27.degree. C.
Growth of a domain is likely to occur when successive areas overlap while
heated above the compensation point. Successive erase pulses which overlap
above the compensation temperature can create a new domain. It is
essential therefore that the medium rapidly cool below the compensation
point to avoid such overlap on successive operations or successive passes
recording or erasing the same spot. Thin film coatings of the recording
medium on the order of 100 nanometers normally cool to room temperature in
considerably less time than required for multiple passes.
After fabrication, the disk is subjected to a saturating magnetic field of
about 20 kilo-oersteds to provide an initial magnetic state.
As shown in FIG. 1, laser 14 is energized by a power source 30. For lower
cost units operating at low to moderate disk speeds, semiconductor diode
lasers, such as the SHARP LT 024MDD diode laser which operates at 782 mm
wavelength and up to 20 milliwatts power, are preferred. Another suitable
diode laser is HLP 1400 operating at 820 mm wavelength available from
Hitachi. For systems requiring more power for operating at higher disk
velocities, an argon gas laser can be used.
With a diode laser, energization for an interval of 10 nanoseconds to 1
microsecond with power levels on the order of 1-20 milliwatts is used to
affect the magnetic state of the recording medium during "write" and
"erase" operations, whereas a lower power level which does not alter the
magnetic state of the medium is used for "read" operations. Nucleating
micrometer size spot domains in a gadolinium terbium cobalt (GdTbCo) film,
for example, can be achieved by locally heating the film with seven
milliwatts of laser power for a 300 nanosecond pulse duration without an
externally applied magnetic field. A spot domain written with a pulse of
300 nanoseconds duration can be completely erased by a succeeding pulse of
100-200 nanoseconds duration at the same power level. With 12 milliwatts
of laser power, a spot domain written with a 100 nanosecond pulse can be
erased by a 30-80 nanosecond pulse; a spot domain written with a 150
nanosecond pulse can be erased with a 35-130 nanosecond pulse; and a spot
domain written with a 200 nanosecond pulse can be erased with a 45-175
nanosecond pulse. A one-shot multivibrator 31 is connected to power source
30 and set to produce a pulse of duration D.sub.1 corresponding to the
longer "write" pulse, and a one-shot multivibrator 32 is connected to
power source 30 and set to produce a pulse of duration D.sub.2
corresponding to the shorter "erase" pulse.
Lens 18 should be configured to focus the beam, at the half power diameter
thereof, to an area having a diameter somewhat less than the diameter of
the region to be heated. In both writing and erasing in accordance with
this invention, the object in selecting the laser pulse power is to heat
an area above the compensation point without significant heating above the
Curie temperature. Thus, localized heating raises the temperature above
the compensation point, and causes a localized decrease in the coercivity,
of substantially only a limited region into which data is to be written.
The data written on a track of disk 10 can be non-destructively read
therefrom by causing a reading light beam of linear-polarized light from
laser 14, energized by a low level power source (not shown in FIG. 1) of
amplitude insufficient to heat an area to a temperature high enough that
the magnetization changes, to be projected toward the disk surface. In the
reflected beam, polarization is rotated in a direction dependent upon the
direction of the net magnetic moment. The polarization of the reflected
beam can therefore be analyzed to determine the binary state of the data
stored in the region being read.
The present invention facilitates spot domain recording and erasing of
previously written information in selected regions of the magneto-optic
medium. The method employs the self demagnetizing field created within a
reheated region by the specially formulated thin film magneto-optic
recording medium (with a compensation temperature in the range of
40.degree. to 140.degree. C. and a Curie point temperature at least
50.degree. C. above the compensation temperature) to reverse the net
magnetization of the region and thus eliminate the requirement for an
externally applied bias magnetic field. The physical processes believed to
be involved will be described hereinafter in conjunction with FIGS. 2a-2h.
In general, when a sufficiently large region is heated above the
compensation point to lower the coercivity of the region, the
demagnetizing field creates a stable domain of reverse magnetic polarity
when the region cools. To erase a previously recorded domain, a lower
energy laser beam is used to create a domain within the prior domain. The
new domain expands and draws in the surrounding domain wall until the
domain walls annihilate one another, thereby erasing the domains.
Control circuits suitable for achieving direct over-write operation
according to the invention are illustrated in FIG. 1. Timing clock pulses,
such as are conventionally derived from a timing track on the disk, are
applied to a terminal 40 whereas binary data to be recorded is supplied to
a terminal 41. Data terminal 40 is directly connected to one input of an
AND gate 33 and is connected to one input of an AND gate 34 via an
inverter 35. Terminal 40 is connected to the other inputs of AND gates 33
and 34. The outputs of AND gates 33 and 34 are connected to one-shot
circuits 31 and 32, respectively.
The direct over-write method of operation according to the invention is
illustrated in FIGS. 1a-1d. A sequence of eighteen timing pulses is shown
in FIG. 1a designating the positions of the data spot locations along a
data track on the disk. Assume that data has previously been recorded as
shown in FIG. 1b wherein domains of reverse magnetic polarity appear at
locations 2, 3, 7, 9, 10, 11, 15 and 16. If on a subsequent pass, the
laser is energized with the larger write pulse from one-shot 31 at
locations 3, 5, 6, 9, 10, 14, 15 and 18, as shown in FIG. 1c, domains of
reverse polarity are recorded as indicated by the shaded areas appear at
corresponding locations on the track as shown in FIG. 1d. Short erase
pulses from one-shot 32 at locations 1, 2, 4, 7, 8, 11, 12, 13, 16, and 17
erase any domains that may have previously been recorded at these
locations.
As can be seen, application of a higher energy "write" pulse results in a
domain of reverse magnetic polarity which can correspond to a "1" state
regardless of the prior magnetic history at the data location. Likewise,
application of a lower energy "erase" pulse results in a "0" state
regardless of prior magnetic history. That is, any existing domain is
erased and if no domain exists, none is formed. Thus, the data pattern can
be directly controlled by a single beam pass according to the pattern of
"write" and "erase" pulses.
With a recording media including gadolinium to provide domain mobility and
terbium to increase coercivity (GdTbFeCo) and a film thickness on the
order of 100 nanometers, re-recording a data spot 1000 times did not
result in any noticeable growth of the domain. Also, multiple erase pulses
do not result in a recorded domain with such a medium.
In some instances re-recording domains can lessen the contrast indicating
some porosity in the domain. The problem can be eliminated by placing a
small permanent magnet 50 (FIG. 1) beneath the film with the field in such
a direction as to enhance the recording process without affecting the
erase process without affecting the erase process (i.e. in opposition to
the pre-magnetized state). Such use of a non-reversible magnetic field
does not adversely affect the operating speed as would be the case with an
external reversible field.
Laser pulses at different energy levels for the "write" and "erase"
operations can be achieved using different power levels with the same
pulse duration. With a GdTbFeCo medium, a domain written with an 800
nanosecond laser pulse at 8.3 milliwatts (0.5 micrometer domain size) can
be erased with a laser pulse of the same duration at a power level of
2.6-7.3 milliwatts. A domain written with an 800 nanosecond pulse at 10.4
milliwatts (about 0.7 micrometer domain size) can be erased at a power
level of 6.8-9.2 milliwatts at the same duration. A domain of a 1.0
micrometer size recorded using a 1000 nanosecond pulse at 7.9 milliwatts
can be erased with a pulse at 2.4-6.9 milliwatts at the same duration. A
domain of a 1.2 micrometer size can be recorded using a 1000 nanosecond
pulse at 10.5 milliwatts and can be erased using pulses at 3.3-7.4
milliwatts at the same duration.
Alternative apparatus for achieving high energy "write" pulses and lower
energy "erase" pulses is illustrated in FIG. 3, wherein the light pulses
produced by laser 14 are of the same duration, but at a higher power level
for "write" operations and at a medium power level for "erase" operations.
The timing pulses at terminal 40 are supplied to a one-shot multivibrator
80 which in turn supplies pulses of controlled duration to AND gates 81
and 82. Data terminal 41 is directly connected to the other input of AND
gate 81 and is connected to the other input of AND gate 82 via an inverter
35. The output of AND gate 81 activates a write power source 91 which in
turn energizes diode laser 14 at the higher power level when activated.
The output of AND gate 82 activates power source 92 which in turn
energizes diode laser 14 at the medium power level when activated. A read
power source 90 is connected to diode laser 14 to energize the laser at a
power level below that which can affect the state of the magnetic
recording medium.
Critical to the write operation according to the invention is to supply
sufficient energy to heat an area above the compensation temperature large
enough to reduce the coercivity, and create a domain of reverse magnetic
polarity upon cooling. Critical to the erase operation according to the
invention is to supply a moderate amount of energy sufficient to create a
domain which can grow and annihilate any surrounding domains but not
sufficient to create a stable domain upon cooling. The energy for the
write and erase operations can be by means of laser pulses of the same
power level at different pulse durations (FIG. 1) or by means of pulses of
the same duration at different power levels (FIG. 3). The energy can also
be supplied using multiple pulses sufficiently closely spaced to create
overlapping areas heated above the compensation temperature.
The process that appears to be physically occurring for the self-inverting
data over-write method (using the apparent self-demagnetizing field of the
thin-film magneto-optic recording media) is illustrated in FIGS. 2a-2h.
Prior to the time at which a first over-write operation is to occur, the
media layer 60 is at an ambient temperature Ta less than the compensation
temperature Tc of the magneto-optic material. The recording regions each
contain one bit of a first set of data. FIG. 2a illustrates that, for the
starting data set with the same data value, e.g., a binary one state, in
each of three sequential regions (N-1), N, and (N+1), the net magnetic
moments (symbolized by the broad arrows 62a-62o) are all directed in the
same (e.g., upward) direction substantially perpendicular to the media
layer surface 60a and are all of approximately the same amplitude. The
amplitude and direction of the net magnetic moment M is established by the
relative amplitudes and direction of the magnetic moment of the individual
components of the magneto-optic alloy. Here, the downwardly-directed
moment 64a of the transition metal (TM) component of the alloy is of
smaller amplitude than the upwardly-directed moment 64b of the rare earth
(RE) alloy component, in each subregion.
As a region N receives energy from the overwriting means (laser 18), the
temperature of that region is raised until the compensation temperature Tc
is exceeded. Because the impingent light beam 15 has a substantially
Gaussian energy distribution, the entire region N is not uniformly heated.
Thus, while the individual alloy component magnetic moments (e.g., moments
62a and 62b) and the net magnetic moments (e.g., net moments 62a, 62b, 62n
and 62o) all remain substantially unchanged in subregions removed from the
region N receiving energy, those subregions nearer to the heated region N
receive energy from the fringes of the beam. Responsive to the increased
temperature, which is less than the compensation temperature in these
other regions (N-1), (N+1), etc., the magnetic moment 64c of the
transition metal TM component is decreased by some amount, which is not as
great as the decrease in the magnetic moment 64d of the rare earth RE
component; the net moment (e.g., net moments 65c' and 64m') of that
subregion is reduced. As the temperature increases, the reduction in the
magnitude of the net magnetic moment (e.g., net moments 64d' and 64l')
continues, responsive to the faster reduction of the RE moment 64fthan the
reduction in the TM moment 64e, with closer location to region N. In some
subregions the compensation temperature is just attained and the reduced
amplitudes of the TM and RE moments 64g and 64h become by definition,
equal; the net magnetic moment 62e', 62k', . . are of zero magnitude (and
define the periphery of the region N). Inward of the regions 62 with zero
net magnetic moment, the subregion temperature exceeds the compensation
temperature; the reduced amplitude of the TM moment (e.g., magnetic
moments 64i, 64k, 64m . . . ) is now larger than the reduced amplitude of
the RE moment (e.g., magnetic moments 64j, 64l, 64m, 62f-62j', . . . ) are
all now reversed, having increasing magnitude but in the opposite
direction (e.g., into the recording layer).
The self-demagnetizing field in opposition to the magnetization in the
center region of FIG. 2b causes the component moment directions to reverse
as shown in FIG. 2c. The component moments in subregions within, but
adjacent to, the region N periphery remain fixed in the former direction
(e.g., as shown by moments 64i and 64j) so that the net moment remains
fixed in the new (now inverted) direction. The component moments in the
more central subregions, however, are direction reversed to the original
direction (e.g., the upward direction, as at net magnetic moments 62g",
62h", 62i", . . . ). As the subregion temperature decreases upon cooling,
after removal/off-switching of the light beam, the amplitudes of the alloy
component magnetic moments increase to their ambient temperature values;
as each subregion passes through the compensation temperature, the net
magnetic moment 62 thereof is decreased to zero. As shown in FIG. 2d, in
each subregion, at some temperature less than the compensation
temperature, the RE magnetic moment (e.g., magnetic moment 64n.degree.)
amplitude is again greater than the amplitude of the TM magnetic moment
(e.g., magnetic moment 64m.degree.) and the direction of the net moment
(e.g., net magnetic moment 62h.degree.) is again in the same inverted
direction. The rest of the region N subregions experience the same
inversion of their net magnetic moments (e g., net magnetic moments
62g.degree., 62i.degree. . . . ). Thus, the subregions of region N all
have net magnetic moments aligned in a direction opposite to the alignment
direction prior to the heating of the region N to a temperature greater
than the compensation temperature. As the opposed moments nucleate a
magnetic wall (as at the periphery subregions 66a and 66b upon opposite
sides of region N), a stable magnetic domain is created, with diameter D,
now storing the new value of the associated data bit.
Referring now to FIGS. 2e-2h, at some later time, a comparison of the data
value (e.g., a logic zero) stored in the domain and the logic value (e.g.,
a logic one) of a new bit of binary data for storage in domain N indicates
that the region N must be over-written. This decision enables the write
laser diode and causes region N to be again heated, in the absence of any
substantial intentional external bias magnetic field, to a temperature in
excess of the compensation temperature. The domain wall, as exemplified by
wall portions 66a and 66b of FIG. 2e is not abruptly destroyed; there is
an inversion of the net magnetic moments 62 of the subregions within
region N, due to the reversal of the alloy component magnetic moment
dominance. Thus, the net magnetic moments 62f'-62j', of those subregions
within the heated region N, are not only modified in amplitude by the
Gaussian energy distribution of the impinging light beam, but are also
inverted in direction, to point upwardly and away from the magneto-optic
material layer. The self-demagnetizing field of the immediately-adjacent
subregions (i.e., the upwardly directed net moments of the subregions 62f'
and 62j") cause a reversal in the local magnetic field in the center
subregion(s), as here represented by subregion 62h", of the heated region
N, as shown in FIG. 2f, so that at least one of the interior subregions
now has the net magnetic moment thereof directed in an again-inverted
direction (e.g., the downwardly-directed net magnetic moment 62h" of a
smaller, region of diameter D', less than region/domain diameter D, within
the larger domain/region N). A second, inner domain wall, as shown by
opposed wall portions 68a and 68b, is now present about the subregion
periphery. Local wall motion causes the portions of the inner wall to
expand to the locations of the associated portions of the outer wall; the
two walls meet and mutually annihilate one another, so that the diameter
D", of the region in which the net magnetic moments 62g"-62i" (see FIG.
2g) are still inverted, is greater than the domain diameter D'. As the
temperature of the region N is decreased to below the compensation
temperature, by cooling after cessation of the heating pulse, the relative
amplitudes of the RE and TM alloy component magnetic moments change and
the subregion net magnetic moments are again all directed in the same
direction (e.g., the upward direction for net magnetic moments 62e-62k, of
FIG. 2h). The data value stored in region N has, therefore, been inverted
(e.g., to a logic one value) from the state of the data value (e.g., the
logic zero value) previously stored in that region.
While presently preferred embodiments have been described herein, many
modifications and variations should be apparent to those skilled in the
art. The scope of the invention is defined in the appended claims and not
by the specific details and instrumentalities presented herein as
illustrations.
* * * * *
|
|
 |
|
 |
Description |
|
