Magneto optic recording medium with silicon carbide dielectric

A magneto optic recording medium comprising a substrate, an amorphous magnetizable rare earth-transition metal alloy layer, a transparent dielectric layer on at least one side of the magnetizable layer, and a reflective surface located to reflect light through the magnetizable alloy layer. The dielectric layer is comprised of silicon carbide of the formula SiC.sub.x, wherein x, the molar ratio of carbon to silicon, is greater than 1. The dielectric layer is preferably deposited by direct current magnetron sputtering at low argon partial pressure from an electrically conductive mixture of silicon carbide and carbon. The medium exhibits similar or improved characteristics over media constructed with present dielectrics, for example, silicon suboxide (SiO.sub.y, y<2).

TECHNICAL FIELD 
This invention relates to magneto optic recording media which employ 
dielectric materials to protect a rare earth-transition metal recording 
material from oxidation or corrosion, enhance signal to noise ratio, act 
as a thermal barrier, or for other purposes. The dielectric material is 
comprised of carbon-rich silicon carbide with properties desirable in this 
application, such as suitable index of refraction, transparency, and 
ability to prevent corrosion. 
BACKGROUND 
Magneto optic recording media are also known by several other names: 
thermomagnetic media, beam addressable files, and photo-magnetic memories. 
All of these terms apply to a storage medium or memory element which 
responds to radiant energy permitting the use of such energy sources as 
laser beams for both recording and interrogation. Such media modify the 
character of an incident polarized light beam so that the modification can 
be detected by an electronic device such as a photodiode. 
This modification is usually a manifestation of either the Faraday effect 
or the Kerr effect on polarized light. The Faraday effect is the rotation 
of the polarization plane of polarized light which passes through certain 
magnetized media. The Kerr effect is the rotation of the plane of 
polarization of a light beam when it is reflected as at the surface of 
certain magnetized media. 
Magneto optic recording media have several advantages over known magnetic 
recording media: 
1. No contact between the medium and a recording head, thus eliminating a 
source of wear; 
2. Using a pulsed laser beam as the writing means, very high density data 
storage is possible; and 
3. With a protective layer on top of a magneto optic layer, the medium is 
affected less by dust than magnetic media. 
In magneto optic recording, data is written into a medium having a 
preferentially directed remanent magnetization by exposing a localized 
area (spot or bit) on the recording medium to an electromagnetic or other 
energy source of sufficient intensity to heat the recording medium above 
its Curie or compensation point temperature and simultaneously biasing the 
medium with a magnetic field. Preferably, the energy source is a laser 
which produces a monochromatic output beam. The magnetic field required to 
reverse the magnetization of the recording medium varies with the 
temperature to which the recording medium is brought. Generally speaking 
for a given material, the higher the temperature, the smaller the required 
magnetic field coercive force. 
The write or record operation for both Curie point and compensation point 
writing is as follows: 
1. The medium is initially in a randomly magnetized state. A domain will 
herein refer to the smallest stable magnetizable region; although in 
common usage, a domain is a uniformly magnetized region of any size. A 
selected area of the medium may be magnetized by exposing it to a 
continuous energy beam and a small magnetic bias field normal to the 
surface of the medium. 
2. A small magnetic bias field oriented perpendicular to the surface or 
plane of the medium, but oppositely directed to the magnetic field applied 
earlier is applied over the entire thin film medium. 
3. With the biasing field in place, a light beam from a radiant energy 
source such as a laser beam is directed toward a selected location or bit 
on the medium where it causes localized heating of the medium to a 
temperature at or above the Curie and/or compensation temperature. When 
the laser beam is removed, the bit cools in the presence of the biasing 
magnetic field and has its magnetization switched to that direction. The 
medium, in effect, has a magnetic switching field which is temperature 
dependent. The magnetic biasing field applied to the irradiated bit 
selectively switches the bit magnetization, with the bit momentarily near 
its Curie and/or compensation temperature under the influence of the 
laser. The momentary temperature rise reduces the bit coercive force. 
In the write operation, the write laser beam is focused to the desired 
diameter (e.g. 1.0 micrometer) onto the surface of the recording medium by 
an objective lens. 
The memory element or recorded bit is interrogated, or read, 
nondestructively by passing a low-power (e.g. 1-3 mW) beam of polarized 
light (e.g. a laser beam) through the bit storage site for a sufficiently 
short time so as not to heat the medium to change its magnetic state. The 
read laser beam is normally shaped to a circular cross section by a prism, 
polarized and focused to the same diameter as the write beam onto the 
recording medium by a lens. When the read beam has passe through the 
recorded spot, it is sent through an optical analyzer, and then a detector 
such as a photodiode, for detection of any change or lack of change in the 
polarization. 
A change in orientation of polarization of the light is caused by the 
magneto-optical properties of the material in the bit or site. Thus, the 
Kerr effect, Faraday effect, or a combination of these two, is used to 
effect the change in the plane of light polarization. The plane of 
polarization of the transmitted or reflected light beam is rotated through 
the characteristic rotation angle .theta.. For upward bit magnetization, 
it rotates .theta. degrees and for downward magnetization -.theta. 
degrees. The recorded data, usually in digital form represented by logic 
values of 1 or 0 depending on the direction of bit magnetization, are 
detected by reading the change in the intensity of light passing through 
or reflected from the individual bits, the intensity being responsive to 
the quantity of light which is rotated and the rotation angle. 
It was previously believed that the signal-to-noise ratio (SNR) or 
carrier-to-noise ratio (CNR) of an erasable magneto optic medium is 
proportional to .theta..times.R.sup.1/2 where .theta. is the angle of 
rotation and R is the reflectivity of the medium. Presently, the 
relationship between CNR and the parameters of a fully constructed magneto 
optic medium is not well understood. The process of optimizing media 
construction appears to be more complicated than simply optimizing 
.theta..times.R.sup.1/2. 
Forty-five decibels in a 30 kHz band width is generally considered the 
minimum CNR acceptable for direct read after write (DRAW) media. The speed 
at which the bits can be interrogated and the reliability with which the 
data can be read depends upon the magnitude of the magneto optical 
properties, such as the angle of rotation, and upon the ability of the 
interrogation system to detect these properties. For purposes of this 
discussion, the noise floor or noise level is measured at the average 
noise level. 
The main parameters that characterize a magneto optic material are the 
angle of rotation, the coercive force, the Curie temperature and the 
compensation point temperature. The medium is generally comprised of a 
single layer or multiple layer system where at least one of the layers is 
a thin film metal alloy composition. Binary and ternary compositions are 
particularly suitable for amorphous metal alloy formation. Suitable 
examples would be rare earth-transition metal (RE-TM) compositions, such 
as: gadolinium-cobalt (Gd-Co), gadolinium-iron (Gd-Fe), terbium-iron 
(Tb-Fe), dysprosium-iron (Dy-Fe), Gd-Tb-Fe, Tb-Dy-Fe, Tb-Fe-Co, 
terbium-iron-chromium (Tb-Fe-Cr), gadolinium-iron-bismuth (Gd-Fe-Bi), 
gadolinium-iron-tin (Gd-Fe-Sn), Gd-Fe-Co, Gd-Co-Bi, and Gd-Dy-Fe. 
Many of the elements which are suitable for the rare earth-transition metal 
alloy layer react strongly with oxygen and other elements which may be 
present in the environment in which the media are used. Furthermore, the 
substrate upon which the alloy layer is deposited may itself contain 
impurities which react with the alloy layer. Thus, materials are deposited 
on one or both sides of the RE-TM thin film to protect it. To be 
effective, such materials must not themselves react with the rare 
earth-transition metal layer or any other layer, must offer chemical and 
physical resistance to degradation by heat, humidity, and corrosive 
chemicals, and must be transparent at the wavelengths used for reading and 
writing of data (typically about 8200 or 8300 angstroms for a laser diode, 
or approximately 6328 angstroms for a helium-neon laser, although other 
wavelengths may be used). A material is "transparent" for the purposes of 
this discussion when it absorbs less than about 20 percent of the 
intensity of an incident light beam at a particular wavelength. 
Presently used dielectrics include silicon suboxide (SiO.sub.y, y&lt;2), 
titanium dioxide, silicon dioxide, cerium oxide, aluminum oxide, and 
aluminum nitride. Most of these materials contain oxygen, which can react 
with the rare earth element in the magnetizable layer and thereby degrade 
media performance. All these materials are dielectrics, i.e., they have 
very low electrical conductivity. This prevents the use of DC magnetron 
sputtering to deposit them on the other layers of a complete magneto optic 
medium. Instead, radio frequency (RF) sputtering, evaporation deposition, 
or reactive sputtering deposition, can be used. 
DISCLOSURE OF INVENTION 
The invention is a magneto-optic recording medium comprising a substrate, 
an amorphous magnetizable rare earth-transition metal alloy layer having a 
transparent dielectric layer on at least one side, and a reflective 
surface located to reflect light through the magnetizable alloy layer, 
wherein the dielectric layer is comprised of silicon carbide of the 
formula SiC.sub.x, wherein x, the molar ratio of carbon to silicon, is 
greater than 1. 
Many substrates can be used. They may be formed of any material which is 
nonmagnetic, dimensionally stable, minimizing radial displacement 
variations during recording and playback. Semiconductors, insulators, or 
metals can be used. Suitable substrates include glass, spinel, quartz, 
sapphire, aluminum oxide, metals such as aluminum and copper, and polymers 
such as polymethyl-methacrylate (PMMA) and polyester. Glass is preferred 
for applications requiring high dimensional stability, while polymers are 
preferred for mass production due to their comparatively lower cost. 
The substrate is typically a disc. Common diameters include 3.5 inches (8.9 
centimeters) and 5.25 inches (13.3 centimeters), although other sizes are 
used. Transparent substrates allow the construction of media in which the 
read and write light beams pass through the substrate before the recording 
layer, then onto a reflector layer, and back again to the recording layer 
after reflection. Such a medium is known as a substrate incident medium. 
When the reflector layer is between the substrate and the recording layer, 
the read and write beams will not be directed through the substrate. Such 
a medium is known as an air incident medium, although generally there is 
at least one layer between the recording medium and the air. 
When a magnetizable amorphous material is deposited on a reflector, it is 
known that the magneto optic rotation is increased because the Faraday 
effect is added to the Kerr effect. The former effect rotates the plane of 
polarization of the light as it passes back and forth through the magneto 
optic layer while the Kerr effect rotates it at the surface of the layer. 
The reflective surface may be a smooth, highly polished surface of the 
substrate itself, or it may be the surface of a separate reflecting layer 
deposited by techniques known in the art such as vacuum vapor deposition. 
The reflective surface or layer usually has a reflectivity greater than 
about 50 percent (preferably 70 percent) at the recording wavelength. 
Deposited reflecting layers usually are about 500 to 5000 angstroms thick. 
Typical reflective surfaces or layers are copper, aluminum, or gold. 
The recording medium thin film typically comprises an alloy of at least one 
rare earth element and at least one transition metal and usually is no 
more than 400 angstroms thick if a reflector is employed; if not, the film 
may need to be as thick as 2000 angstroms to produce the same magneto 
optic effects, as the Faraday effect will not be present. If it is too 
thin, generally 50 angstroms or less, the magneto optic film may not 
absorb enough light in the write mode. Sufficient coercivity to create a 
stable memory should be about 500 Oersteds (Oe), but a range of 2000 to 
3000 Oe is generally used. 
Oxidation of the magnetizable RE-TM layer is believed to be a major cause 
of loss of media performance. 
A transparent layer can be deposited on one or both sides of the 
magnetizable amorphous film. When it is located between the reflecting 
layer and the magnetizable amorphous film, it is known as the intermediate 
layer. In this position, dielectric materials are preferred, as they are 
known to protect the alloy layer from reacting with the reflecting layer 
or any impurities in it. A dielectric layer also provides a thermal 
barrier, reducing heat conduction from the magnetizable amorphous film to 
the reflector layer, thereby reducing the laser power required to write 
data in the magnetizable amorphous film. The intermediate layer is 
generally 0 to 300 nanometers thick. Such an intermediate layer should 
have an index of refraction greater than about 1.2, preferably between 2.0 
and 3.0. An intermediate layer with a high index of refraction allows the 
magneto optic rotation angle to be significantly increased by interference 
enhancement. 
Interference enhancement may also occur when a second transparent layer is 
deposited on the other side of the magnetizable amorphous thin film. Such 
a layer will be called a barrier layer. Media having one interference 
layer (either an intermediate or barrier layer) plus the MO and reflective 
layers are referred to as trilayer media. Media having both an 
intermediate layer and a barrier layer are called quadrilayer media. When 
the barrier layer is constructed from a dielectric material, it also is 
characterized by an index of refraction greater than 1.2, although it need 
not be the exact same material as the intermediate layer. The index of 
refraction should not be so high, however, as to produce too much 
reflection from the interface of the barrier and substrate layers, if the 
barrier layer is located adjacent a transparent substrate on which the 
polarized light is first incident (substrate incident structure). The 
barrier layer is usually between about 30 and 200 nanometers thick. 
In cases where the dielectric layer is in between the recording film and 
the reflecting layer or surface and there is no barrier layer (trilayer 
construction), it is beneficial to add a transparent passivating layer 
over the recording film layer. Passivation is the change of a chemically 
active metal surface to a much less reactive state. The transparent 
passivating layer is basically the same as the previously described 
transparent dielectric barrier layer but thinner, typically up to about 
100 angstroms thick. As in the case of the transparent dielectric layer on 
the other side of the MO film, the passivating layer must protect the 
recording film from oxidation or corrosion due to excessive heat, 
humidity, or chemical reaction with impurities. It need not obtain the 
same optical effects (e.g., .theta. enhancement) as the thicker barrier 
layer. It is possible to combine the functions of the barrier layer and 
the passivating layer into a single layer comprised of a transparent 
dielectric material, and selecting the thickness to provide interference 
enhancement. Such a layer is still known as a barrier layer.

DETAILED DESCRIPTION OF THE INVENTION 
The magneto optic amorphous thin films can be made by known thin film 
deposition techniques, such as sputtering, evaporation and splat cooling. 
One preferred process is cathodic sputtering. Typical known sputtering 
conditions for amorphous thin films are: initial vacuum less than 
1.times.10.sup.-5 torr; sputtering pressure of from 3.times.10.sup.-2 to 
2.times.10.sup.-2 torr; pre-sputtering of a sputtering source of material 
to clear the surface thereof; substrate temperature of 30 degrees to 100 
degrees C.; and a noble gas (usually argon) partial pressure. 
In the cathodic sputtering process, gas ions bombard the solid alloy target 
cathode in the sputtering chamber, dislodging metal atoms by transferring 
the momentum of the accelerated ions to the metal atoms near the surface 
of the target. The substrate is placed at the anode, and the metal alloy 
atoms traverse the space between the anode and cathode to deposit on the 
substrate. 
In the triode sputtering process, a thermionic cathode, known as an 
emitter, is added to the sputtering chamber between the anode and cathode. 
This allows the gas plasma to be maintained at much lower pressures than a 
direct current glow discharge, even in a magnetic field or magnetron. 
Typically an argon plasma can be maintained at 4.times.10.sup.-3 to 
6.times.10.sup.-4 torr gauge pressure. This process enables the sputtered 
atoms to strike the substrate at a higher energy than they would at a 
higher pressure since there are fewer argon ions in the space between the 
target and substrate to interfere with the motion of the sputtered atoms, 
increasing the mean-free-path. 
Magnetically enhanced sputtering, in which a magnetic field is applied in 
the sputtering chamber perpendicularly to the electric field, further 
reduces the pressures needed to sputter and thereby increases the 
mean-free-path. This is because the magnetic field deflects electrons into 
following spiral-like paths with greater distances to travel to reach the 
anode. The longer path increases the probability of collision with the gas 
atoms. These collisions produce the gas ions which dislodge the sputter 
target atoms, hence an increase in probability of gas collision increases 
the sputtering rate. Another feature of magnetic enhancement is that 
electrons are confined to the ionized gas plasma and produce less heating 
of the substrate by electron bombardment. This feature is a benefit when 
it is desired to use substrates with comparatively low melting points. 
Currently used dielectric materials are deposited by several methods which 
have disadvantages. Evaporation deposition requires much lower vacuum 
pressures than the sputtering techniques used to deposit the RE-TM alloy 
films; uniform film deposition over a wide area is difficult; and the 
deposition rate is difficult to control because it is an exponential 
function of evaporation boat temperature. Radio frequency (RF) sputtering 
produces excessive heat at the substrate, making some inexpensive plastic 
substrates unusable; and the RF signals can interfere with computer 
controlled manufacturing processes. 
Reactive sputter deposition requires the addition of a reactive gas to the 
chamber, which can contaminate the other deposition processes; and the gas 
may react at the sputter target surface, forming a nonconductive film 
which interferes with the sputtering. 
Direct current (DC) magnetron sputtering offers superior deposition rate 
control and deposited film thickness when compared to evaporation 
techniques. Nonreactive DC magnetron sputtering (i.e., no reactive gases 
present) reduces the contamination of other processes carried out in the 
same deposition chamber. Temperatures generated at the substrate are low 
enough to permit the use of plastic substrates. There is less 
electromagnetic interference with control equipment than is produced by 
the RF methods. Therefore, the DC magnetron sputtering process is 
preferred for depositing the dielectric layers (either the intermediate or 
barrier layers, or both). 
DC magnetron sputtering requires a sputtering target which is electrically 
conductive. Assuming a target current of 50mA/cm.sup.2 and that an induced 
through-target voltage drop greater than 50 volts is unacceptable, a 
maximum target material resistance of 1000 ohm/cm.sup.2 is required. For a 
target thickness of 0.3 cm, the permissible resistivity is approximately 
3300 ohm-cm or less. 
A suitable material for nonreactive DC magnetron sputtering is electrically 
conductive silicon carbide, available from the Standard Oil Company, 
Structural Ceramics Division, under the trademark Hexoloy. A grade of the 
material, identified by this manufacturer as SG, is produced by blending 
approximately 95 percent SiC and 5 percent graphite powder by weight and 
adding suitable binders. The blended powders are extruded to form a sheet 
or rod, and then sintered in high temperature graphite furnaces. The final 
product consists of SiC grains in a matrix of electrically conductive 
porous graphite. The electrical conductivity at 20 degrees C is between 
0.2 and 300 ohm-cm, depending upon the dopants used. 
Electrically conductive silicon carbide can be used as a DC sputtering 
target without bonding to a backing plate. The outer portion of the 
material has higher carbon content, which has been found to produce 
sputtered films with carbon/silicon ratio strongly determined by 
sputtering time. A pre-sputtering procedure to clean the target is used to 
improve the uniformity of the films produced. 
The thin films produced with a silicon carbide and graphite sputter target 
have the chemical formula SiC.sub.x, wherein x, the molar ratio of carbon 
to silicon, is greater than 1. The carbon-rich silicon carbide films also 
have detectable concentrations of other elements, which are believed to be 
due to the binders used in a particular blend of powders. Generally 
speaking, the more electrically conductive the SiC/graphite sputtering 
target is, the higher the value of x in the deposited film. Excessively 
conductive blends produce films with large x values (e.g., 3.0 or more), 
but they lose transparency and thus are less acceptable for magneto optic 
media applications. 
FIG. 1 is a Auger Electron Spectroscopy (AES) spectrum of a transparent 
dielectric layer deposited on a substrate from a pre-sputtered target. The 
target material was Hexoloy SG SiC/graphite. The sputter targets used were 
0.25 inch (6.4 centimeter) thick by 12 inches (30.5 centimeters) long by 5 
inches (12.7 centimeters) wide. The spectrum indicates the presence of 
carbon, boron, silicon, nitrogen, and oxygen in detectable concentrations. 
Using the peak intensities and standard sensitivity factors known in the 
art, the atomic concentration of the dielectric was estimated as 
Si(35%)C(51%)B(7%)N(5%)(2%), which yields a value of x=(0.51./0.35)=1.47. 
The binders are the suspected source of boron and nitrogen, while the 
oxygen is believed to come from contamination of the vacuum chamber during 
sputtering. Because oxygen may react strongly with one or more elements in 
the amorphous rare earth-transition metal alloy layer, terbium for 
example, the oxygen concentration should be minimized. 
It is also desirable to perform the DC magnetron sputtering at low noble 
gas partial pressures, typically 0.01 torr or less. The resulting media 
show less decrease in coercivity with time (a measure of media storage 
stability) than media produced by sputtering at higher pressures. 
The magneto optic recording media produced in the reduction of this 
invention to practice were substrate incident, quadrilayer media. The 
substrates were glass or polycarbonate. The amorphous rare 
earth-transition metal (RE-TM) alloy used for the recording layer 
comprised approximately 69 atomic percent iron, 23 atomic percent terbium, 
and 8 atomic percent cobalt. The RE-TM layer was deposited by magnetically 
enhanced triode sputtering. The reflector layer comprised an aluminum-2% 
chromium alloy, and was deposited by nonreactive DC magnetron sputtering. 
Both transparent dielectric layers were deposited by DC nonreactive 
magnetron sputtering. All layers were sputtered at 1.times.10.sup.-3 torr 
gauge pressure of ultra pure (99.999 percent minimum purity) argon. 
The relative thicknesses of the magnetizable amorphous magneto optic film 
and the transparent dielectric intermediate layer in the trilayer 
construction, and the intermediate dielectric and barrier dielectric 
layers and magnetizable amorphous film of the quadrilayer construction, 
are selected to yield a magneto optic angle of rotation exceeding that of 
the medium without the added layers. 
The effect of the thickness of the intermediate layer on media 
characteristics was studied. Glass substrates were sputtered with 200 
angstrom thick barrier layers, 230 angstrom thick RE-TM layers, varying 
thickness intermediate layers (in the range of 250 to 450 angstroms), and 
1500 angstrom thick reflector layers. Both the barrier and intermediate 
layers were comprised of a Sic.sub.x (x&gt;1) transparent dielectric. 
Reflectivity was essentially linear with dielectric layer thickness, 
ranging from approximately 16 percent at 250 angstroms to 20 percent at 
450 angstroms. Carrier-to-noise ratio (1.4 micron bit size, 30kHz 
bandwidth) peaked at nearly 53 db at 300 angstroms. The rotation angle, 
.theta., decreased essentially linearly from approximately 0.65 degrees at 
250 angstroms thickness to approximately 0.20 degrees at 450 angstroms 
thickness. 
The effect of the thickness of the barrier layer on media characteristics 
was also studied. Glass substrates were sputtered with varying thickness 
barrier layers (in the range of 290 to 430 angstroms), 250 angstrom thick 
RE-TM layers, 360 angstrom thick intermediate layers, and 800 angstrom 
thick reflector layers. Both the barrier and intermediate layers were 
comprised of a SiC.sub.x (x&gt;1) transparent dielectric. Reflectivity 
decreased rapidly with increased barrier layer thickness, ranging from 
approximately 28 percent at 290 angstroms to 18 percent at 430 angstroms. 
Carrier-to-noise ratio (1.4 micron bit size, 30kHz bandwidth) generally 
increased with increasing thickness, peaking at approximate 53 db at 430 
angstroms. The rotation angle, .theta., increased essentially linearly 
from approximately 0.60 degrees at 290 angstroms thickness to 
approximately 0.85 degrees at 430 angstroms thickness. 
The carrier-to-noise ratio (CNR) of media made in accordance with this 
invention is relatively large, examples having been measured at 
approximately 50 db when measured with a laser diode at a wavelength of 
about 8300 angstroms. For representative embodiments of the invention, the 
threshold power for a laser diode in a write mode has been found to be 
approximately 4mW, which is acceptable because it is greater than the 
desired read laser power, typically 1-3 mW. Bit Error Rate (BER), a 
measure of the amount of digital data lost due to degradation of the RE-TM 
layer with time, for certain of the inventive media is on the order of 
10.sup.-5. 
EXAMPLE 1 
To compare the invention to media prepared with the silicon suboxide common 
in the art, two series of quadrilayer media were prepared. The substrates 
were identical and were made of injection molded polycarbonate. The 
silicon carbide dielectric layers were DC sputtered as described above, 
and the silicon suboxide layers were thermally evaporated using techniques 
known in the art. Eight media were constructed with silicon suboxide 
(SiO.sub.y, y&lt;2) dielectric layers, and fifteen media with the silicon 
carbide layers of this invention. The following table compares the average 
results of the two series. Data were taken on tracks near the inside 
diameter of the media. Carrier-to-noise ratio (CNR) (1.4 micron bit size, 
30kHz bandwidth) data were taken with an 8300 angstrom laser diode at 
three different write power levels, as shown. Bit error rate (BER) 
measurements were made after 800 hours exposure to 80 degrees C. and 90 
percent relative humidity. 
TABLE 1 
______________________________________ 
Write 
Dielec- 
Thres- 
tric hold CNR (db) Background 
Material 
(mW) 6mW 7mW 8mW Noise (db) 
BER 
______________________________________ 
SiO.sub.y 
3.8 50 51 51 -62 4 .times. 10.sup.-5 
SiC.sub.x 
3.2 48 50 50 -61 4 .times. 10.sup.-5 
______________________________________ 
EXAMPLE 2 
The resistance to thermal degradation over time of quadrilayer media with 
polycarbonate substrates and the carbon-rich silicon carbide dielectrics 
was also compared to that of media prepared with silicon suboxide layers. 
For example, the carrier-to-noise level of run 422, made with SiC.sub.x, 
was approximately 95 percent of initial value after exposure to an 
elevated temperature of 100 degrees C. for 1300 hours between measurements 
(CNR was measured at room temperature). A similar comparison for run 217, 
made with SiO.sub.y, showed a normalized CNR of approximately 95 percent 
of initial value after 220 hours exposure to 115 degrees C. 
EXAMPLE 3 
Another measure of media stability is the change in coercivity of the RE-TM 
layer with time. Three samples, identified as runs 587, 595, and 585, were 
constructed. They were trilayer construction comprising a glass slide 
substrate, a 200 angstrom SiC.sub.x dielectric barrier layer, a 500 
angstrom FeTbCo RE-TM layer, and a 200 angstrom SiC.sub.x dielectric 
intermediate layer. The dielectric layers of different runs were sputtered 
at different argon partial pressures to test the effect of this parameter 
on media stability. Each dielectric layer of a given run was sputtered at 
the same pressure. The media were exposed to 115 degrees C. to accelerate 
their aging. Measurements of the coercivity of the RE-TM layer were made 
at room temperature after various times, as shown below. 
TABLE 2 
______________________________________ 
Sputter Normalized Coercivity With 
Pressure Time (Hours) (1.0 at 0 hours) 
Run (torr) 16 44 134 164 
______________________________________ 
587 1 .times. 10.sup.-2 
.48 .39 .27 -- 
595 5 .times. 10.sup.-3 
.55 -- -- .45 
585 8 .times. 10.sup.-4 
.73 .70 .70 -- 
______________________________________ 
Other samples from runs 587 and 585 were aged at 80 degrees C. and 90 
percent relative humidity for 230 hours. After the exposure, the media 
were inspected with a magneto optical looper, a device for measuring 
rotation angle as a function of applied magnetic field. The run 585 medium 
exhibited the ability to magnetize a bit perpendicular to the plane of the 
RE-TM layer, and in the direction opposite that of the RE-TM layer 
adjacent to it. The run 587 medium did not exhibit this ability. 
FIGS. 2 and 3 are transmission electron microscope (TEM) views of samples 
from runs 587 and 585, respectively, enlarged by factor of 200,000. Layers 
20 and 30 are the glass substrates, layers 21 and 31 are the 200 angstrom 
SiC.sub.x dielectric barrier layers, and layers 22 and 32 are the RE-TM 
layers. The TEM sample preparation process removed the 200 angstrom 
dielectric intermediate layer of each sample, as well as much of the RE-TM 
layer 22 of the run 587 sample, leaving empty space at the lower portion 
of each figure. Inspection of the figures shows a more uniform and dense 
structure in the dielectric layer 31 of run 585 than is visible in the 
dielectric layer 21 of run 587. This experiment established that 
sputtering of the dielectric layer should be accomplished at as low an 
argon partial pressure as possible, preferably below 5.times.10.sup.-3 
torr. 
EXAMPLE 4 
To establish the effect of the carbon/silicon ratio, x, on the optical 
properties of the SiC.sub.x, a series of runs was made in which a 
SiC.sub.x layer was deposited at 8.times.10.sup.-4 torr onto glass slides. 
The carbon or silicon content of the films was adjusted by adding carbon 
or silicon, respectively, to the Hexoloy SG silicon carbide/graphite 
material described above. The x value was measured by Auger Electron 
Spectroscopy. Optical properties of the dielectric layers were measured 
using a substrate incident 8300 angstrom laser. The results are presented 
in Table 3. 
TABLE 3 
______________________________________ 
Run R T A t n k x 
______________________________________ 
521 .42 .50 .04 474 3.0 .09 1.21 
506 .38 .53 .05 530 2.8 .09 1.57 
524 .34 .58 .04 480 2.7 .08 1.67 
626 .34 .49 .17 488 2.8 .20 2.10 
625 .28 .51 .21 456 2.7 .30 2.39 
______________________________________ 
R is the reflection coefficient 
T is the transmission coefficient 
A is the absorption coefficient (Note that R+T+A.noteq.1.0 due to 
measurement and rounding errors) 
t is the thickness in angstroms 
n is the index of refraction 
k is the extinction coefficient (Note that n and k were calculated from R, 
T, A, and t using known relationships) 
x is the carbon/silicon ratio 
As stated earlier, it is desirable to maintain the absorption of the 
dielectric layer below about 20 percent to ensure that the layer is 
sufficiently transparent for use in a magneto optic medium. It is also 
desireable to maintain the index of refraction between about 2.0 and 3.0, 
particularly in the case of an intermediate layer. This experiment 
established a preferred carbon/silicon ratio below approximately 2.4, more 
preferably between about 1.2 and about 2.0. 
Although the media of this invention are erasable, they may be used in the 
same applications as write-once or non-erasable media. A two-sided medium 
is also possible by combining two one-sided media through means known in 
the art. One may also groove the magneto optic recording media to aid in 
locating recording tracks. 
While certain representative embodiments and details have been shown to 
illustrate this invention, it will be apparent to those skilled in this 
art that various changes and modifications may be made in this invention 
without departing from its true spirit or scope, which is indicated by the 
following claims.