This invention relates to an improved process for the preparation of high coercivity yttrium iron garnet thin films useful for magnetic and magneto optic recording.
Yttrium iron garnet (YIG) thin film with suitable doping of bismuth (Bi), cerium
(Ce) or germanium (Ge) have been prepared for possible use in magnetic recording.
The magnetic bit size recorded is important for this application - smaller the
possible bit size higher will the density of information that can be recorded on these
films during magnetic or magneto-optic recording. For getting smaller bit size, one
has to aim for thinner films with high coercivity (Hc) to withstand the demagnetizing
fields of the smaller bits. Gamma iron oxide and iron containing garnets are
commonly used materials for magnetic recording due to their favourable properties.
Terbium-iron-cobalt based metallic thin films are being used as magneto-optic
recording material commercially, but they suffer from the disadvantage that they
oxidize over the long run resulting in loss of information stored on them. Thin films
based on yttrium iron garnets (YIG) doped with bismuth and/or aluminium are under
development for use as magneto-optic recording materials due to their oxidation
resistance.
The YIG films are usually made by the Radio Frequency (RF) sputtering or Liquid Phase Epitaxy (LPE) technique using glass, quartz or Gadolinium Gallium Garnet (GGG) substrates. RF sputtering of oxide thin films involves deposition of
the garnet film using RF discharge plasma inside a vacuum system and a target of
composition same or nearly same as that of the required film. The process has
many disadvantages like slow deposition rate leading to low throughput, use of
vacuum techniques requiring longer turn around time and difficulty in controlling the
composition of the film under production environment. The LPE method requires
dipping a single crystalline GGG substrate inside a molten saturated solution
containing the garnet phase at high temperatures and reducing the temperature of
the molten solution very slowly. After the epitaxial growth of the garnet phase from
the solution, the GGG substrate is withdrawn from the melt. The LPE method is
slow, expensive and requires single crystals of GGG, which are available only in
small sizes. Therefore, LPE method is used only for small size recording media and
is not usually used for magnetic recording or magneto-optic recording applications.
The most extensive work reported in literature is from Prof. Abe's group at Tokyo Institute of Technology, Tokyo, Japan. M.Gomi, KKurishima and M.Abe in J. Magn. Soc. Jpn., v.11 (1987) pp.309 reported in-situ epitaxial growth of bismuth (Bi), aluminium (Al) and cobalt (Co) substituted YIG films on GGG substrates by RF sputtering. M.Gomi, KSatoh and M.Abe in Jpn. J. Appl. Phys., v.27 (1988) pp.L1536 report growth of cerium (Ce) substituted YIG thin films on GGG substrates with better magneto-optic properties than Bi substituted YIG films by RF sputtering. M.Abe and M.Gomi in Proc. Mater. Res. Soc. Symp., v.150 (1989) pp.121 describe growth of doped garnet films on glass substrates made by RF sputtering and their
applications. Enhanced magneto-optic properties in Ce substituted YIG films grown
on GGG substrates by RF sputtering is described by M.Gomi, H.Furuyama and
M.Abe in Jpn. J. Appl. Phys., v.29 (1990) pp.L99. All -these YIG films had
coercivities of 30-80 kA/m. The Metal Organic Chemical Vapour Deposition
(MOCVD) technique as described in J. Appl. Phys., v.79 (1996) pp.953-955 has
been used by the National Physical Laboratory, New Delhi, a constituent laboratory
of Council of Scientific and Industrial Research, New Delhi for the first to make YIG
thin films. These films made by MOCVD and uniformly doped with cerium and
cobalt showed a coercivity of 100-175 kA/m.
The main object of the present invention is to provide an improved process for preparation of high coercivity yttrium - iron garnet thin films useful for magnetic and magneto-optic recording, which obviates the drawbacks as mentioned above.
Yet another object of the present invention is to provide an improved process for preparation of thin films of yttrium-iron garnet (YIG) or doped YIG with high coercivity (>200 kA/m) using the metallo-organic chemical vapour deposition (MOCVD) technique.
Yet another object of the invention is to provide an improved process for preparation of YIG thin films which are useful for magnetic recording media or magneto optic recording media.
Still another object of the invention is to provide a process, which is faster and easy to scale up for production of these media.
In the process of the present invention, the chemical vapour deposition is carried out in a vessel made out of quartz or stainless steel which can be evacuated and externally or internally heated. The chamber is connected to a number of sub-chambers each containing the heated source metallo-organic compound (dipivaloyalmethanates of Y, Fe, Ce and Co or similar compounds with high vapour pressure) of one element. The vapours of metallo-organic compounds along with a carrier gas is flown through the deposition chamber in a sequential manner after
passing the gases through a mixing chamber. A substrate of glass, quartz or
gadolinium gallium garnet (GGG) is kept at an angle varying between 10-60° to the
direction of the flow of the gas inside the deposition chamber.
Accordingly, the present invention provides an improved process for the
preparation of high coercivity yttrium iron garnet thin films useful for magnetic and
magneto-optic recording, which comprises :
a) depositing yttrium iron garnet (YIG) films of desired thickness at a
temperature in the range of 500-800°C on a suitable substrate kept at
an angle of 10-60° using known metallo-organic chemical vapour
deposition technique,
b) heating the YIG film so obtained in step (a) to a temperature in the
range of 850-950°C for a period in the range of 10-30 minutes to
stabilize the garnet phase of the YIG film,
c) depositing a cobalt oxide (CoO) overlayer of desired thickness on the stabilised YIG film so obtained in step (b) using known metal organic chemical vapour deposition (MOCVD) technique, followed by annealing, if desired, of the said YIG film at a temperature in the range of 400-800°C for a period in the range of 2-8 hours. In an embodiment of the present invention, the YIG film deposited may be pure or doped with cobalt (Co), chromium (Cr), bismuth (Bi) or aluminium (Al) in the concentration range of 0-10 atomic percent.
In another embodiment of the present invention, the substrate used for the present invention for deposition of YIG film may be such as glass, quartz .gadollinium gallium garnet (GGG).
The critical parameters for achieving high coercivity are thickness of the
overlayer of oxide of magnetic materials such as cobalt oxide (CoO) overlayer, rate
of deposition of this overlayer and the temperature and time of the deposition and
heat treatment after the deposition of the overlayer using the known MOCVD
process. The thickness of the CoO overlayer is controlled by controlling the gas flow
rate inside the MOCVD chamber and the temperature of the heated cobalt (Co)
Control of the CoO overlayer thickness and subsequent heat treatment lead to
controlled distribution of cobalt at the interface and also inside the YIG layer leading
to high coercivity as high as 285 kA/m in the YIG films . The films grown by RF
sputtering as reported by other workers show coercivity in the range of 30-80 kA/m. source.
The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the present invention.
Example-1
Effect of CoO Overtayer Thickness
YIG films of 400nm thickness were deposited over a heated quartz substrate
kept at temperature of 590°C using a MOCVD reactor described above at a chamber
pressure of 50 mTorr under an ambient containing argon and oxygen in equal
proportion by volume and then annealed at 900°C in an oxygen atmosphere for 300
minutes. Thereafter, CoO oxide layer was grown over the YIG film using the same
MOCVD set up described above operating at a substrate temperature of 590°C
under pressure of 50 mTorr with an ambient containing argon and oxygen in equal
proportion by volume. Deposition thickness of CoO overlayer over stabilised YIG
film was achieved by changing the precursor amount and the deposition time during
the CVD growth. CoO overlayer thickness has a profound effect over magnetic
parameters of bilayer film. Deposition of a CoO layer thickness of 15-20nm had no
effect on the coercivity of the stabilised YIG film which remains around 15-20 kA/m.
However, deposition of about 200-280nm thick CoO layer increased this value to
190-200 kA/m. A further increase of CoO thickness to 400-450nm caused
enhancement of coercivity to 270-285 kA/m for the bilayer media. The ratio of Hc in
directions perpendicular to and parallel to the film plane also increased with thickness from 1.003 to 1.12.
Example-2
Effect of CoO Deposition Rate
YIG films of 400nm thickness were deposited over a heated quartz
substrate kept at temperature of 590°C using a MOCVD at a chamber pressure of
50 mTorr under an ambient containing argon and oxygen in equal proportion by
volume and then annealed at 900°C in an oxygen atmosphere for 300 minutes.
Thereafter CoO layers were deposited on these YIG films under pressure of 50
mTorr with an ambient containing argon and oxygen using the same MOCVD set up
described above operating at a substrate temperature of 590°C in equal proportion
by volume. Deposition thickness of CoO overlayer over stabilised YIG film was
achieved by changing the precursor amount and the deposition time during the CVD
growth. Deposition rate of CoO overlayer is critical to realising the magnetic media
with high coercivity. Characteristically low deposition rates of 5-10nm/min. were
employed to achieve enhanced coercivities. If the deposition rate was increased to
25-30nm/min. the expected increase in coercivity could not be realised. Typically for
a 450nm thick CoO overlayer, Hc increased from 15-20 kA/m to 190-285 kA/m if the
CoO was deposited at the rate of 10nm/min., whereas the increase in Hc was only to
35 kA/m if the CoO was deposited at the rate of 30nm/min. The CoO deposition rate
was varied by varying the precursor vaporisation temperature and gas flow rates.
These parameters were 150°C and 10 seem respectively in the preferred mode of CoO deposition.
Example-3
Effect of Post Deposition Annealing
YIG films of 400nm thickness were deposited over a heated quartz substrate
kept at temperature of 590°C using a MOCVD reactor described above at a chamber
pressure of 50 mTorr under an ambient containing argon and oxygen in equal
proportion by volume and then annealed at 900°C in an oxygen atmosphere for 300
minutes. Thereafter. 200 nm thick CoO layers were deposited on these YIG films
under pressure of 50 mTorr with an ambient containing argon and oxygen using the
same MOCVD set up described above operating at a substrate temperature of
590°C in equal proportion by volume. Deposition thickness of CoO overlayer over
stabilised YIG film was achieved by changing the precursor amount and the
deposition time during the CVD growth. Coercivity enhancements could not be
observed and coercivity was only 8 kA/m for the bilayer CoO/YIG film; In such a
case, improvement in the magnetic properties of CoO/YIG bilayer film media could
be achieved by annealing them at temperatures of 500 to 900 °C in oxygen ambient
for period of 6 hours. Coercivity could be enhanced to 20 kA/m by a 700°C anneal
for 5 hours or to 35 kA/m by a 900°C anneal for 6 hours. These enhanced
coercivities were generally lower than those obtained where proper CoO thickness
was used.
Subjecting the as made CoO/YIG bilayer film media with CoO layer thickness of 460nm displaying optimum magnetic coercivity to further high temperature anneal in a post CVD deposition state was undesirable for film as this invariably resulted in deterioration of the magnetic parameters. Typically, when optimised CoO film was obtained displaying high coercivity, annealing this film between 500-900° C caused reduction in coercivity from 285 kA/m to 200 kA/m for a 500 ° C anneal and to 100 kA/m for a 900 ° C anneal.
Typical values of magnetic properties - saturation magnetization (Ms) and coercivity (Hc) for YIG films prepared by various methods are given below:
Type of film Ms Hc
(kA/m) (kA/m)
RF Sputtered YIG 140 30-80
MOCVD YIG doped with Cr 120 170-220
MOCVD YIG with CoO overcoat
As deposited pure YIG 5-10 2-10
After YIG stabilisation at 900° C 20-40 15-20
After CoO overlayer deposition 80-120 190-285
After heattreatment of film with overlayer. 30-60 80-100
The main advantages of the improved process of the present invention are:
1. Provides a YIG thin film with very high coercivity (as high as 285 kA/m),
which is useful in reducing the recording bit size if one uses these films as
magnetic or magneto-optic recording media. Such high coercivities in these
films have not been obtained by anyone else.
2. It is a much simpler and higher throughput process for production as
compared to the conventional RF sputtering or LPE process normally used to
fabricate these thin films of pure YIG or doped YIG.

We claim:
1 . An improved process for the preparation of high coercivity yttrium iron garnet
thin films useful for magnetic and magneto-optic recording, which
comprises of:
a) depositing yttrium iron garnet (YIG) films of desired thickness at
temperature in the range of 500-800°C on a suitable substrate kept
at an angle of 10-60° using known metallo-organic chemical
vapour deposition technique,
b) heating the YIG film so obtained in step (a) to a temperature in the
range of 850-950°C for a period in the range of 10-30 minutes to
stabilize the garnet phase of the YIG film,
c) depositing a cobalt oxide (CoO) overlayer of desired thickness on the
stabilised YIG film so obtained in step (b) using known metal organic
chemical deposition(MOCVD) technique, followed by annealing, if
desired, of the said YIG film at a temperature in the range of 400-
800 ° C for a period in the range of 2-8 hours .
2. An improved process as claimed in claim 1, wherein the YIG film deposited is pure or doped with cobalt (Co), chromium (Cr), bismuth (Bi) or aluminium (Al) in the concentration range of 0-10 atomic percent.
3.. An improved process as claimed in claims 1 and 2, wherein the substrate used is such as glass, quartz gadollinium gallium garnet (GGG).
4. An improved process for the preparation of high coercivity yttrium iron garnet
thin filmsfor magnetic and magneto-optic recording substantially as herein
described with reference to the examples.