Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
lo Field of the Invention
The invention relates to the field of magnetic garnet ccmpositions
and more particularly to means of adjusting the maqnetic properties of
the garnet compositions.
2. Prior Art
In order to produce normal magnetic bubble domains in garnet compo-
sitions, it is necessary to reduce magnetization of a garnet such as
yttrium iron garnet (YIG) (Y3Fe5O12) by substitution of other ions for
either the yttrium or the iron. It is common practice in the prior art
to substitute non-magnetic ions for iron ions in order to reduce the
magnetization of the material. Iron ions occupy two different types of
lattice sites within a garnet crystal. These are the octahedral and
tetrachedral lattice sitesO Substitution of non-magnetic ions on the
tetrahedral iron lattice sites lowers the magnetization of the material~
while substitution of non-magnetic ions on the octahedral iron lattice
sites raises the magnetization. ~he Neel temperature (the temperature
at which the garnet material ceases to act as a magnetic material) is
reduced by substitution for iron on either type of lattice site and the
more substitution there is ~or iron the more the Neel temperature is re-
duced. A high Neel temperature is desirable for extended operating tem~
perature range of bubble domain memories.
Aluminum and gallium are the commonly used non-magnetic ions
which substitute on both octahedral and tetrahedral lattice sites with
the result that in order to produce a desired reduction in the magnetiza-
tion o~ the garnet composltion, a larger amount of substitution ~ox iron
must take place than would be required if the ions substituted only on
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the tetrahedral iron lattice sites. ThiS ~reater substitution for iron
results in a larger reduction in the Neel temperature than would other~
wlse result.
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The problem of large substitution and resulting large reduction
in the Neel temperature has been minimized through the use of substitution
ions which have a strong preference for the tetrahedral iron lattice
sites Germanium, silicon and vanadium ions appear to substitute almost
exclusively on the tetrahedral iron lattice sites with a consequent
minimum reduction in the Neel temperature of the garnet material for a
given reduction in magnetization of the material.
Unfortunately, the quantity of germanium, silicon or vanadium
substitution is dependent on the growth rate of the garnet, melt composi-
tion and other factors which are difficult to control. Consequently
germanium, silicon or vanadium substituted garnet compositions are diffi-
cult to grow with precisely repeatable magnetic properties. Many appli-
cations for garnet thin films such as magnetic bubble domain devices
require that there be a minimal variation in the magnetic properties
from film to film. Because of various influences on the amount of sub-
stitution of germanium for iron in a garnet composition such close toler-
ances have bee~ difficult of achievement.
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SUMMARY OF THE INVE~ION ~,
The inventlon overcomes the proble~s ~ film to film variations
of magnetic prvperties of germanlum~ silicon or vanadium substituted
garnet compositions by inclusion in the garnet composltion of 6mall
quantitles o~ ad~ustment-permittin~ ions whose dlstr~bution between
tetrahedral iro~ lat-tice sites and octahedral iron lattice sites can be
re~ersibly cha~ged by post-growth heat trestment of the garnet material.
~t is pre~erred to utillze ions which demonstrate a decided but not
exclusive prefere~ce for the tetrahedral iron lattice sites ~ince this
allow~ po~t ~ro~th adJust~ent with a minimum additional reduction in the
~eel temperature. The percentage of the ad~ustment-permitting ion which
~s located on tetrahedral lattice sites is a ~unction o~ the post-growth
thermal history of a garnet material. When a garnet film is annealed
at hlgh temperatures, the percentage o~ the ad~ustment-permitting ions
which are located on tetrahedral lattice ~ites decrea~es and produces
a corre~ponding increase in`the magneti2ation of the film. Thus, by -
the addition o~ small quantit1es of sd~ustment-permitting ions to the
garnet composition, the magnetlc properties of the compositlo~ may be
ad~usted after growth through the anneal~ng of the films.
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The invention can be defined as a method of providing
a magnetic garnet bubble domain material having desired
magnetic characteristics suitable for use in magnetic bubble
domain systems which require substantially identical magnetic
characteristics from production lot to production lot, said
method comprising: selecting a substituted magnetic yttrium
iron garnet composition for the bubble domain film which
includes ions of at least two elements as substituents for
iron, the ions of a first of said elements being a non-
magnetic ion selected from the group consisting of germanium,
; siIicon and vanadium which substitutes ~or iron substantially
exclusively on the tetrahedral iron lattice site of the
garnet in order to obtain a desired magnetization with a
minimum reduction in the Neel temperature of the magnetic
garnet, the ions of a second of said elements being a non- ~
magnetic ion selected from the group consisting of aluminum ~.
and gallium which substitutes for iron on both the octrahedral
and tetrahedral lattice sites and whose distribution between
the octrahedral and tetrahedral lattice sites can be controlled
by post growth annealing of the garnet film, the quantity of
said second ion being sufficient to allow post growth adjust-
ment of the magnetic properties of the film by annealing; :
including ions of additional elements each of which partially ;::
, substites for yttrium to tailor the overall characteristics
`~ of the magnetic garnet bubble domain film so as to yield an
effective bubble domain film; growing the film on a mono- ::
crystalline gadolinium gallium garnet substra~e; measuring
the post growth magnetic properties of the garnet film; and
annealing the film at a temperature and for a period of time
which modifies the magnetic characteristics of the film to
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the extent necessary to achieve the desired ~haracteristics.
The invention also consists of a magnetic garnet
bubble domain composite comprising: a monocr~stalline
gadolinium gallium garnet substrate; a monocrystalLine garnet
magnetic bubble domain fi].m epitaxially disposed on said
substrate, said film having desired magnetic characteristics
suitable for use in magnetic bubble domain systems which can
be modified after growth in order that the magnetic
characteristics of a first production lot which differ from
lC those of a second production lot may be modified to be sub-
stantially identical to those of the second lot; said magnetic ...
bubble domain film being a substituted magnetic yttrium iron
garnet which includes ions of at least two elements as :~
substituents for iron, the ions of a first of said elements
being a non-magnetic ion which substitues for iron substantially :.
exclusively on the tetrahedral iron lattice site of the garnet
in order to obtain a desired magnetization with a minumum
reduction in the Neel temperature of the magnetic garnet, the
; ions of a second of said elements being a non-magnetic ion
which substitutes for iron on both the octrahedral and
tetrahedral iron lattice sites of the garnet and whose dis-
tribution between the octrahedral and tetrahedral lattice sites
. can be controlled by post growth annealing of the garnet film,
the quantity of said second ion being sufficient to allow
post growth adiustment of the magneticproperties of the
film by annealing; said first ion selected from the group con~
sisting of germanium, silicon and vanadium; said s~cond ion -~
selected from the group consisting of gallium and aluminum, said
film composition including addltional ions each of which
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partially substitutes for yttrium to tailor the overall
characteristics of the magnetic garnet bubble domain film
so as to yield an effective bubble domain film; said
second ion being present in a formula amount of substantially
5~ of the formula amount of said first ion.
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DE~AIL3D Dæ CRlPTIO~ OF THE PREFERRED EMBODIMENT
An exemplary film in accordance with the invention comprises
Yl.95smo.locao.95Fe4.ooGeo.95Alo.o5ol2 A fllm of this nominal composi-
tion may be grown from a melt comprising 7.0434gm Y203; 1.3854gm S~ 03;
19.7112gm CaO; 20.3014gm GeO2; 185.218~gm F~ 03; 0.6000gm A1203;
78.9227gm ~23; 1513.54gm PbO. This compo~ition results in a decreased
magnetization which i6 suitable for the formation of magnetic bubble
domalns. Such a film grown at a temperature of' 911 degrees centigrade
on a gadolinium galllum garnet sub6titute was caFable o~ supporting
bubble domains having a diameter o~ 5.9 micrometer with a oollap~e field
o~ 62 Oe. Appropriate temp~rature annealing of the film can increase or
decrease the bubble diameter and correspondingly decrease or increase the
collapse ~ield. Table I tabulates the bubble diameter and collapse field
a~ a i~unction o~ the last annealing temperature. A period of approxima'e-
ly eight hours at the specified te~perature was utilized to anneal the
~ilm.
TABLE 1
A~n~ 6 5A rer~ e ~C
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Strlp width or bubble diameter 7.o 6-5 6.o 5 9 5.2
(~m)
20 Collapse ~ield (Oe) 5 52 61 62 71
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It will be understood, that other non-magnetlc ions having a sub-
stantially exclusive preference for tetrahedral lattice sites suck as
sillcon and vanadium may be utilized instead of the germanium and other
ad~ustment-permitting ions such as gallium may be utilized instead ~
25 the aluminu~ of the exemplary embodime~t. It is preferred that the ad~ust-
ment-permdtting lons have a substantial preference for the tetrah@~ral
lat~ice siie in order ~o minimize the ~eel temperature reduction which is
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induced by the adjustment producing ions. However, it will be under~
stood that adjustment permitting ions which do not have a strong prefer- -
ence for the tetrahedral site may be utilized with a consequent sacrifice
in the Neel temperature.
Another composition in accordance with the invention which is
suitable for use as a bubble domain material is Yl 88Euo 20CaO 92Fe4 03
GeO 92Alo~o5 12
If it is desired to increase rather than decrease the magneti7-
ation of the garnet composition through substit.ution, elements such as
scandium, indium and chromium which apparently occupy only octrahedral
lattice sites may be utilized to achieve the increase in magnetization.
Adjustability of the magnetic properties of the garnet is preferably
- obtained by inclusion of a small percentage of adjustment-permitting
(non-magnetic) ions such as tin and titanium which e~hibit a substantial
preference for the octahedral iron lattice sites but whose distribution
between octahedral iron lattice sites and tetrahedral iron lattice sites
can be reversibly changed by post~growth heat treatment of the garnet
material ~owever, adjustment~permitting ions which do not have a
substantial preference for octahedral iron lattice sites may be utilized,
althou~h their use will result in an additional reduction in the Neel
temperature of the garnet.
This technique is use~ul both with g æ net films and bulk garnet
material.
Where cations having a valence other than ~3 are substituted
into the garnet composition, a charge compensating ion should also be
substituted to maintain charge neutrality.
A technique which allows post-growth adjustment of the magnetic
properties of germanium substituted type garnet compositions has been
described, as have several advantageous compositions~ Those skilled in
the art may be able to vary the specifics of the preferred ~mbodiments
; without departing from the scope of the in~ention which is de~ined by
the appended claims.
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