Note: Descriptions are shown in the official language in which they were submitted.
32
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CROS_~REFE~NCE TO ~EL~TED ~PPLIC~T~ON
Reference is made to the copending Canadian patent
application of Rodney D~ Henry, Paul J. Besser, and Robert G.
Warren entitled CH~RACTERISTIC TEMPERATURE-I)ERIVED H~RD BUBBLE
SUPPRESSION, bearing Serial Number 222,154~ filed on March 14,
1975, and assigned to the common assignee~ This related
application corresponds to U.S. Patent 3,946,372 issued on
March 22, 1977.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to materials in which single
wall magnetic domains can be generated and, more particularly,
to materials suitable for the selective generation of normal,
without hard, single wall magnetic domains.
2. Brief Description of the Prior Art
It is well known in the art to use magnetic materials
such as garnets and orthoferrites with intrinsic and/or
induced (by shape, stress or growth~ uniaxial anisotropy to
generate single wall magnetic or "bubble" domains. Typically,
the bubble domains are generated by applying a suitable bias
field perpendicular to a layer of magnetic bubble domain
material. The normal bubble domains that are induced in such
a material exist over a narrow range of bias field values,
typically about 15 Oersteds in garnet materials, and propagate
in the direction of an applied bias field gradient. However,
in certain materials, bubble domains may be formed that exist
over a much larger range of bias field values, e.g.~ as much
as approximately 40 Oersteds in garnets. In addition, these
unusual bubble domains, termed hard bubbles, have low
mobilities and propagate at an angle to the applied bias field
gradient. Because of such properties, the presence of hard
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.~ . .
61~
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bubbles may render the bubble domain material unsuitable
for use in circuits and devices.
As discussed in U.S. Patent No. 4,001,793, inventors
Rodney D. Henry and Paul J. Besser, entitled MAGNETIC
BUBBLE DOMAIN COMPOSI~E WITH HARD BUBBLE SUPPRESSION, and
assigned to the common assignee, several techniques are
available for suppressing the formatlon of hard bubble
domains. These techniques utilize an extra magnetic
layer, or an implanted region, along the bubble domain
layer to form an extra domain wall between or along the
ends of the domains in the bubble domain layer.
Suppression techniques may be highly ef~ective. For
example, the aforementioned United States Patent No.
4,001,793 teaches a highly effective technique in which an
extra magnetic layer is magnetized along a direction per-
pendicular to the magnetization directions in the bubble
domain layer to form an extra wall, termed a "90~ cap",
along the domains. However, the prior art techniques
; require additional structures and/or processing steps. As
may be appreciated, it is desirable to have a hard bubble
suppression technique that eliminates the cost in time and
money of such additional structures and steps.
SUMMARY OF THE INVENTION
The present invention comprises a sheet of magnetic
garnet, having the formula (YGd)3(FeGa)5O12~ substantially
parallel to a ~1103 plane, and having an easy axis of
magnetization along a <110> direction normal to the {110¦
plane, for generating normal single wall magnetic domains.
Another aspect of the invention comprises a stratified
magnetic composite for generating normal bubble domains,
3 ~
C
`-\ 106~ 0~
comprising:
a monocrystalline, non-magnetic ~arnet substrate
having a deposition surface substantially parallel to a
{110~ plane; and a layer of monocrystalline, magnetic
garnet bubble domain material having the formula
(YGd)3(FeGa)512 formed on said deposition surface
substantially parallel to the ~110} plane such that an
easy axis of magnetization is along a <110> direction
normal to the {110} plane.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a partial, cross-sectional view of a
bubble domain composite embodying the principles of the
present invention.
Figure 2 is another, enlarged, partial cross-sectional
view of the bubble domain layer of the composite of Figure
1 schematically illustrating the domain wall thereof.
Figure 3 is a cross-sectional representation of the
bubble domain
3a -
` ~619~
~-~and wall of Figure 2, taken along the llnes 3~3, and showing
the orientation of the atomlc magnetic moments associa~ed with
.
the domain wall.
DETAILED DESCRIPTION
Referring now to Figure 1, there is shown a partial,
cross-sectional representation of a bubble domain composite,
designated generally by the reference numeral 10, constructed in
accordance with the prlnciples of~the present invention. The
bubble domain composite 10 comprises a substrate 11 which :Ls,
and has a deposition surface 15 which is, substantially parallel
to a {110} plane. ~ layer 12 of bubble domain material is formed
on the substrate deposition surface 15 substantially parallel to
the {110} plane such that an easy axis of magnetization is along
a <l10~ direction normal to the {110} plane. Bubble domains
13 (only one is shown), i.e., cylindrical-shaped regions which
- are enclosed by individual domaln walls and are magnetized
anti-parallel to the magnetization of the layer 12, are generated
within the layer upon the applica~ion of a suitable bias field,
Hb, perpendicular to the plane thereof.
The substrate 11 typically comprises a monocrystalline
oxide material, e.g., a metal oxide such as a non-magnetic
garnet. As used here, the term "non-magnetic garnet" refers
to garnet materials containing no iron or insufficient iron
to supply the magnetic characteristics necessary for the
formation of bubble domains. The non-magnetic garnets are
considered to be oxides designated by the general formu~a
J3Q512' where J is at least one element selected from the
lanthanide series of the Periodic Table, lanthanum, yttrium,
magnesium, calcium, strontium, barium, lead, cadmium, lithium,
sodium, and potassium. The Q constituent is at lease one
element selected from gallium, indium, scandium, titanium,
061~1~Z
,~. `,
vanadium, chromi~m, silicon, germanium, manganese,
rhodium, zirconium, hafnium, molybdenum, niobium,
tantalum, tungsten and aluminum.
The bubble domain layer 12 typically comprises a
monocrystalline layer of magnetic garnet having the
formula (YGd)3(FeGa)512
As is well known in the art, the magnetic garnets have
growth- or stress-induced noncubic anisotropy. This
property is utilized to form bubble domains by providing
- 10 an induced easy axis of magnetization approximately normal
to the plane of a magnetic garnet layer. In presently-
used garnet bubble materials, this induced easy axis is
made to coincide with one of the crystallographic
(intrinsic) easy axes, and, more specifically, with one of
the <111> axes. In this case, the induced anisotropy has
uniaxial symmetry.
The present invention utilizes the orthorhombic
symmetry of the induced magnetic anisotropy associated
with ¦110~ planes. This orthorhombic symmetry is
discussed in 'iMagnetic Oxide Films" by J. E. Mee, G. R.
Pulliam, J. L. Archer and P. J. Besser, IEEE Trans.
Magnetics, Vol. MAG-5, p. 717 (1969). In accordance with
the present invention, hard bubbles are suppressed by
providing an easy magnetization direction along a <110>
direction, speci~ically along a <110> direction which is
perpendicular to the ~1103 plane of the magnetic garnet
layer 12.
L
~~`~ln this case, axes or two different degrees of "hardness" are
established in the plane of the layer 12. These latter two
axes will hereinaf~er be termed "medium" and "hard" axes.
The consequence of the variation of anisotropy with
direction in the ~ilm plane, as it relates to hard bubble
suppression, is shown in Figures 2 and 3. Figure 2 is an enlarged,
partial, cross-sectional view of the bubble domain layer 12
in the vicinity of the single bub~le 13, showing a cylindrical
domain wall 14. Figure 3 is a cross-sectional representation
of the bubble 13 and the domain wall 14 of Figure 2, schematically
illustrating the orientation (by arrows) of the individual
atomic magnetic moments (spins) in the domain wall 14 at a
point in the center of the wall and midway between the top and
bottom surfaces of the bubble layer 12. Because of the
anisotropy in the film plane resulting from the orthorhombic
symmetry, the spins in the wall prefer to lie along the medium
axis as shown. Thus, the extra degrees of freedom available to
spins in the case of uniaxial anisotropy (all directions in the
plane equally hard) are eliminated in this case.
The configuration shown in Figure 3 is precisely
that found in orthoferrite materials, where no hard bubbles
are observed, and is hypothesized to be that induced by other
hard bubble suppression techniques. Calculations have confirmed
that alignment with the medium axis is preferred for the {110}
layer 12. That is, for sufficient difference in the magnetic
"hardness" of the medium and hard axes, the magnetic mvments
(spins) in the domain wall 14 tend to align parallel to the
medium axis. See "Stress Related ~all Energy Variations in
Garnet Films", by G.R. Pulliam and F.A. Pizzarello, Magnetism
and Magnetic Materials - 1972, AIP Conf. Proc. No. 10, American
Institute of Physics, New York, p. 413 (1973).
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-~ It should be pointed out that, although garnets are
used as an example, this invention is not restricted to
such material.
Referring again to Figure 19 in geoeral, the
bubble domain layer 12 may be epitaxially grown on the substrate
11 using growth te~hniques such as liquid phase epitaxy (LPE)
and chemical vapor deposition (CVD). CVD is particularly
suited to growing garnet layers w~th ~110> easy axes perpendicular
to the plane of the film. Using CVD, the materials used for the
substrate 11 and r~ubble domain layer 12 are selected such that
their lattice constant mismatch provides stress-induced anisotropy
with the requisite ~lla~ easy axis perpendicular to the plane
of the layer. The use of CVD and lattice constant mismatch to
produce bubble domains is taught in U.S. patents 3,728,152,
3,745,046 and 3,788,396, issued April 17, 1973, July lO, 1973
and January 29, 1974~ respectively, all of which are assigned
to the common assignee. The teachings of these paten~s, albiet
relating to the uniaxial anisotropy associated with ~ easy
axes, are applicable to the present invention~
The effectiveness of the orthorhombic symmetry of
{110} deposits in suppressing hard bubbles was demonstrated by
growing {110} and {111} bubble domain layers 12 of composition
(YGd)3 ~FeGa)5 12 on Gd3Ga5012 substrates 11. ~For the
garnets and other cubic materials, planes parallel to
crystallographic planes such as {110} and {111} planes have
like numbered axes, i.e., cllo> and <11l~ , perpendicular there-
to). Specifically, CVD was used to grow {111} and {110} layers
2.5 0.5 1.OFe4.00l2 on respective {111} and {110}
Gd3Ga5O12 substrates 11.
The sample {111} and {110} composites 10 were
tested for the presence or absence of hard bubble domains by
~determining the range of values of the bias fleld, ~Hb
~oersteds), which w~s necessary for bubble collapse. A
collapse field range of 2 oe. or less indicate~ the existence
of normal bubhles without the presence of hard bubbles. At
near room temperature, 20C, the bias field range was <2 oe.
for the {110} composite and considerably above 2 oe, about
15 oe., for the {111} composite. Accordingly, it was concluded
that normal bubble domains, but n~t hard bubb:Le domains,
had been generated in the {110} composite and that both normal
and hard bubbles had been generated in the {lll} composite.
It should be noted that the {110} and the {111}
composites generated both hard and normal bubbles at temperatures
below about 20C and 60C, respectively. Thus, the
orientation-derived hard bubble suppression of the Y2 5Gdo 5
Gal oFe4 012 composition is temperature-dependent. See the
aforementioned copending application serial number entitled
CHARACTERISTIC-TEMPER~TURE-DERIVED HARD BUBBLE SUPPRESSION, to
Henry, Besser and Warren.
This temperature dependence of the hard bubble
suppresslon is to be expected for certain compositions. The
values of the anisotropy energy along the medium and hard
axes depend on material parameters which are temperature
sensitive. Thus, in certain compositions the difference
between the two directipns may be reduced at some temperature
so that the condition of Figure 3 is no longer maintained.
The proper choice of material parameters will allow this
temperature to be placed outside the operating range.
Consideration of the results derived in the
aforementioned teachings of "Magnetic Oxide Films" by
Mee et al and "Stress Related Wall Energy Variations in Garnet
Films'l by Pulliam et al shows that the conditions producing
~6~ Z
a strong stre~s-induced orthorhombic anisotropy whLch is suitable
for bubble domain formatlon in a ~110> layer, such as layer
12, (Figure 1) are:
(1) A 111 C O
l o o 1 1 1 , 2 K 1 J
(3~ l2A~ > ¦~100~ A111l
where Aloo and Alll are the magnetostriction coefficients,
Kl is the anisotropy constant, and a is the s~ress. Conditions
~1) and ~2) are necessary to make the <110> direction normal
to the plane of the layer 12 they easy axis, so that bubble
domains can be formed. The in-plane anisotropy responsible
Eor the medium and hard axes (Figure 3) is a result of
condition (3). If these conditions are well satisfied over
the temperature range of interest, the formation of hard bubbles
should be suppressed.
An example of a garnet material which would be
expected to satisfy the above conditions over a wide temperature
range is gallium or germanium-substituted, yttrium ytterbium
lron garnet of the approximate formula Yl 5Ybl 5(Ga or Ge)l 0
F~4-012- Here Alll < . Aloo ~ 0, and Kl is ~ 0 and of small
magnitude. Practically speaking, the hard bubble suppression
of such a material can be expected to be independent of
temperature.
Thus, there has been described a magnetic bubble
domain layer which utilizes crystallographic orientation to
suppress the formation of hard bubble domains. ~xemplary
bubble domain materials and a composite have been described.
However, the scope of the invention is limited only by the
claims appended hereto.
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