CA1061902A - Orientation-derived hard bubble suppression - Google Patents

Abstract

ORIENTATION-DERIVED HARD BUBBLE SUPPRESSION
Abstract of the Disclosure Normal single wall magnetic or "bubble" domains are generated in a layer of bubble domain material without generating hard bubble domains by establishing a <110> easy axis of magnetization perpendicular to the plane of the layer.

Description

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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 - 2 - ~
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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,

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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.

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~~`~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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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A sheet of magnetic garnet, having the formula (YGd)3(FeGa)5O12, substantially parallel to a {110}
plane, and having an easy axis of magnetization along a <110> direction normal to the {110} plane, for generating normal single wall magnetic domains.

2. A stratified magnetic composite for generating normal bubble domains, comprising:
a monocrystalline, non-magnetic garnet 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)5O12 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.

3. A stratified magnetic composite as set forth in claim 2, wherein said magnetic layer is Y2.5Gd0.5Ga1.0Fe4.0O12.

4. A stratified magnetic composite as set forth in claim 2 or claim 3 wherein said non-magnetic garnet substrate has the general formula J3Q5O12, 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 and the Q constituent is at least one element selected from gallium, indium, scandium, titanium, vana-dium, chromium, silicon, germanium, manganese, rhodium, zirconium, hafnium, molybdenum, niobium, tantalum, tungsten and aluminum.

5. A stratified magnetic composite as set forth in claim 2 or claim 3 wherein said non-magnetic substrate is Gd3Ga5O12.