CA1180805A - Device for propagating magnetic domains - Google Patents

Abstract

ABSTRACT
"Device for propagating magnetic domains".

A device for propagating magnetic domains (6), comprising a monocrystalline non-magnetic substrate (1) of a material having a garnet structure and a layer (2) of an iron garnet carrying magnetic domains and grown epitaxial-ly on the non-magnetic substrate (1). In the dodecahedral lattice sites the iron garnet comprises at least a bismuth ion and a rare earth ion selected from the group consisting of lutetium, thulium, ytterbium, whereby it combines a very high uniaxial anisotropy with a high domain mobility, which properties make the device extremely suitable for the propagation of magnetic domains having diameters from approximately 1 to approximately 2 /um under the influence of comparatively low driving fields.

Description

PHN 9795 1 25--5~1981 ~Devic e for prop~gating magnetic dornains".

The invention rela-tes -to a device for propagating magnetic domains, including a monocrys-talline non-magnetic substrate bearing a layer of an iron garnet capable of supporting local enclosed magnetic domains, said layer having a uniaxial magnetic anisotropy induc~d substantial-:ly by growth and having been caused to grow epi-taxi.ally on the non-magnetic substrate, said iron-garnet bei.ng of -the class o~ iron garnet materials comprising ir,~ the dodecahedral sites o~ the garnet lattice at least a large and a small occupant.
I~ magnetic "bubble" domain devices it holds that the smaller the bubble diameter, tne larger th.e in-formation storage density which can be achieved. Iron garnet 'bubble domain materials are pre~erred in bubble do-main technology because small diameter bubble domains are stable in these materials. For a bubble domain material which must be use~ul for -the manu:facture of bubble domain devices, it is important that the bubbles ~ormed ,in the material should have a h.igh wall mobility so that compara~
~ tibely small driving :~ields can ca.use rapid bubble movements.
T~lis property permits the use o~ high ~requencies t~ith low energy dissipQtion, It :Ls a:Lso lmport;ant that -t;he ma,gnetic bubble domain mat~:rials shou:l.d have a h.ig:h unia,xial an:isotropy.
This proves to be necessary to avoid spontaneous nucleation o:f bu'bbles. This is o~ great importance ~or reliab.le in-f'ormation storage and processing-within the bubble domain material.
The overall uniaxial anisotropy (Ku) may have con-tribu-tions o~ stress or strain induced (Ku) and of ,,rowth-induced (Ku) terms. This rneans tha-t KU ~ KU -~ Kg ( 1 1 3~

PHN 9795 -2~ 25-~_1981 In the usual bubble domain rnaterials, Ku is rn~Linly determined by -the grow-th-induced term. In choosing ions -to occupy dodecahedral sltes in the lattice of' a bu'h'ble garnet material in order to increase the gro~th-induced anisotropy, the choice in the past was restricted to magnetic rare ear-th ions, because -the accepted theory f`or growth~anisotropy demanded the use of` magnetic ions. However~ the magne-tic rare earth ions used provide a contrib-ution to the damping, so that this choice does no-t lead to an op-timum domain mobility. It is even so that the srnaller the -bu'bble domain becomes, -the more damping ions ha~e to be incorporat-ed to reali~e the required high uniaxial anisotropy.
Netherlands Patent ~pplication 7514832 discloses a bubble domain device in which the bubble domain material comprises lanthanum and lutetium in dodecahedral sites so as to ensure the high bubble domain wall mobility which is desira'ble for operation at high f`requencies of` bu'bble domairl devices. A f`ilm of this known material proves to have a growth-induce~ uniaxial anisotropy (Ku) of 6800 erg/cm3, which is suff`icient to enable s-table device behaviour with a bubble domain cross-section down to a minimum of 4 /um.
The high growth-induced uniaxial anisotropy (I~u) of films of this known material is ascri'bed to -the com-bination of lanthanum (the largest of the rare earth ions) with lutetium (the srnallest o~ the rare earth ions), while the high bu'bble domaln wall mo'bili-ty :is a result of the ~act that'both lanthanum and :Lutetium do not contribute to the ~amping or only contribute to a sma:L:L exten-t. ~Iow-e~er, a dLsadvanta,~e o~ th:is mater:ia'l :is that lanthanum can he incorporated in the garnet lattice only to a res-tricted extent, as a result of which the ef`fect of` the combination of a large rare earth ion and a small rare earth ion in the dodecahedral latt:ice sites caNno-t be -used optically.
It has surprisingly been found that the occupation of dodecahedral sites by a non-magnetic ion not belonging to the class of -the rare ear-th ions, namely bismuth, iII
combinatiorl w:ith small rare earth ions, leads to a material PHN 9795 3~ 2~ 5-'1981 having a comparab:Le mobility to the known rnaterial but with an approximately l0 x higher uniaxial anisotropy, so -that it is sui-ta'ble ~or use in bu'bble domain de~ices having bub'hle domains with a dlameter as small as 0.~ /um.
As small rare earth ions in combinatlon with bismuth may be utilized lutetium, ytterbium an~ thulium.
Layers o~ iron garnet with a com'bination of bismu-th ions and small rare earth ions in dodecahec]ral sites can ~e epitaxially grown on various substrates, in lD whieh matehing o~ the lattice constant takes place by a suitable choiee o~ the ratio large ion/small oceupant in the docLecahedral sites. Grow-th has generally been on Gd3Ga~01~ (lattice constant aO = 12.38 A.) ,~u-t other materials which may be utiled are e.g. Eu3Ga5012 (aO - 12.40 ~) Sm3Ga5012 (aO = 12.43 ~) and Nd3Ga5012 (aO = 12.50 ~) or mixed crystals thereof. A face parallel to -the crystallographic (111) ~ace may serve as a deposition face.
In those eases in whieh the darnplng o~ the above-deseribed iron garnet nrlaterial with Bi ions occupyinga part of -the dodecahedral sites is smaller than is in ~act necessary ~'or the applieation in ~iew, on0 has the liberty o~ subs-tituting, if` desired, damping ions in a part o~ t'he dodeca'hedral s:ites. If, ~or e;~arnple, Sm or Eu is used f'or this purpose, the un:Laxial anisotropy constant may bc ~urther lncreased (by appro~:imately -15%) ~
A pre~erred mater:ial for minirrlizing the growth-indueed an:Lsotropy is Bi, Y, M ~3 GayFe5_y whereill is Lu ancL/or Trn and/or Yb. With a fi~ed Ga content in the layer, the anisotropy constant of layers in which Lu ~ Tm or Yb) is gradually replaeed entirely by Lu -turned out to reach a maximum at a Lu : Y weight ratio in ~he me:it o~ approximately 'I : 1, which corresponds wi-th a Lu : Y ratio in the iron garnet layer o~ approximately 'I : 2 e:Lements other -than gallium can be substitutecL ~or iron to reduee the magr1etization o~ the resulting garnet la~er,and a gerLera:L ~orlmlla ~or ttliS material is ~i, Y, M~3 QyFe5~y0l2, wh~rein Q is a non-rnagnetic ion g3~

P~IN 9795 ~~~ 2~ 19~1 which preferably occupie.s tetri~hedral lattice sites, 0 ~ y ~ 5, and ~5-~) is suf~iciently large in order tha-t the material '~e magne-tic at -the operating -tempera-ture~
When a subs~iitution is realized in the iron sites ~ith an ion having a charg~ of more than ~3, charge compensation may require that a charge-compensa-ting ion be incorporated in the dodecahedra~si-tes~ so tha-t a material is provided o:~` the composition {Bi, Y, M~3_z Jz Qy F~5_yO12, J is a charge-compensating ion having a charge o~ ~1 or ~2 and which prefera'bly occupies dodecahedral si-tes, Q is a non-magrletic ion ha~ing a charge of more than ~3, 0 ~ z ~ 3, and 0 ~ y ~ 5~ In this case also, -the material mus-t be magnetic at the operating temperatureo~ -the device.
For growth on a rare earth-gallium garnet sub-.strate, the inventi.on makes i-t possible to choose a nominal composition of` the bubble domain layer which provides a minimum deviation ( ~C1.6 x 10 3 nm~ betwe0n the lat-tice constant o~ the bubble domain layer and the lattice con-stant of the substrate, as a resul-t o~ which -the stress or strain in the Pilm is maintained at a su:~ficiently small value to restrict the possi'bilit~ of cracking and tearing of the layer. As appears .from the formula which indica-tes the nominal composition of the present bub'ble ,aomain materials~ there is started from the assumption that bismu'th~ yttrium, lutetium, thulium and ~tterbium e~cllls:i~ely substltute in clodecahecL:ral lattice sites. I-t has 'been ~ound, howe~er, t'hat in the present materials a small part of the smal.L rare earth ions subst:ihl-tes in octa'hcdraL 9i~es :i:n t:he :Latt:ice, ~hic:h g:ives :rise to an imp:ro~ecl temperature dependence of both the satura-tion magnetisation and the collapse field.
The invention will be described in greater detai.l~ by way o~ example, with re~erence ~o the ~o].lowing examp:Les and the drawing.
Figure 1 is a graphic representation of the way i.n which the ad.apta~on o:f the lattice constant o.~ a bismuth-containing bu'bble clomain layer to -the la-ttice constant o:f a GGG-su'bstxa-te (denoted by ~ a) depends on PHN 9795 -5- 2,~5~'19$1 the weight ratio Y203/Lu203 in the mel-t and on the growth temperature Tgo Figure 2 shows dlagrammatically a 'bu'bble domain device.
Films o~ the nominal composition (Biz~xLu3 x ~) (Fe5 yGay) 12 were made to grow ~rom a melt by liquid phase epitax~ techniques while using a PbO/Bi203 flux.
In this case x was varied ~rom 0 to '1.2 and ~ was varied between 0.-1 and 0.7 on the one hand by varying the ratio Y203/Lu203 in -the melt and on the other hand by growing layers at dif~`erent grow-th -ternperatures with a given ratio Y203/l;u203 in the mel-t. (The lower -tile temperature of the meltl the more Bi is incorporated in the layer.) ~t is always possible -to find such combinations of Y203/Lu203 in the melt and growth temperature T that the grown layers have a lattice constant which di~fers by con-siderably less than 1.6 x 10 3 ~Lm ~rom the lattice constant of -the substra-te on which -the layer is grown. A difference in lattice constant of 1.6 x 10 3 nm has been assumed as -the limit within which layers o,f good quality can be grown withou-t cracks or -tearsO All this is explained in t'he case o:f the use o~ Gd3Ga5012 substrates with refererlee to Figure 1~ in which the area between the solid lines in-dicates the conditions in which good layers were de~posited on the relevant substra-tes without cracks or tears.
The -top line :Lnd:icates ln what c:ircumstances layers were ~ormod w:ith a m:is~:it ~ a o~' approx:i1nately ~'1.6 x '10 3 nm (these layers were in i;ension),and the bottom line indicat es in what c:ircu111stances :Layers were formed with a misfit ~ a o~ approximate:Ly -'1.6x'l0 3 nm (-these layers were in compression).
The layers were rnade to epitaxially grow on sub-strates immersed horizontally in -the melt at temperatures 'between 680 annd 970C ~or periods varying from 0.5 - 5 35 minu-tes9 the substrates 'being rotated at 100 r.p.m~,the direction o~ rotation being reversed a~ter every 5 revolut:ions. The layer thicknesses varied f`rom 0.5 to 4 /um.
_ 3~

P~IN 9795 -6- 25 ~ 1981 EXAMPLE I.
F~r the growth of a layer having the nominal composition ~Bi, Lu)3 (Fe, Ga)5O12, the f`ollowing oxides were weighed out in -the ~ollowing quantities:
Bi2O3133.47 g P'bO3'19.71 g LU23 2.35 g Ga23 4O13 g Fe23 29.85 g The mixture was melted and heated to a temperature of` 723C, A Gd3Ga5O12 substrate having a (11'l) oriented depos:ition ~ace was dipped in the melt, and a 2 /um thick layer hacl deposited on it in 3 minutes.
EXAMPLE II.
- For t'he growth of a layer having the nominal c~omposition (Bi, Y~ Tm)3 (Fe, Ga)5 12~ the following oxides were weighed out in the followi~g quantities:
Bi23 133.47 g PbO 319.71 g Y2O3 1OO35 g Z 3 5 g Fe23 29.85 g Ga23 2.'l3 g The mixture was melted and hea-ted to a temperature o~ 855C. A Gd3Ga5Ol2 substrate llavlng a (111) oriented cleposition ~ace ~as clippecl in the melt, and a 1.'16 /um thick lLyer had deposited on it in I mimlte.
EXAMPL,E :CIC
For the growth ol` a layer having the nominal corllposit:ion (Bi~ Y, Lu)3 (Fe~ Ga)5O12~ t~le ~ollo~ing ox:ides were weighed out in t'he ~ollo~ing quan-tities:
''l33.~7 g PbO 3'19.71 g Y2O3 'l.O35 g Ga2O3 2.13 ~`
Fe23 29.85 g Lu O l-5 g PHN 9795 7 2~5-1981 The mix-ture was mel-ted and'heated to a temperature of 828 C. A Gd3Ga5012 su'bs-trate having a (1'l1) oriented deposition face was dipped in the melt~ and a layer having a thickness of 1.96 /um had deposited on it in 1 minute.
EX~MPLE IV
For -the growth o~ a layer having -the nominal compo5ition (B;, Y, Lu)3 (Fe, Ga)5012, the ~ollo~ lg oxides we~re weighed out in the following quantities:
Bi23133. 47 g 10 PbO3't9.71 g Y2031.035 g LU23 2.00 g Fe2329.85 g

2 3 3 g The mi~ture was melted and heated -to a temperature of 810~C. A Gd3Ga50l2 substrate having a (111) oriented deposition facewas dipped in the melt, and a layer having a thickness of 2.38 /um had deposited on it in 45 seconds~
EXAMPLE V
For the growth of a layer having the nomirlal composition ~Bi~ Y, Lu ~ (Fe~ Ga)5012, the following oxides were weighed out in -the following quanti-ties:
~i23l33. L~7 g 25 PbO3~9.7l g Y203'l.635 g 2o31.200 g 2329.85 g Ga23 1l.5 g The mixture was melted and heated -to a temperature of 766 C. An Sm3Ga5012 substrate (lat-tice constan-t aO = 12.432 ~) ha~ing a ( 111 ) oriented depos,ition ~`ace was dipped in the melt for 1-1- minu-tes producing a layer ha~ing a thickness of 3.80 /um.
The said layers hacl the following~ properties:

PHN 979~ -8- 25-~-1981 TABLE
Layer No. i I ~ III . IV ¦ V
~ a (~) ~0.001 _o.007~ 0.00'l-0.002~0.00'l B(/um) 1.6 1 o.83 2.25 2~63 5 K (erg.cm 3) 3.5~104.36x104 5.2x10 5.4x10 7.9x104 ~ H (Oe) j 8 '16 3 1 3 8 4 ~MS(Gauss) 1~ 821 791 1 471 427 (m sec Oe ) ~ l _ __ In the above Table, B is the s-table s-trip domain width~ .K~ is the uniaxial anisotropy cons-tant, ~ H :is the ferromagnet:ic resonance line width at 10 GHz, 4~r M
is the satura-tion magnetization and /u is the bubble domain mobility.
The unia~ial aniso-tropy cons-tan-ts of the resulting layers were determined by means o.f a torsion magnetome-ter.
Values up to 5.4'x 104 erg/cm3 were thus realized for (Bi, Y, Lu~3 (~e, Ga)5012 films on GGG, while i-t has been ~ound that these values can be approxima-tely 1.> x as large 20 for the same films on SGG.
Elerewith a ne~ -type o:f bubble domain mater:Lal has been provided with - also as regards ~ine ~idth and mo'bility - properties which make it excep-tionally sui.table for use in bubble domain propagation devices ~ith 1 to 2/um 25 bu'bbl.e domains. 'L`hose ski:Lled in the presellt tecl~lc~)gy will be capab:l.e o~ varying the composition o~` the bubble cdoma:ir.~ layer wh:;le using -the gene:ral composition ~B:i~ Y~ M)3_zJz Qy Fe5_yO.!2 withou-t clepart:ing I'rom I;he scope o:f the p:reserlt :invelltion. Consequent:Ly, the 'E~amples llave 30 'been glven only 'by ~ay Or illustration and are hence not limiting.
In one embodiment in accordance wi-th the invention, a substrate 'I and a bu'b'ble domain layer 2 ~or the ac:tive storage ancl movement of magnetic clomains have a common 35 i.nter~ace 3 9 each bei.ng characterized'by a special. nQ-ture a:nd 'by an above-clescribed mutua:l. relationship. The ~I.ayer 2 h.as an upper surf'ace 4 rernote :frorn the interface 3, -the sur:~ace l~ bear:ing certa:in convent:ional elelrlents for the

3~

PHN 9795 9 ~_l981 excita-tion propagation and sensing o~ domains. The l<ayer 2 for the storage or rnovement.o~' magnetic domains, generally speaking, may 'be the place of any of the va~ious processes for digital logi.cs, as -these were elaborz.-tely descri.bed in Patent Specifications and o-ther techni.cal literature. For exan1ple, reference may be made -ro The Bell System Technical Journal XLVI, No. 8~ 1901-1925 ('19~7) which comprises an article enti-tled "Prope.rties and Device Applications of Magnetic Domains in Ortho~erri~7 es"~
Figure 2 of the accornpanying drawing shows a rather simp].e con:E`ig~lration which represents only a.
f`ragment of a normally larger construc-tion comprising a layer 2 for storage and movement of magnetic domains and various conventional elements for the excitation7 movement and sensing of magnetic domains. Figure 2 may be considered to represent a shift register 5 in which., according to the invention, a layer 2 of a magnetic material having a high uniaxial magnetic anisotropy and high domain 7.0 mobility is used~ The easy axis of magneti~a.tion o~ the layer 2 is perpendicular to the surface 4. The general magnetiza-tion condition of the layer 2 ls denoted by minus signs 'lO wh:ich i:ndicate the li:nes of magnetic :E`lux directed perpendicu:Lar to -the sur:race L~. Magne-tic ~lux lines s:ituated inside the domains and directed oppositely are :indicated 'by plus sig:ns~ :E`or exalllplc the plus sign 6 with:Ln condLLcto:r loop 7, Conductors 'l2, 'l3 and 'I Ll governed 'by a domain transmltter 9 can be connected to or 'be present in the irnrnediate pro~irnity of the surface 4 of the layer 2 for magnetic domains~ in a pre-viously chose:n usual manner.
The conductors 'l2, 'l3 and 'I L~ are coupled respectively to successive triads of conduc-tive loops, for example, the loops 8, 8a~ 8b of a ~'irs-t o:E` such a -triad, e-tc. An array o~ rows and colurnns o~ such multiple l.oop arrangeme:nts is often used .in storage systems~ A mag:net:ic b:ias fi.eld ~or stabil:i~lng axided domai.ns is provided in a conven-tional manner, :~or exarllple, by using o:E` a coil or coi:l.s (:not ?~

PHN 9795 -10- ~5~5 l981 shown) surrounding the substrate-bubble domain layer con-figura-tion, or by the use of permanent magnets During operation of the device the magne~ic domains are e~cite~ by means o~ a conventional domain generator 20 combined with a loop 7 which is sub-stantial]y coaxial with a loop 8. A stable, cylindrical domain, ~or example, the position of the dornain indicated by the plus sign 6, can be propagated in incremental s-teps from the location of the loop 8 to the location of the loop 8a, then to that of loop 8b, etc., by successive excitation of the conductors 12, 13 and 14 etc. by the domain propagator 9. ~hen a propagated magnetic domain reaches loop 8n, it can be detected by means of domain sensor 21. It will be obvious that other digital logic func-tions can easily be carried out while using the same ~nown methods as those ~lich are used in the example of the shift register 5.
Flnally, bubble domain layers ~ere deposited from one melt in a thickness of approximately 1 /um on a ~GG
substrate ~lattice constant a = 12.38 ~) 7 a SGG s~bstrate (aO ~ 12.43 ~) and a ~GG s-ubstrate (aO = 12.50 ~)0 ~y varying the growtn temperatures (these were 832 C, 742 C
and 699 C, respec-tively) it wa~ ensured that the lattice parameter o~ each layer was~dapted as much as possible to the :Lattice parame-ter of the substrate on which it was deposited. The rnelt contained 0.9 g of` Y203, l.O g of Lu203 and 2 g of Ga203 a-tld furthtl hacl the salrle compositioll as that of` example ~. This experirnen-t cdemonstrates that~ by means o~ the invention, bubble domain layers with very h:igh uniaxial anisotropy constants (these were 6 x 10 erg~cm 3, 9.l2 x lO erg.crn 3 and 1.1-~ x 105 erg cm~~3 ~cspeetively) in combination with high wall mobililies for which low line widths (these were 4 Oe 7 4 Oe and 1 Oe, respectively)are characteris-tic , are possible.

Claims (8)

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

1. A device for propagating magnetic domains (6), including a monocrystalline non-magnetic substrate (11) bearing a layer (2) of an iron garnet capable of supporting local enclosed magnetic domains, said layer having a uni-axial magnetic anisotropy induced substantially by growth and having been caused to grow epitaxially on the non-mag-netic substrate (1), said iron garnet being of the class of iron garnet materials comprising in the dodecahedral sites of the garnet lattice at least a large and a small occupant characterized in that the iron garnet consists essentially of a material which comprises in the dodecahedral sites at least a bismuth ion and a rare earth ion selected from the group consisting of lutetium, thalium and ytterbium.

2. A device as claimed in Claim 1, provided with first means for magnetically biassing said layer to stabilize said domains, second means (7) for exciting such magnetic domains, third means (8n, 21) for detecting the presence of such magnetic domains, and fourth means (8, 8a, 8b, 9) for propagating such magnetic domains.

3. A device as claimed in Claim 1, characterized in that the non-magnetic substrate material has a first cha-racterizing lattice parameter a0 and that the magnetic domain-carrying iron garnet has a second characterizing lattice parameter a1, where -1.6 X 10-3 nm < a0 - a1 < +
1.6 x 10-3 nm.

4. A device as claimed in Claim 3, characterized in that the non-magnetic substrate material is represented by the formula RE3 Ga5O12, wherein RE is at least one element selected from the group consisting of Gd, Eu, Sm and Nd, and has a lattice parameter a between 1.238 and 1.250 nm.

that the iron garnet consists essentially of a material which also includes yttrium in the dodecahedral lattice sites.

6. A device as claimed in claim 5, characterized in that the iron garnet may be represented by the formula {Bi, Y, M} 3 (Fe, Q)5O12' wherein M is Lu and/or Tm and/or Yb, Q is Ge, Si, Al or Ga.

7. A device as claimed in Claim 1, 5 or 6, character-ized in that the iron garnet consists essentially of a material which also includes samarium and/or europium in the dodecahedral lattice sites.

8. A device as claimed in Claim 6, characterized in that the weight ratio of Y : M in the iron garnet is between 0 and 2.5 : 1.