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DEVICE APPLICATIONS OF GARNET THIN FILMS
Ph. Coeure
To cite this version:
Ph. Coeure. DEVICE APPLICATIONS OF GARNET THIN FILMS. Journal de Physique Colloques,
1985, 46 (C6), pp.C6-61-C6-68. �10.1051/jphyscol:1985610�. �jpa-00224848�
JOURNAL DE PHYSIQUE
Colloque C6, suppl6rnent au n09, Tome 46, septernbre 1985 page C6-61
D E V I C E A P P L I C A T I O N S O F G A R N E T T H I N F I L M S
Ph. Coeure
C. E.A., I . R.D. I . , LETI/CRM, 85 X, 38041 G r e n o b Z e C e d e x , F r a n c e
R6sum6 - Les 15 derniGres annees o n t
ete
t r e s fructueuses pour l e developpe-m e d i s p o s i t i f s r P a l i s e s avec des f i l m s minces de grenats. Les p r i n c i p a u x
domaines d ' a p p l i c a t i o n sont l e s memoires
i
b u l l e s magnetiques, l e s m6moiresoptiques, l e s d i s p o s i t i f s d ' a f f i c h a g e , l e s d i s p o s i t i f s hyperfrequences
i
ondesmagnGtostatiques, mais Pgalement d ' a u t r e s domaines t e l s que l e s composants optiques i n t e g r e s pour l e s tPlecomnunications sur f i b r e optique. L ' o b j e c t i f de c e t t e revue e s t d'examiner l e s dPveloppements r e c e n t s des composants en r e l a -
t i o n avec l e s propriPtPs souhaitees pour l e s f i l m s de grenat : a n i s o t r o p i e
magnetique, aimantation, r o t a t i o n Faraday, 1 argeur de r a i e de resonance f e r r o -
magnetique, etc... Les composants seront d e c r i t s e t l e s problemes techniques
r e s t a n t
i
resoudre seront d i scutPs.
Abstract
-
The past 15 years have been v e r y f r u i t f u l i n t h e f i e l d o f develop-ment o f devices making use o f garnet t h i n f i l m s . The major device areas are t h e magnetic bubble memories, t h e magneto-optical d i s p l a y s and p r i n t e r s , t h e o p t i c a l memories, t h e magnetostatic surface wave f i l t e r s , but, a1 so, o t h e r devices such as i n t e g r a t e d o p t i c s components a r e o f c u r r e n t i n t e r e s t . The pur- pose o f t h i s review i s t o examine t h e r e c e n t developments o f components i n r e -
l a t i o n t o t h e s u i t a b l e p r o p e r t i e s o f t h e garnet f i l m s : magnetic anisotropy,
magnetization, Faraday r o t a t i o n , FMR l i n e w i d t h , e t c .
..
The components w i l l bedescribed, as w e l l as t e c h n i c a l problems s t i l l t o be solved.
I - INTRODUCTION
The garnet system has been known since 1958 /1/ t h e best-known m a t e r i a l i s y t t r i u m - i r o n garnet, Y3Fe5012, o r Y I G , which i s used f o r microwave a p p l i c a t i o n s . Up t o 1970, t h e garnet s i n g l e c r y s t a l s where obtained i n t h e f o r m o f b u l k m a t e r i a l s used t o make small spheres f o r microwave purpose. At t h a t time, t h e enthusiasm f o r t h e l a r g e
scale manufacture o f magnetic bubble memories made i t p o s s i b l e t o develop t h i n garnet
f i l m s from l i q u i d phase e p i t a x y technique. This powerful. technique allowed t h e f a b r i - c a t i o n o f many garnet m a t e r i a l s f o r new a p p l i c a t i o n s . To date, t h e major device areas are magnetic bubble memories, magneto-optical d i s p l a y s a n d p r i n t e r s , o p t i c a l storage, microwave f i l t e r s and i n t e g r a t e d o p t i c s components.
The purpose o f t h i s paper i s t o review t h e recent developments of components
i n r e l a t i o n w i t h t h e s u i t a b l e p r o p e r t i e s o f garnet f i l m s .
I 1
-
THE GARNET MATERIALSThe r a r e e a r t h (R.E.) i r o n garnets havethe f o l l o w i n formula : {R.E.31CFe21(Fe3) 01
where
{
,
[1
,
and ( ) stand f o r dodecahedra1 (cq, octahedral ( a ) and tetrahedra?( d l s i t e s r e s p e c t i v e l y /2/. The c o u p l i n g between t h e s u b l a t t i c e s i s antiferromagne- t i c , as shown below f o r t h e "heavy" rare-earths".
C6-62
JOURNAL
DE PHYSIQUEt
T
magnetization v e c t o r s3 ~ e ~ +2 Fe3+
1
si!ed S i t e a 3 S i t e R . E . ~ + cI
Fig. 1---
The most important c h a r a c t e r i s t i c o f t h e garnets i s t h e p o s s i b i l i t y t o a d j u s t t h e i r composition and t h e r e f o r e t h e i r magnetic p r o p e r t i e s thanks t o t h e s u b s t i t u t i o n s o f desired i o n s on s i t e s ( c ) , (a), o r ( d l . Table 1 Fe3+ s i 4 + Ge4+ Ga3+ AI 3+
v5+
S i t e dThe magnetization i s changed by p l a c i n g non-magnetic i o n s on t h e t e t r a h e d r a l s i t e :
i n c r e a s i n g t h e amount o f Ga3+, A13+, 6e4+, o r ~ i 4 + i o n s wi 11 decrease t h e magnetiza-
t i o n . A l t e r n a t i v e l y , i n c r e a s i n g t h e amount o f S C ~ + o r 1n3+ on t h e octahedral
s i t e s w i l l r a i s e t h e magnetization. The i n c o r p o r a t i o n o f ~ 1 3 + , 6a3+, ~ i 3 f i s s t r a i g h t
forward w h i l e t h e use o f Si4+ o r Ge4+ r e q u i r e s t h e simultaneous a d d i t i o n o f a d i v a - l e n t c a t i o n l i k e Ca2+ f o r charge compensation.
I o n s u b s t i t u t i o n s are a l s o used t o a d j u s t o t h e r magnetic p r o p e r t i e s ( a n i s o t r o p y
f i e l d , c o e r c i v i t y , magnetostriction, etc... and t o o p t i m i z e o p t i c a l p r o p e r t i e s
(Faraday r o t a t i o n and l i g h t absorption).
...
Fe3+ T i 4+ sn4+zr4+
co2+ sc 3+ sb5+...
S i t e a b - - - ^ - - - L - - - L - - -The m a t e r i a l requirements are a l s o met thanks t o t h e v e r s a t i l i t y o f t h e f a b r i c a t i o n
techniques : e s s e n t i a l l y a l l magnetic garnet f i l m s r e p o r t e d t o date have been grown
by t h e L i q u i d Phase Epitaxy (LPE) technique /3/. y3+ ~ a 2 + ~ b 2 + B i 3+ ~ r 3 + ~ d ~ + T b3+ Gd3+ ~ m 3 + S i t e c
This method i s v e r y powerful because i t leads t o h i g h q u a l i t y , very r e p r o d u c i b l e t h i n f i l m s . The LPE growth process i s now w e l l standardized among t h e v a r i o u s workers i n t h e f i e l d . The LET1 procedure which i s f a i r l y t y p i c a l i s as f o l l o w s : t h e f i l m s
are grown on [l 1 11 s u r f aces o f gad01 in i u m g a l 1 i.um garnet (GGG ) p o l i s h e d substrates
under isothermal c o n d i t i o n s . A f o u r
-
pronged p l a t i n u m h o l d e r i s used t o supportf i v e wafers.
(mclbi t
\
4 M b i t s
---
-
-
The substrates are i n t r o d u c e d i n t o t h e furnace chamber and lowered t o j u s t above t h e s o l u t i o n surface f o r preheating. B a f f l e s and r e f l e c t o r s are p o s i t i o n e d above t h e platinum c r u c i b l e t o o b t a i n t h e optimum temperature p r o f i l e . A h o r i z o n t a l d i p p i n g technique a t 60 rpm i s used d u r i n g growth. The composition s u i t a b l e f o r t h e a p p l i c a - t i o n i s obtained from supersaturated s o l u t i o n s . The s a t u r a t i o n temperature i s i n t h e
range 750-950°C and t h e f i l m s are grown i s o t h e r m a l l y under a supercooling o f
-
25°C.The growth o f a 1-micron t h i c k f i l m takes about 1 minute
.
The GGG s u b s t r a t e i s nowa v a i l a b l e commercially i n t h e form o f p o l i s h e d wafers as l a r g e as 10 cm i n diameter and w i t h a d e f e c t d e n s i t y lower than 1 per cm2.
I 1 1
-
MATERIAL FOR HIGH DENSITY BUBBLE MEMORIESSince t h e i n t r o d u c t i o n of magnetic bubble memories (MBM) i n 1967 /4/ r a t h e r remarka- b l e developments have been achieved. Several companies ( I n t e l , Motorola, F u j i t s u ,
Saqem
. .
.
) are s e l l in g one megabit bubble memory devices rn ade from ( YSm) (FeCaGeIgarnett h i n f i l m s supporting 2pm-diameter bubbles. MBM systems o f f e r several advantages over o t h e r k i n d o f memories.These i n c l u d e t h e absence o f moving p a r t s , h i g h r e l i a b i l i t y , non v o l a t i l i t y of data, and easy maintenance. I n bsbble memories t h e i n f o r m a t i o n i s stored i n t h e f o r m o f c y l i n d r i c a l (bubble) domains whose magnetization i s t h e r e v e r s e o f t h a t o f surrounding area. T h e i r presence ( " 1 " ) o r absence ( " 0 " ) i n s p e c i f i c places corresponds t o b i n a r y d i g i t s s t o r e d a t those l o c a t i o n s /5/. MBM a r e s t i l l q u i t e ex- pensive, so, i n order t o decrease t h e p r i c e o f devices t o an a t t r a c t i v e c o m e r c i a l le-
vel, say 3 KC per b i t ( f i g . 2), i n d u s t r i a l companies a r e f a b r i c a t i n g 4 M b i t s memory
chips w i t h garnet f i l m s supporting 1 micron bubbles /6/. E p i t a x i a l growth on t h e
( 1 11 ) plane o f t h e GGG s u b s t r a t e produces t h e SO-cal le d "growth induced ani sotropy necessary t o s t a b i l i z e t h e magnetic bubbles. T h i s anisotropy i s a t t r i b u t a b l e t o t h e mi- x i n g o f l a r g e and small i o n s on t h e dodecahedra1 s i t e s : Sm i s t h e l a r g e i o n commonly used w h i l e Tm and Lu serve as t h e small ions. Bubbles can be obtained o n l y when : where Ku i s t h e u n i a x i a1 ani sotropy constant, Hk t h e ani sotropy f i e l d and 4 xMS, t h e
s a t u r a t i o n magnetization. The bubble diameter i
..
s7approximately given bySubmicron bubbles are w e l l w i t h i n t h e c a p a b i l i t i e s o f t h e garnet systems. Table I 1 gives examples o f garnetswhich are already a b l e t o support bubblesdown t o 0,5 micron.
A : Y0.5Sm0.6Lu1 .6Gd0.3Fe4.5Ga0.5012
: Sm0.75LU2.~Gd0.25Fe4.6Ga0.401 2
: Sm0.9LU1.9Gd0.2Fe4.8A10.2012 Table I 1 - Submicron Bubble Garnets
2
The l i m i t o f present technology seems t o be i n t h e o r d e r o f 32 Mbits/cm where
0.35 micron bubbles are needed. This can be obtained by t h e use o f ( S ~ , L U ) ~ ( F ~ S C \ O ~ ~ garnets. Scandium i o n s occupy octahedral s i t e s and would b r i n g both a decrease
i n t h e exchange s t i f f n e s s A and an increase i n magnetization MS. To overcome t h e
Bubble diameter Sample name S t r i p w i d t h w(pm) F i l m t h i c k n e s s h (pm) Bubble c o l l a p s e f i e l d Ho
(Ce)
S a t u r a t i o n i n d u c t i o n 4 n M, (GI Anisotropy f i e l d Hk (Oe) Curie temperature Tc ("C) 0.7 pm ( 8 ~ b / c m P ) B 0.7 0.8 540 1050 2500 233...
1 ~m ( 4 ~ b / c m 2 )--lblt-denslt~l---
A 1 .O 1.1 450 870 21 00 220 0.5 pm (16 Mb/cmZ) C 0.5 0.6 730 1400 2800 253JOURNAL DE PHYSIQUE
fundamental l i m i t a t i o n due t o t h e concept o f bubble domain remories one has r e c e n t l y
proposed a new s o l i d s t a t e magnetic memory named Bloch l i n e memory
/6/.
The "1" and"0" a r e s t o r e d i n t h e s t r i p e domain w a l l s ( f i g . 3 ) i n s t e a d o f u s i n g t h e well-known presence o r absence o f c y l i n d r i c a l "bubble" domains. T h i s new technology i s expected t o f a c i l i t a t e t h e production o f memories w i t h c a p a c i t i e s o f t h e order o f one g i g a b i t
on a s i n g l e c h i p measuring about one square centimeter /7/.
GGG s u b s t r a t e
1
Fig. 3-
B l o c h - l i n e memory'I I I I
/loch w a l lv
- 7 e r t i c a l Bloch 1 i neI V
-
GARNETS AS MATERIALS FOR OPTICAL DEVICESMagneto-optical e f f e c t s i n garnet c r y s t a l s have been t h e s u b j e c t o f l a r g e i n t e r e s t since t h e m i d - s i x t i e s . Possible a p p l i c a t i o n s i n c l u d e o p t i c a l storage, displays,
p r i n t e r s and fi
her-optics
telecommunications. Before any d e s c r i p t i o n o f p o t e n t i a1a p p l i c a t i o n s we s h a l l present t h e e s s e n t i a l features o f t h e magneto-optical materials.
IV-1
-
Optimum composition o f a magneto-optical garnet f i l mThe primary requirement i s t h a t t h e r e must be t h e g r e a t e s t c o n t r a s t between l i g h t beams t h a t have passed through areas m a g n e t i c a l l y p o l a r i s e d i n opposite d i r e c t i o n s
( f i g . 4 ) . The d i f f e r e n c e A 1 i n i n t e n s i t y i s l a r g e i f t h e r a t i o Q = 2 Q F / ~ i s l a r g e :
QF i s the6Faraday r o t a t i o n per metre,
a i s t h e absorption c o e f f i c i e n t per metre,
Q i s t h e f i g u r e o f m e r i t o f t h e m a t e r i a l
.
Many s t u d i e s
/8,9/
have shown t h a t t h e o p t i c a l c o n t r a s t i s g r e a t l y enhanced by subs-t i t u t i n g bismuth a t dodecahedra1 s i t e s i n t h e l a t t i c e . The general formula o f a gar- n e t used f o r an o p t i c a l a p p l i c a t i o n i s
RE3-xBix(FeGa)501
where t h e bismuth content i s between x = 0.5 and x = 1.5. G a l l i u m i s used t o a d j u s t
t h e magnetization. A l a r g e bismuth c o n c e n t r a t i o n changes magnetic p r o p e r t i e s l i k e anisotropy f i e l d , magnetization, compensation temperature, and Curie p o i n t . The consequence i s t h a t t h e optimum composition depends on t h e a p p l i c a t i o n and can be r e -
l a t i v e l y complex, as i t i s i n t h e case o f bubble memories.
IV-2
-
L i g h t s w i t c h i n g d i s p l a y s and arraysThe s w i t c h i n g c e l l s shown on f i g . 4 can be c o n f i g u r e d as a l i n e a r a r r a y t h a t can
be used f o r electrophotographic p r i n t i n g
/lo/.
When arranged i n a m a t r i x t h e c e l l sc o u l d form t h e basis f o r d a t a d i s p l a y s /11/. The p r i n c i p l e o f r e c o r d i n g i s as f o l -
lows : t h e garnet composition i s chosen so as t o o b t a i n an anisotropy f i e l d H which
i s h i g h a t room temperature and decreases r a p i d l y when t h e m a t e r i a l i s heate6 (fig.5). So, t h e d i r e c t i o n o f magnetic p o l a r i z a t i o n can be reversed i f an e x t e r n a l magnetic f i e l d and a h e a t i n g p u l s e are a p p l i e d t o t h e m a t e r i a l a t t h e same time. Called LISA f o r l i g h t s w i t c h i n g a r r a y t h e device proposed by P h i l i p s Research Laboratories i n
Hamburg /11/ has very a t t r a c t i v e f e a t u r e s : t y p i c a l s w i t c h i n g time i s 20 ,us, up t o
2000 o p t i c a l p a t t e r n s can be generated per second, and t h e r e s o l u t i o n i s t y p i c a l l y 300 dots/inch. An A4 page can be exposed and p r i n t e d w i t h i n o n l y 2 ,us. The design o f
a s w i t c h i n g c e l l i s shown on f i g . 6. The temperature i s r a i s e d by an e l e c t r o n i c c u r -
r e n t p u l s e a p p l i e d t o t h e r e s i s t o r and t h e a p p l i e d magnetic f i e l d i s i n t h e r e g i o n o f 0.03 Tesla. Besides p r i n t i n g t h i s k i n d o f device c o u l d l e a d t o d i s p l a y a p p l i c a t i o n . A two-dimensional s w i t c h i n g device o f 256 x 256 p i x e l s which operates i n t h e same
C6-65
way as the LISA array has been achieved by P h i l i p s r e s e a r c h e r s . A 16 K-pixels device was r e c e n t l y t e s t e d by P u l l i a m e t al / 1 2 / f o r use as a s p a t i a l l i g h t modulator i n o p t i c a l processing a p p l i c a t i o n s . plane of Polarization light blocked light incident light transmitted
analyzer (polarizing foil)
- i r o n - g a r n e t switching cell
polari2er (polarizing toil)
4 TT Ms (Gauss)
H K (Gauss)
temperature (k)
Fig. 4 - Magneto-optical switching
array /11/ Fig. 5 - Magnetization Ms and anisotropy field
Vs temperature for a
(GdBi)3(FeGaAl)5O12 garnet
For these applications it could be necessary to grow garnet films with a figure of merit as large as 4 degree/db.This can be obtained with a (GdTmBi h(FeGa).5O12qarnet as shown by Ferrand et al /13/. substrate magnetic c o i l
quartz
layer
"metal conductor resistance layer side t o p F i g . 6 - D e t a i l s of a magneto-optical s w i t c h i n g c e l l / 1 1 /IV-3 - Disk storage
Optical d i s k storage u n i t s are g e n e r a l l y c l a s s i f i e d i n two types : read only and r e -w r i t a b l e types ( f i g . 7 ) . Read only o p t i c a l d i s k storage corresponds t o d i g i t a l audio disk and a r c h i v a l media on which records are not e r a s a b l e . On the other hand, i n the case of r e w r i t a b l e d i s k s i t i s p o s s i b l e t o w r i t e , r e a d , erase, and w r i t e again d a t a , using a laser beam. Several r e a d - o n l y - m a t e r i a l systems are already a v a i l a b l e i n the market p l a c e . U n t i l recently,many people have argued t h a t erasable o p t i c a l storage was n e i t h e r needed or d e s i r e d . However, recent developments i n both magneto-optical / 1 4 / and phase change / 1 5 / m a t e r i a l s have r e s u l t e d i n renewed enthusiasm. This i n t e -r e s t can be a t t -r i b u t e d t o :
1 . The technology base ( i n c l u d i n g GaAlAs semiconductor l a s e r s ) which were provided by read-only o p t i c a l disk storage.
JOURNAL D E PHYSIQUE
h i g h power 1 aser a r c h i v a l media
/
DIGITAL-
AUDIODISK\
low power GaAl As l a s e r(compact d i s k )
/
VIDEO DISK\
He Ne LasersFig.
7
-
O p t i c a l memories2. The development o f amorphous m a t e r i a l s w i t h l a r g e magneto-optic K e r r r o t a t i o n
l e a d i n g t o g r e a t l y improved s i g n a l t o n o i s e r a t i o , i n t h e order o f 50 db i n a 30 KHz bandwith.
The p r i n c i p l e o f r e c o r d i n g i s s i m i l a r t o t h a t used f o r d i s p l a y s and arrays. A weak
magnetic f i e l d (- 0.02 T) i s produced and, a t t h e same time, t h e temperature o f t h e
m a t e r i a l i s increased by t h e l a s e r beam focused on t h e d i s k . As a r e s u l t t h e c o e r c i - ve f i e l d o f t h e magnetic l a y e r decreases and t h e d i r e c t i o n o f magnetization i s r e - versed. The diameter o f t h e area t o which t h e l a s e r beam i s a p p l i e d can be reduced t o about 1 micron. Hence, t h e r e c o r d i n g d e n s i t y is-4x107 bits/cm2, t h a t i s an order o f magnitude h i g h e r t h a n those o f magnetic storage. Read o u t i s g e n e r a l l y based on
magneto-optical Kerr r o t a t i o n since t h e 1 i ght i s r e f l e c t e d f r o m t h e metal1 i c storage.
Some 40 000 A4 documents can be recorded on a 30 cm diameter d i s k .
The magnetic f i l m i s u s u a l l y made from amorphous a l l o y s . Examples are TbFeCo a l l o y s which are already i n development. Among t h e m a t e r i a l s which have p o t e n t i a l i t i e s t o be a good medium, t h e gdrnets are very a t t r a c t i v e because t h e y e x h i b i t l a r g e magneto- o p t i c a l r o t a t i o n , and, i n c o n t r a r y t o amorphous metals are chemically stable.
I n order t o o b t a i n u n i f o r m magnetic t h i n l a y e r on amorphous substrates l a r g e r than 5 inch, i t i s necessary t o choose a s u i t a b l e d e p o s i t i o n technique. A drawback o f LPE i s a need f o r l a t t i c e -matched s i n g l e c r y s t a l l i n e substrates. Furthermore, GGG subs-
t r a t e s are l i m i t e d i n s i z e and very expensive (150 $ a 4 inch-wafer i n 1985).
For these reasons o t h e r techniques have t o be i n v e s t i g a t e d . RF diode s p u t t e r i n g seems t o be s u i t a b l e since (GdBi)3(FeA1)5012 garnets have been obtained w i t h p r o p e r t i e s s i m i -
l a r t o those o f LPE f i l m s w i t h t h e same compositions /16/.
IV-4
-
I n t e g r a t e d o p t i c a l devicesIt i s known s i n c e t h e work o f Tien e t a1 /17/ t h a t LPE i r o n garnet f i l m s are s u i t a b l e
f o r o p t i c a l waveguides between 1.1 pm and 4-5 pin wavelength. I n t h i s magneto-optical
waveguide Faraday and Cotton-Mouton e f f e c t s cause mode conversion between TE and TM guided modes. These p r o p e r t i e s can be used t o b u i l d modulators as w e l l as non r e c i - procal devices such as i s o l a t o r s and c i r c u l a t o r s which c o u l d be v e r y u s e f u l f o r f i b e r -
o p t i c communication systems /18,19/. However, t h e e f f i c i e n c y o f t h e mode conversion
and t h e r e f o r e t h e performance o f devices are s t r o n g l y a f f e c t e d by t h e o p t i c a l b i r e -
fiingence i n t h e f i l m . It was r e c e n t l y demonstrated t h a t t h i s b i r e f r i n g e n c e i s com-
posed o f a stress-induced p a r t and o f a growth-induced p a r t /20/. I t means t h a t a
h i g h e f f i c i e n c y mode conversion remains a problem t o be solved, s p e c i a l l y i n bismuth garnet f i l m s f o r which t h e o r i g i n e o f t h e growth induced anisotropy i s n o t y e t c l e a r - l y understood.
V
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MAGNETOSTATIC-WAVE DEVICES FOR MICROWAVE SYSTEMSFor years, engineers have been t r y i n g t o f i n d a planar technology t o make microwave devices e a s i e r t o be b u i l t than Y t t r i u m I r o n Garnet (YIG) spheres used as resonators
i n microwave systems. Since 1 t o 50 m i c r o n - t h i c k YIG f i l m s can be grown w i t h low de-
f e c t densi ty,impressive good experimental r e s u l t s have been obtained i n t h i s f i e l d
/21-25/. These devices depend on t h e propagation o f slow, d i s p e r s i v e wave a t micro-
wave frequencies i n a low l o s s f e r r i m a g n e t i c m a t e r i a l . Presently, YIG f i l m s grown on GGG substrates by t h e l i q u i d - p h a s e e p i t a x y technique are c h a r a c t e r i z e d by a f e r r i - magnetic resonance l i n e w i d t h AH comparable t o those o f YIG spheres, say 0.5 oersted a t 10 GHz. A basic c o n f i g u r a t i o n o f a d e l a y l i n e based on forward volume waves propagation i s shown on f i g . 9. Magnetic Field H, Signal Out u Dielectric Substrate Signal In
Fig. 9
-
Basic c o n f i g u r a t i o n o f a magnetostatic wave delay /25/When a RF f i e l d i s a p p l i e d t o t h e i m p u t transducer a l o c a l p e r t u r b a t i o n o f t h e
spins occurs, which then propagates through t h e Y I G f i l m v i a t h e c o u p l i n g o f adja-
cent magnetic moments. T h i s wave reaches t h e output transducersome-time l a t e r and induces a c u r r e n t i n t h e m i c r o s t r i p . A t y p i c a l r e s u l t i s shown on Table 3 /25/.
Thickness o f t h e GaYIG f i l m : 18.7 microns
Center frequency 840 MHz Bias f i e l d Ho 85 oersteds Del ay 150 ns Bandwi t h 120 Mhz
...
Table 11'1Without changing t h e b i a s f i e l d H
,
t h e same devices can be used as d i s p e r s i v edelay l i n e s which are a t t r a c t i v e ?or p u l s e compression a p p l i c a t i o n s . Multielement
g r a t i n g transducers are used t o make narrow band f i l t e r s : bandwith as narrow as
15 MHz were obtained f o r t h e S ( 2 t o 4 GHz), C ( 4 t o 8 GHz), X ( 8 t o 12 GHz) and K
(18 t o 27 GHz) frequency bands. One-part and two-part tunable resonators can be f a b r i c a t e d u s i n g p e r i o d i c etched-groove g r a t i n g s as s e l e c t i v e r e f l e c t o r s /22/.
V I
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CONCLUSIONThis r a p i d survey o f t h e a p p l i c a t i o n s o f garnet t h i n f i l m s shows t h a t a l a r g e number o f devices making use o f these m a t e r i a l s are s t i l l i n development. Among them, t h e most promising are magneto-optical displays, o p t i c a l memories and i n t e g r a t e d o p t i c a l devices. The magnetostatic wave devices makes them v e r y c o m p e t i t i v e w i t h o t h e r micro- wave device technology such as YIG spheres i n f i l t e r s and o s c i l l a t o r s . The market o f these components i s j u s t t a k i n g o f f . On t h e contrary, t h e production and demand f o r magnetic bubble memories cannot be s a i d t o be low. I n a d d i t i o n t o a constant r a t e of increase i n demand f o r NC machine t o o l s , r o b o t i c s , m i l i t a r y equipments, t h e i r use i s r a p i d l y i n c r e a s i n g i n POS t e r m i n a l s and p o r t a b l e computers. The worldwide market f o r MBM was approximately 200 m i l l i o n s d o l l a r s i n 1984 and w i l l become a 300 m i l l i o n s d o l l a r s market d u r i n g 1987. I n conclusion, one can say t h a t garnet t h i n f i l m s a r e ve- r y good examples o f new m a t e r i a l s s u c c e f u l l y developed t o meet t h e needs o f e l e c t r o - n i c i n d u s t r y and have opened t h e way t o Targe markets i n t h e f i e l d .
JOURNAL DE PHYSIQUE
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