AMORPHOUS MAGNETIC FILMS FOR NON-STORAGE APPLICATIONS

(1)

HAL Id: jpa-00224879

https://hal.archives-ouvertes.fr/jpa-00224879

Submitted on 1 Jan 1985

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

AMORPHOUS MAGNETIC FILMS FOR NON-STORAGE APPLICATIONS

H. Lachowicz, A. Milewski, J. Wenda

To cite this version:

H. Lachowicz, A. Milewski, J. Wenda. AMORPHOUS MAGNETIC FILMS FOR NON- STORAGE APPLICATIONS. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-181-C6-188.

�10.1051/jphyscol:1985631�. �jpa-00224879�

(2)

30URNAL DE PHYSIQUE

Colloque C6, supplement au n°9, Tome 46, septembre 1985 page C6-181

AMORPHOUS MAGNETIC FILMS FOR NON-STORAGE APPLICATIONS

H.K. Lachowicz, A. Milewski and J. Wenda

Institute of Physios, Polish Academy of Sciences, al. Lotnikow 32/46, 02-668 Warszawa, Poland

^Institute of Electron Technology, Warsaw Technical University, ul. Koszykowa 75, 00-662 Warszawa, Poland

Solid State Physics Dept., Academy of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Krakow, Poland

Résumé - Des possibilités d'application, autres que pour l'enregistrement, des couches amorphes d'alliages terres rares-métaux de transition et métal de transition-métalloide sont présentées. On considère en particulier les dispo- sitifs générateurs d'ondes acoustiques de surface dans lesquels les films amorphes peuvent servir à l'accord des paramètres du dispositif. Une utili- sation possible des films comme senseurs thermiques est aussi présentée.

Abstract - Possible non-storage applications of amorphous films of the rare earth-transition metal and transition metal-metalloid alloys are presented.

Particular attention is given to the surface acoustic wave devices in which amorphous films can serve to tune the device parameters. A possible utilization of these films as thermal sensors is also presented.

1. INTRODUCTION

Research on amorphous magnetic films has been triggered by the paper of Chaudhari et.al. [1], published in 1973, showing that sputtered films of the Gd-Co alloys can exhibit perpendicular anisotropy and that magnetic bubbles with submicron diameters can be created in these films.

Since this discovery, intense research on amorphous films of rare earth-3d transition metal (RE-TM) alloys has been discernible. However, this activity was mainly oriented towards applications of the RE-TM films as storage media for thermomagnetic beam addressable memories, since some technological limitations made it rather questionable to utilize these films in bubble memories.

Though the main effort has been concentrated on the above-mentioned applications, there are,nevertheless, other possible ways of utilizing the amorphous RE-TM films, e.g. in the surface acoustic wave (SAW) devices, as Hall sensors, etc.

The rapid development of metallic glasses, which took place during the last decade has, despite being concentrated on ribbons, also induced an interest in amorphous TM-M films [where M-metalloid). At the beginning, research on these films was concentrated on their fundamental physical properties, but shortly afterwards it has been shown that the TM-M amorphous films can also be utilized in devices, e.g. as thermal sensors, magnetic mirrors in ring-laser gyroscopes, etc.

A survey of the possible non-storage applications of both the RE-TM and TM-M amorphous films will be given in this paper. Attention will be focused on the utilization of these films in the SAW-devices. However, other possible applications will also be briefly indicated, in particular, those which have been developed by the authors' colleagues from collaborating institutions.

2. MAGNETICALLY TUNED SAW-DEVICES

SAW-devices are at present widely used for signal processing in the frequency band of 30 MHz up to 1.5 GHz, giving a cheap and efficient solution in this field of application.

Deposition of a magnetostrictive film onto the surface of a piezoelectric substrate (usually LiNb03, LiTa03 or quartz, though Bij2CeO20 also seems to be a potential candidate for this application, owing to its good piezoelectric properties) between the interdigital transducers in a SAW-device (its geometry is

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985631

(3)

JOURNAL DE PHYSIQUE

I/ \ rnaqnetostrictive film

F i g . 1 - iviagnetically v a r i a b l e SAW-delay l i n e ( a f t e r r e f . 1 2 1 ) .

snown i n Fig 1 ) i s a technique which g i v e s a p o s s i b i l i t y of changing t h e v e l o c i t y of t h e propagating S k i by a p p l i c a t i o n of an e x t e r n a l d.c.-magnetic f i e l d . These cnanges a r e due t o t h e m a g n e t o e l a s t i c i n t e r a c t i o n w i t h i n t h e f i l n and mechanical coupling between t h e f i l m and t h e p i e z o e l e c t r i c s u b s t r a t e .

The p o s s i b i l i t y of v a r y i n g t h e SAW-velocity i s d e s i r a b l e because of t h e c o r r e c t i o n of t h e t e c h n o l o g i c a l s c a t t e r ( a f t e r f a b r i c a t i o n t h e e l e c t r i c a l parameters of t h e d e v i c e a r e f i x e d , s i n c e t h e y depend s o l e l y on t h e geometry and soacing of t h e t r a n s d u c e r s , and on t h e e l e c t r i c , e l a s t i c , and p i e z o e l e c t r i c p r o p e r t i e s of t h e sub- s t r a t e and t h e e l e c t r o d e s ) . Because of ageing e f f e c t s (ageing r a t e s o f ( 1 + 1 0 ) ~ 1 0 - 6 p e r y e a r a r e common [ 2 ] ) a p o s s i b i l i t y of p e r i o d i c trimming of t h e d e v i c e parameters t o t h e i r nominal v a l u e s is a l s o r e q u i r e d . The p o s s i b i l i t y of tuning can a l s o be

d e s i r a b l e t o compensate f o r t h e temperature v a r i a t i o n of t h e device parameters.

On t n e o t h e r hand, t h e r e a r e a p p l i c a t i o n s of t h e SAW-devices where some degree o f tuning i s r e q u i r e d , u s u a l l y r a p i d and u n p r e d i c t a b l e .

Though t h e r e a r e w e l l known methods of t u n i n g t h e SAW-devices, f o r example by means of a c o n t r o l l e d v a r i a t i o n of t h e e l e c t r i c charge on t h e s u r f a c e of t h e semi- conducting l a y e r l o c a t e d on t h e p i e z o e l e c t r i c s u b s t r a t e , t h e use of a magneto- s t r i c t i v e f i l m seems t o be a v e r y promising technique, mainly because o f i t s t e c h n o l o g i c a l s i m p l i c i t y .

2 . 1 . Requirements f o r f i l m s

H i s t o r i c a l l y , a p o l y c r y s t a l l i n e Ni-film was one of t h e f i r s t m a t e r i a l s e v a l u a t e d f o r magnetic tuning o f a Ski-device [ 3 ] . The changes of SAW-velocity o b t a i n e d f o r Ni-films a r e i n t h e range of 0.02+0.07% b u t t h e i n t e n s i t y of t h e b i a s f i e l d neces- s a r y f o r o b t a i n i n g t h e s e changes i s w i t h i n t h e lkOe range.

The dependence of t h e v e l o c i t y v a r i a t i o n upon t h e f i l m parameters i s r a t h e r complex. T h e o r e t i c a l approaches, simple and more r i g o r o u s , have been developed t o p r e d i c t t h e dependence of t h e SAW-velocity on t h e a p p l i e d magnetic f i e l d ( s e e e . g . [ 4 ] [ 5 ] ) . The common r e s u l t o b t a i n e d i s t h a t most of t h e v a r i a t i o n of t h e v e l o c i t y o c c u r s i n t h e range of f i e l d v a l u e s i n wliich a uniform r o t a t i o n of t h e f i l n magnetization t a k e s p l a c e and t h e s e changes a r e due t o t h e change of t h e e l a s t i c moduli of t h e m a g n e t o s t r i c t i v e f i l m ( t h e s o - c a l l e d AE-effect;.

T h i s r e s u l t i s n o t s u r p r i s i n g s i n c e t h e v e l o c i t y of bulk a c o u s t i c waves i s given by V = ( c e f f / p ) l l 2 , where p i s t h e d e n s i t y of t h e m a t e r i a l i n which an a c o u s t i c wave i s propagating and ceff i s t h e e f f e c t i v e e l a s t i c s t i f f n e s s c o n s t a n t , dependent on t h e a c o u s t i c wave mode and t h e s t r u c t u r a l symmetry o f t h e m a t e r i a l .

An a n a l y s i s of t h e m a g n e t o e l a s t i c wave e x c i t a t i o n i n an amorphous f i l m , performed by F u j i i e t a l . [ 6 ] on t h e b a s i s o f a s i m p l e o n e - d i m e n s i o n a l s t r i n g model, has shown t h a t t h e maximum change of t h e sound v e l o c i t y i s o b t a i n e d when t h e d i r e c t i o n of magnetization forms an a n g l e of 450 with t h e d i r e c t i o n of sound propagation. To achieve t h i s e f f e c t , changes of t h e f i l m magnetization should occur a s a r e s u l t of t h e r o t a t i o n . This could be t h e c a s e i f one c r e a t e s i n t h e f i l m an a n i s o t r o p y ( p o s s i b l y a low one) with t h e i n - p l a n e easy a x i s p e r p e n d i c u l a r t o t h e e x t e r n a l f i e l d d i r e c t i o n . Such a n i s o t r o p y can be induced by means of a thermomagnetic t r e a t m e n t . However, c a r e should be taken when u s i n g t h i s procedure s i n c e ,

according t o t h e a u t h o r s ' e x p e r i e n c e , t h e induced a n i s o t r o p y can have a r o t a t i o n a l c h a r a c t e r ( s e e e . g . [ 7 ] ) with t h e time c o n s t a n t i n t h e range of 51100 s e c .

A r e l a t i o n , l i n k i n g t h e AE-effect with t h e magnetic and e l a s t i c parameters of t h e materia1,was given by Kersten [ S ] h a l f a c e n t u r y ago

(4)

where Es and E a r e t h e Young's modulus a t t h e s a t u r a t e d and a t t h e g i v e n f i e l d , r e s p e c t i v e l y , - m a g n e t i c p e r m e a b i l i t y , A s - s a t u r a t i o n m a g n e t o s t r i c t i o n , MS - s a t u - r a t i o n m a g n e t i z a t i o n .

Although n o t u n i v e r s a l l y a p p l i c a b l e , e q . (1) g i v e s a r e a s o n a b l e a p p r o x i m a t i o n o f t h e e x p e r i m e n t a l r e s u l t s . Fukamichi e t a l . [ g ] have shown t h a t i n t h e c a s e o f un- a n n e a l e d samples o f t h e Fea3P17 and Fe8f,fi14 m e t a l l i c g l a s s e s , t h e o b s e r v e d and c a l - c u l a t e d ( u s i n g e q . ( 1 ) ) v a l u e s o f t h e A E - e f f e c t a r e i n r e a s o n a b l e agreement, a s can b e s e e n i n T a b l e 1. For s e l e c t i o n o f t h e t y p e o f f i l m t o be u s e d i n a SAW-device,

T a o l e 1 - C a l c u l a t e d and measured v a l u e s o f AE/ED ( a f t e r r e f . [ g ] )

where ED i s t h e Young's modulus a t z e r o f i e l d (demagnetizing s t a t e ) .

t h e m a t e r i a l w i t h t h e most f a v o u r a b l e p a r a m e t e r s c a n b e found by u s i n g eq. ( l ) , l o o k i n g f o r t h e m a t e r i a l e x h i b i t i n g t h e l a r g e s t AE-effect p o s s i b l e . A s c a n e a s i l y be n o t i c e d by c o n s i d e r i n g t h i s e q u a t i o n , f i l m s w i t h l a r g e m a g n e t o s t r i c t i o n and low m a g n e t i z a t i o n would b e t h e b e s t c h o i c e i f we want t o f u l f i l t h e r e q u i r e m e n t s f o r t h e maximum p o s s i b l e A E - e f f e c t .

Tine Tb-Fe amorphous a l l o y s show r e l a t i v e l y l a r g e m a g n e t o s t r i c t i o n (As = 3 1 0 ~ 1 0 - ~ f o r amorpilous TbFe2 [ 1 0 ] , w h i l e f o r a c r y s t a l l i n e compound o f t h e same c o m p o s i t i o n t h i s v a l u e i s a l m o s t s i x t i m e s l a r g e r ) . S i n c e t h e s e a l l o y s behave l i k e f e r r i m a g n e t s

( t h e i r m a g n e t i c s t r u c t u r e i s o f t h e s p e r i m a g n e t i c t y p e ) , c o n d i t i o n s f o r a low m a g n e t i z a t i o n c a n e a s i l y b e a c h i e v e d by a p r o p e r s e l e c t i o n o f t h e f i l m c o m p o s i t i o n

( s e e F i g . 7 ) . Though Yanlaguchi e t a l . [ l 1 1 have shown t h a t t h e u s e o f t h e TbFe2 amorphous f i l m i n t i l e SAW-device makes i t p o s s i b l e t o a c h i e v e t h e l a r g e s t e v e r r e p o r t e d change o f t h e SAN-velocity, AV/V = -0.27%, t h i s v a r i a t i o n i s o b t a i n a b l e f o r r e l a t i v e l y h i g h i n t e n s i t i e s o f t h e m a g n e t i c f i e l d ( o f t h e o r d e r o f 4kOe) b e c a u s e o f t h e g i a n t a n i s o t r o p y , o f s i n g l e - i o n o r i g i n , p r e s e n t i n t h e s e f i l m s . A p p l i c a t i o n o f a f i e l d o f s o h i g h a n i n t e n s i t y i s u n a c c e p t a b l e from t h e s t a n d p o i n t o f c o n s t r u c - t i o n o f t u n e d SAW-devices, b e c a u s e o f t h e o v e r - s i z e d m a g n e t i c b i a s f i e l d c i r c u i t r y which s h o u l d b e d e s i g n e d f o r t h e minimum power consumption and f a s t s w i t c h i n g .

T h e r e f o r e , t h e e f f o r t h a s c o n c e n t r a t e d on s e a r c h i n g f o r t h e m a t e r i a l s which a r e much s o f t e r m a g n e t i c a l l y .

I t i s w e l l known t h a t f e r r o m a g n e t i c m e t a l l i c g l a s s e s o f t h e Fe-based a l l o y s sllow, i f p r o p e r l y a n n e a l e d , a n e x t r a o r d i n a r i l y low a n i s o t r o y . T y p i c a l v a l u e s o f magneto- s t r i c t i o n f o r t h e s e a l l o y s a r e o f t h e o r d e r o f 30x10-' ( s e e e . g . T a b l e 1 ) . Use o f t h e e v a p o r a t e d Fe-B amorphous f i l m s i n t h e SAlY-device h a s shown t h a t t h e t u n i n g e f f e c t i s o b t a i n a b l e w i t h i n 10 Oc o f tile b i a s f i e l d , b u t t h a t i t s maximum v a l u e d o e s n o t exceed 0.03% [Z]. Though AV/V i s a n o r d e r o f magnitude lower t h a n i n t h e c a s e o f t h e Tb-Fe f i l m , t h i s v a l u e i s n e v e r t h e l e s s a d e q u a t e f o r many a p p l i c a t i o n s . I t t h e r e f o r e seems t h a t amorphous f i l m s o f t h e Fe-based a l l o y s a r e t h e

b e s t c a n d i d a t e s f o r t h e m a t e r i a l s t o b e u t i l i z e d f o r m a g n e t i c t u n i n g o f t h e SAW-devices .

Both t h e r m a l e v a p o r a t i o n and s p u t t e r i n g t e c h n i q u e s c a n be a p y l i e d t o produce a m a g n e t o s t r i c t i v e l a y e r on t h e s u r f a c e o f t h e p i e z o e l e c t r i c s u b s t r a t e o f t h e

SAW-device. However, a p p l i c a t i o n o f s p u t t e r e d f i l m s g i v e s b e t t e r r e s u l t s s i n c e t h i s t e c h n i q u e i n t r o d u c e s lower i n t r i n s i c s t r e s s e s . The s t r e s s g i v e s r i s e t o e f f e c t i v e a n i s o t r o p y v i a t h e m a g n e t o e l a s t i c i n t e r a c t i o n s , which r e s u l t s i n t h e e a s y a x i s o f m a g n e t i z a t i o n b e i n g i n t h e f i l m p l a n e o r p e r p e n d i c u l a r t o i t , depending on t h e s i g n of b o t h t h e s t r e s s and t h e m a g n e t o s t r i c t i o n . Some good e v i d e n c e f o r t h e p r e s e n c e o f t h i s e f f e c t i s shown by t h e r e s u l t s o f a n n e a l i n g o f a Fe-B amorphous f i l m s p u t t e r e d

A l l o y Fe83P17 Fe86B14

P O

670 3500 ( ~ 1 0 - 6 ) 1s

30 31

AE ' % ) c a l c .

0.09 0.40 (x101i:;n/cm2)

1.35 1 . 7 3

'5) AE ^meas.

0.11 0 . 4 3

(5)

C6-184 JOURNAL DE PHYSIQUE

onto a Lildb03-substrate, shown i n Fig. 2 i n t n e form of t n e SAN-velocity dependence on t h e S i a s f i e l d f o r d i f f e r e n t temperatures of t h e annealing performed i n an argon atmosphere (using t h e measuring c o n f i g u r a t i o n shown i n Fig.3). Note t h a t t h e i n d i v i - dual curves follow t h e shape of t h e AE/E dependence on t h e magnetic f i e l d t s e e e . g .

[ 1 2 ] ) . As can be seen i n Fig.2, t h e f i e l d a t which t h e v e l o c i t y change achieves i t s maximum value s h i f t s with a n i n c r e a s e of t h e annealing temperature t o lower i n t e n s i - t i e s , a s can be expected, whereas t h e maximum value o f AV/V i s r a t h e r weakly dependent on t h i s temperature.

150°C

-0.005 - 125OC

100 O C

W

l

as sputtered

-0.01ot

Fig. 2 - SAW-velocity change v s . applied magnetic f i e l d f o r d i f f e r e n t annealing temperature f o r a 0 . 7 u m %orphous Fe-B f i l m on YZ-LiNb03. The frequency i s 69 EItlz.

An i n c r e a s e o f t h e f i l m t h i c k n e s s gives a g r e a t e r v e l o c i t y v a r i a t i o n s i n c e a l a r g e r f r a c t i o n o f tile a c o u s t i c energy i s c a r r i e d w i t h i n t h e f i l m . A disadvantage of such a procedure i s a n i n c r e a s e o f a t t e n u a t i o n and d i s p e r s i o n . Therefore, t h e optimum f i l m thickness i s adjusted by a compromise and i s u s u a l l y o f t h e o r d e r of 1 urn.

To g e t t h e b e s t performances t h e f i l m parameters have t o be known ,

i n p a r t i c u l a r , those appearing i n eq. ( 1 ) . Since Young's modulus does not change s i g n i f i c a n t l y with t h e film'composition f o r t h e Fe-based a l l o y s , t h e important parameters a r e and A s .

However, care should be taken when the magnetostriction of t h e f i l m has t o be measured. A s a general r u l e f o r t h i s purpose one uses t h e experimental methods which make i t p o s s i b l e t o determine j u s t one magnetostriction constant, namely A s . This parameter p r o p e r l y d e s c r i b e s t h e m a g n e t o s t r i c t i v e p r o p e r t i e s of t h e f i l m only i f t h e medium i s i s o t r o p i c . As has been shown e a r l i e r , t h e amorphous f i l m s t o be u t i l i z e d i n t h e SKii-devices have t o e x h i b i t an anisotropy i n t h e f i l m plane. By con- s i d e r i n g an a n i s o t r o p i c amorphous f i l m , Szymczak [l33 has sliown t h a t t h e magneto- e l a s t i c t e n s o r B i j k l f o r such a m a t e r i a l has t h e form ( w r i t t e n i n t h e Voigt n o t a t i o n ) determined by -/mm p o i n t symmetry

Where B3i = -Bll-B12, = 1/2(Bll;B12j, = -2B13.

I n t h i s c a s e , t h e magnetoelastic l n t e r a c t l o n s a r e determined by f o u r independent parameters i n s t e a d o f t h e one s u f f i c i e n t f o r a n i s o t r o p i c medium. The f i r s t i n d i r e c t i n d i c a t i o n t h a t l i n e a r magnetostriction could have an a n i s o t r o p i c c h a r a c t e r was given by Twarowski e t a l . j14] f o r t h e case of amorphous r . f . - s p u t t e r e d Gd-CO f i l m s . Experimental evidence f o r t h i s behaviour llas been ,oiven by Zuberek e t a l . [IS], who i n v e s t i g a t e d t h e Fel-xBx amorphous f i l m s by means of t h e s t r a i n modulated f e r r o -

(6)

magnetic resonance (SMFMR) method, which allows one t o c a l c u l a t e more t h a n one m a g n e t o s t r i c t i o n c o n s t a n t . The r e s u l t s o b t a i n e d i n t h i s experiment (shown i n Table 2j i n d i c a t e a c o n s i d e r a b l e d e p a r t u r e o f t h e m a g n e t o s t r i c t i o n from i s o t r o p i c behavior, s i n c e f o r i s o t r o p i c m a g n e t o s t r i c t i o n t h e following r e l a t i o n should be f u l f i l l e d : B l l / B 1 2 _{- -} -- = -2.

Table 2 - Measured v a l u e s of B11/B12 The SMFMR method i s r a t h e r incon- v e n i e n t f o r s t a n d a r d measurements.

f o r Fel-,BX amorphous f i l m s ( a f t e r r e f . [l51 l . Though other methods allow one to determine only one m a g n e t o s t r i c t i o n c o n s t a n t - A s , t h e y should neverthe- l e s s be used t o e s t i m a t e t h e magneto- s t r i c t i v e behaviour of t h e f i l m s of i n t e r e s t .

Among t h e known methods, t h e c a n t i l e v e r - c a p a c i t a n c e one, developed by Klokholm [ 1 6 ] , seems t o 6 e t h e most s u i t a b l e (see e . g . 1171). However, i t h a s r e c e n t l y been shown t h a t t h e s o - c a l l e d small-angle magnetization r o t a t i o n method [l81 can be used s u c c e s s f u l l y i f t h e f i l m i s deposited onto a p o l y e s t e r s u b s t r a t e .

To determine t h e change of sound v e l o c i t y i n a m a g n e t i c a l l y tuned SAW-device one should use t h e c i r c u i t shown i n F i g . 3 . This change i s c a l c u l a t e d a s a f u n c t i o n of

F--- 3 investiaated SAW- delay Line

I

vector

voltmeter recorder

Fig. 3 ,- Measuring c o n f i g u r a t i o n of SAW-velocity cnange.

t h e e x t e r n a l magnetic f i e l d from t h e measured phase d i f f e r e n c e between t h e s i g n a l s obtained a t t h e o u t p u t s of two symmetric SAW-delay l i n e s , one of them with t h e depo- s i t e d f i l m , both placed on t h e same p i e z o e l e c t r i c s u b s t r a t e . A s a r e s u l t of t h i s d e s i g n , t h e i n f l u e n c e o f t h e temperature i s c a n c e l l e d . The d e t a i l s of t h e measuring s e t - u p a r e d e s c r i b e d elsewhere j191.

2.2. An a p p l i c a t i o n of m a g n e t i c a l l y tuned SAW-devices

A s has been s a i d e a r l i e r , tuning of t h e SAW-devices i s d e s i r e d i n o r d e r t o c o r r e c t t h e t e c h n o l o g i c a l i n a c c u r a c i e s , t h e changes of parameters r e s u l t i n g from t h e ageing e f f e c t s and t o c o n t r o l t h e temperature v a r i a t i o n .

The f i r s t two of t h e s e requirements can e a s i l y be f u l f i l l e d by applying a permanent magnet of s u f f i c i e n t i n t e n s i t y , which w i l l i n f l u e n c e t h e sound v e l o c i t y v i a t h e d e p o s i t e d f i l m . A proper p l a c i n g o f t h i s magnet g i v e s t h e r e q u i r e d amount of t u n i n g . Control of t h e temperature v a r i a t i o n of t h e SAW-device parameters can be r e a l i z e d by u s i n g an electromagnet i n c o n j u n c t i o n with a tl~ermocouple s e n s o r . This s o l u t i o n seems t o b e a good a l t e r n a t i v e t o keeping t h e SAW-device i n a tllermostat.

Magnetically tuned SAW-devices c a n be used i n systems i n which a s m a l l , b u t r a p i d and u n p r e d i c t a b l e amount of tuning i s necessary. A good example of these a p p l i c a t i o n s i s t h e adaptive antenna a r r a y where t h e d i r e c t i o n of t h e beam i s guided by means of m a g n e t i c a l l y tuned SAW-delay l i n e s [ 2 ] . An a t t r a c t i v e a l t e r n a t i v e t o conventional approaches seems t o be a m a g n e t i c a l l y tuned SAW-oscillator capable of t r a c k i n g a high speed Doppler t a r g e t [ 2 ] .

Since t h e SAW-device can a c t a s a f i l t e r with a very narrow frequency band, i f only t h e p a t t e r n of t h e i n t e r d i g i t a l t r a n s J u c e r i s p r o p e r l y d e s i p e d , t h e device can

(7)

C6-186 JOURNAL DE PHYSIQUE

be u t i l i z e d i n an o s c i l l a t i n g c i r c u i t , shown i n F i g . 4 . A l a b o r a t o r y d e s i g n of a 69 MHz o s c i l l a t o r based on t h i s i d e a has shown a behaviour which seems promising from t h e p o i n t o f view of t h e p o s s i b l e u t i l i z a t i o n of t h e s e d e v i c e s i n frequency s y n t h e t i z e r s . The generated s i g n a l , a s d i s p l a y e d on a wobbulator s c r e e n , i s shown i n F i g . 5 . Use of tile harmonics of t h e f u n d m e n t a l frequency, p o s s i b l e i n t h e case of

output

l I - ^A

F i g . 4 - SAW-oscillator c i r c u i t .

F i g . 5 - Generated s i g n a l i n c i r c u i t designed a s i n F i g . 4 .

a s p e c i a l d e s i g n o f i n t e r d i g i t a l t r a n s d u c e r p a t t e r n s , makes it ? o s s i b l e , a c c o r d i n g t o t h e a u t h o r s ' e s t i m a t e , t o g e n e r a t e f r e q u e n c i e s of up t o 1 . 5 GHz. i-lowever, f a b r i c a - t i o n of t h e p a t t e r n r e q u i r e s a very high accuracy (of t h e o r d e r of 0 . 1 umj, which can only be achieved by means of e l e c t r o n o r x-ray l i t h o g r a p h y . A p p l i c a t i o n of an amorphous filrn i n t h i s d e v i c e allows one t o t u n e t h e frequency and/or t o compensate f o r t h e temperature v a r i a t i o n of t h e frequency.

3 . THERVAL SENSORS

A simple experiment performed [20] on t h e domain s t r u c t u r e of t h e RE-34 amorphous f i l m s has shown t h a t i n a f e r r i n a g n e t i c f i l m with p e r p e n d i c u l a r a n i s o t r o p y , a sudden r e v e r s a l of magnetization, whicit could be expected when t h e temperature p a s s e s through t h e compensation p o i n t , occurs o n l y i f t h e e x t e r n a l applied f i e l d Ha i s o f s u f f i c i e n t i n t e n s i t y . This i n t e n s i t y i s given by I M ~ ~ - M ~ ~ ~ ~ .Ha>Ea, where MRE and Mnq a r e t h e m a g n e t i z a t i o n s of t h e RE, and TM-subnetworks, r e s p e c t i v e l y , Ea-the energy, d i r e c t l y r e l a t e d t o t h e p e r p e n d i c u l a r a n i s o t r o p y energy.

This phenomenon m a n i f e s t s i t s e l f c l e a r l y i f one s t u d i e s t h e temperature dependence of t h e H a l l v o l t a g e . I t i s w e l l known t h a t t h i s v o l t a g e changes i t s s i g n when t h e temperature of t h e f i l m passes through tile compensation p o i n t ( s e e e . g . [ 2 0 ] j . S i n c e t h i s e f f e c t i s u s u a l l y observed under c o n d i t i o n s of a high, a p p l i e d f i e l d , t h e thermal h y s t e r e s i s which can be p r e d i c t e d by analyzing t h e i n e q u a l i t y given above has not been n o t i c e d . More d e t a i l e d i n v e s t i g a t i o n s performed on t h e Ho-CO amorphous f i l m s [Zl] have shown t h e e x i s t e n c e of thermal h y s t e r e s i s of t h e Hall v o l t a g e . ( F i g . 6 ) . Tne width of t h e h y s t e r e s i s loops i s f i e l d - d e p e n d e n t , a s could be expected

Fig. 6 - Thermal h y s t e r e s i s loops of t h e H a l l v o l t a g e ( a f t e r r e f . [ Z l ] ) .

(8)

C6-187

when one considers the inequality discussed above and the known temperature dependence of the net-magnetization of the ferrimagnetic film (Fig. 7 ) .

The effect described above can be utilized in sensors as a safety catch for switching off a device which may be damaged on over- heating and switching it on after it has cooled down to a safe temperature, as has been suggested in [21] . The temperature range required can easily be adjusted by selecting the film composition and/or the magnetizing field.

It has been shown that the Fe-B and Fe-P amorphous alloys exhibit Invar behaviour in a wide temperature range below the Curie temperature [9]. Invar materials have extraordinarily small thermal expansion coefficients; e.g. a = 2.3x10""

(Of40°C) in the case of F eS 5B1 5 [9], whereas for crystalline Fe, for example,

a = 11.7xl0-6 (20°C) . This behaviour, along with their excellent mechanical properties, provides a unique opportunity to utilize amorphous films in bimetallic structures. In addition, use of sputtering or evaporation techniques can easily solve the problem of combining amorphous and crystalline layers. Since sensitivity of bimetallic sensors is proportional to the difference of thermal expansion coefficients of the component used, one can expect a significant improvement of this parameter when utilizing amorphous films instead of conventional materials.

The idea described above has recently been realized (Stobiecki and Stobiecki, to be published) in the form of a bimetallic structure composed of a Fe-B amorphous film sputtered onto a crystalline Al-foil (os = 25*10-6). The very high sensitivity of this structure to small temperature changes has been confirmed.

This result allows us to expect that bimetallic structures utilizing amorphous TM-M films will find their way into various precise control instruments.

Fig.7 - Saturation magnetization vs.

Co-concentration for Gdj_xCox amorphous films (room temperature).

4. OTHER POSSIBILITIES OF APPLICATION

Amorphous films of both types, RE-TM and TM-M, can be utilized as magnetic mirrors in ring-laser gyroscopes [22], giving protection against the undesirable effect which manifests itself as the lock-in of the two light beams to a common frequency.

Since amorphous films exhibit an extraordinarily large spontaneous Hall effect they can be applied as sensitive Hall detectors.

More sophisticated use of these films has also been reported, e.g. the propagation of bubbles created in the films, using the so-called "domain-drag effect" [23].

ACKNOWLEDGMENT

The authors wish to gratefully acknowledge the support given for this work by the Government Grant PR-3.19.

Thanks are due to a number of colleagues from the authors' Institutes as well as.

from collaborating institutions, in particular, to Dr. H.Jankowski, Dr. F.Stobiecki, Dr. A.Kulak, Mr. J.Sokulski, and Mr. J.Samula, for their cooperation in this work.

A word of special thanks is given to Prof. H.Szymczak for a critical reading of the manuscript.

References

[1] P.Chaudhari, J.J.Cuomo, R.J.Gambino, IBM J.Res .Dev. 11_,66 (1973).

[2] D.C.Webb, D.W.Forester, A.K.Ganguly, C.Vittoria, IEEE Trans.Magn. MAG-15, 1410 (1979) .

[3] A.K.Ganguly, K.L.Davis, D.C.Webb, C.Vittoria, Electron.Letters, U_,610 (1975).

[4] D.C.Webb, D.M.Forester, A.K.Ganguly, C.Vittoria, J.Appl.Phys. 47,2696 (1976).

[5] M.Levy, H.Yashida, J .Magn.Mag .Mat. 3S_,139 (1983).

(9)

C6-188 JOURNAL DE PHYSIQUE

[6] T.Fujii, T.Kamura, T.Miyama, Id.Inoue, Jap.J.App.Phys. 22,79 (1983).

[7] R.F.Soohoo, Magnetic Tnin Films, Harper and Row, New Y G ~ , 1965, p.125.

[a] M-Kersten, Z.Phys. 85,708 (1933).

[g] K.Fukamichi, T.;vlasumoto, M. Kikuchi, IEEE Trans .Magn. MAG-15, 1404 (1979) .

[l01 A.E.Clark, H.T.Savage, IEEE Trans.Sonic and Ultrason. c, 50 (1975).

[l11 Ivl.Yamaguchi, K.Y.Hashimoto, H.Kogo, Z<l.Naoe, IEEE TransAagn. MAG-16, 916 (1980).

l121 B.S.aerry, W.C.Pritchet, Phys.Rev.Letters, 34, 1022 (1975).

[l31 H.Szymczak, J .E4agn.?dagn.;4at. 15-18, 585 (1980j .

[l41 K.Twarowski, H.K.Lachowicz, M.Gutowski, H.Szymczak, Phys.Stat.So1. (a), 63, ¹⁰³

(1281).

[l51 R.Zuberek, F.Stobiecki, J.Wosik, Phys.Stat.So1. (a), 82, K177 (1984).

[l61 E.Klokholm, IEEE Trans-Magn. MAG-12, 819 (1976).

[l71 H.K.Lachowicz, H.Szymczak, J.Magn.Mag.i.lat. 41, 327 (1984).

[IS] K.Narita, J.Yamasaki, H.Fukunage, IEEE ~ranGvlagn. W G - 1 6 , 435 (1980).

[l91 J.Samula, Elektronika, nr 9, 32 (1984), in Polish.

[20] H.K.Lachowicz, IEEE Trans .Magn. !.TAG-20, 1417 (1984).

[21] H.Ratajczak, I .Go<ciaCska, Phys .Stat.Sol. (a), 73, 633 (1982).

[22] J.J.Krebs, W.G.Maisch, G.A.Prinz, D.W.Forester, IEEE Trans. on klagn. ;vlAG-16, 1179 (1980 j .

[23] J .C .DeLuca, R .J .Gambino, A.P .Malozemoff, IEEE Trans .on Magn. MAG-14, 500 (1978).