MATERIAL CONSIDERATION FOR VERTICAL BLOCH LINE MEMORY

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MATERIAL CONSIDERATION FOR VERTICAL BLOCH LINE MEMORY

G. Ronan, W. Clegg, S. Konishi

To cite this version:

G. Ronan, W. Clegg, S. Konishi. MATERIAL CONSIDERATION FOR VERTICAL BLOCH LINE MEMORY. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-127-C6-130.

�10.1051/jphyscol:1985622�. �jpa-00224869�

JOURNAL DE PHYSIQUE

Colloque C6, supplCment a u n09, Tome 46, s e p t e m b r e 1985 page C6-127

M A T E R I A L CONSIDERATION FOR V E R T I C A L BLOCH L I N E MEMORY

G. Ronan, W. Clegg and S. oni is hi+

Department of EZectrieaZ Engineering, University of Manchester, Manchester M I 3 9PL, U . K .

+~epartment o f EZeetricaZ Engineering, Kyushyu University, Fukuoka, Japm

Resume - Un modPle simplifi6 de la paroi s6parant deux domaines est employe afin d'envisager les exigences d'un materiau qui conduiraient 2 1'6laboration d'un film de grenat 2 bulles typiquement adapt6 aux besoins sp6cifiques de la memoire VBL. Un materiau

large Q, faible epaisseur (h "

et grand a

paraitrait pouvoir optimiser la force gyrotropique du champ aqissant et mini- miser l'attraction VBL paire-paire non-desiree. Cependant, pour minimiser l'amplitude du puits de potentiel et celle du champ agissant, il apparaitrait avantageux d'accroitre la separation entre bits jusqu'2 une valeur de l'ordre de

Sw.

Abstract - ^A s i m p l i f i e d model o f a domain w a l l is employed t o c o n s i d e r some o f t h e m a t e r i a l r e q u i r e m e n t s t h a t would t a i l o r a t y p i c a l bubble g a r n e t f i l m t o t h e s p e c i f i c needs o f

memory.

large Q, low t h i c k n e s s ( h =

h i g h a m a t e r i a l would a p p e a r t o o p t i m i s e d r i v e f i e l d g y r o t r o p i c f o r c e and minimise t h e u n d e s i r a b l e

p a i r - p a i r a t t r a c t i o n . Even s o , t o minimise t h e r e q u i r e d p o t e n t i a l w e l l and d r i v e f i e l d a m p l i t u d e s , it would a p p e a r e x p e d i e n t t o i n c r e a s e t h e b i t s e p a r a t i o n t o be o f t h e o r d e r o f 0 . 8

I n t r o d u c t i o n

I n t h e proposed

memory ( 1 ) t h e p r e s e n c e o r absence o f a p a i r o f n e g a t i v e

i n j e c t e d i n a s t r i p e domain w a l l act as a b i n a r y

o r

A p e r i o d i c in-plane f i e l d p o t e n t i a l w e l l s t r u c t u r e d e f i n e s t h e b i t p o s i t i o n and a p e r p e n d i c u l a r d r i v e f i e l d p r o v i d e s a g y r o t r o p i c f o r c e s u f f i c i e n t t o p r o p a g a t e a VBL p a i r o u t of one p o t e n t i a l w e l l i n t o t h e n e x t . For t h i s d i s c u s s i o n we assume a V8L is r e p r e s e n t e d as a p o i n t magnetic c h a r g e and c o n s i d e r t h e in-plane f i e l d component p a r a l l e l t o t h e w a l l a r i s i n g from convergence ( o r divergenc,e) o f m a g n e t i s a t i o n i n t h e f l a n k i n g Bloch w a l l r e g i o n s . This s o c a l l e d 'a-charge' g i v e s rise t o a non l o c a l e f f e c t which i n t e r a c t s w i t h s i m i l a r p o l e s at d i f f e r e n t p o s i t i o n s o n t h e w a l l . I f we c o n s i d e r a

as a l i n e w i t h r e g i o n s o f w a l l magnekisation a n g l e -0 and

on e i t h e r f l a n k t h e n t h e magnetic moment o f t h e

i s simply

I n c l u d i n g t h e w a l l width, wA, and t h e m a t e r i a l t h i c k n e s s , h , t h e n t h e t o t a l magnetic c h a r g e , m, is g i v e n b y 2rfMAh.

Our model t h e r e f o r e assumes a p o i n t c h a r g e as opposed t o t h e c h a r g e d i s t r i b u t i o n which would r e q u i r e a v e r y much more d e t a i l e d s t u d y , and i g n o r e s v a r i a t i o n o f c h a r g e through t h e t h i c k n e s s . ( T h i s l a t t e r p o i n t s h a l l be c o n s i d e r e d i n c o n c l u s i o n .

The magnetic f i e l d p a r a l l e l t o t h e w a l l r e s u l t a n t from t h i s p o i n t c h a r g e i s m / r 2 , and t h e f o r c e a c t i n g on t h e c h a r g e due t o a magnetic f i e l d is m.H.

A t t r a c t i v e Force Between VBL P u

Let u s c o n s i d e r t h e e f f e c t i v e f i e l d r c s ~ r l t a n t from a p a i r o f n e g a t i v e

as shown i n F i g u r e 1. The f i e l d at p a d i s t a n c e ^x from t h e c e n t r e o f t h e

p a i r is

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985622

J O U R N A L D E PHYSIQUE

Fig. 1. VBL p a i r s i n adjacent b i t p o s i t i o n s at a s e p a r a t i o n of r .

fJ t)

RA nn

f o r t h e VBL spacing, nA,

With r e f e r e n c e t o Figure 1 t h e a t t r a c t i v e f o r c e on a pair o f VBL a d i s t a n c e r from t h e i n i t i a l p a i r i s given by,

where is t h e f i e l d a t p. Therefore

which f r o m equation ( 2

is

S u b s t i t u t i n g A

L / ~ Q and A

c/2lrQ

are l e f t with

p o t e n t i a l W e l l Restoring Force

I f we assume a s i n u s o i d a l p o t e n t i a l -1.1 o f t h e form

a , = K , c o s ~ x ... ^{( 9 )}

then i n a similar manner t o equ. 4 t h e r e s t o r i n g force on a VBL p a i r is FR = -2n2MAAh K, S i n 2

r r

which i s a maximum at x = r / 4 and x

3r/4. I d e a l l y a s F a t t a l/r4 it would be p r e f e r a b l e t o have t h e ^maximum r e s t o r i n g f o r c e a t x

and

r reducing t h e required amplitude o f H , 'and t h u s t h e magnitude o f t h e d r i v e f i e l d .

simple s o l u t i o n would be a t r i a n g u l a r p o t e n t i a l w e l l as s h a l l be discussed a t a later d a t e . For t h e p r e s e n t , however, we s h a l l consider t h e c a s e o f maximum r e s t o r i n g f o r c e a t x

-

r and w i t h t h e s u b s t i t u t i o n s o f equ. 6 and 7,

Drive F i e l d Gyrotropic Force

The g y r o t r o p i c f o r c e a s s o c i a t e d with a perpendicular d r i v e f i e l d is more ambiguous

as it is d i r e c t l y r e l a t e d t o t h e density of VBL i n t h e domain wall. The g y r o t r o p i c

JOURNAL DE PHYSIQUE

f o r c e on a VBL is ( 2 )

vy is t h e domain w a l l v e l o c i t y which from t h e m o b i l i t y of a 'hard' w a l l ( i - e . , r

2 n A )

is

where Hz is t h e sum of perpendicular components of e f f e c t i v e f i e l d s , i . e . , demagnetising, w a l l c u r v a t u r e and applied f i e l d s . Our i n i t i a l assumption f o r fat*

however was t h a t r > > n A which would suppose t h a t t h e wall would move a t a h i g h e r v e l o c i t y although not t h a t o f a ' s o f t ' w a l l given by

As a < < ! t h e r e would appear t o be a considerable u n c e r t a i n t y as t o t h e magnitude of t h e g y r o t r o p i c f o r c e (although t h i s may be estimated by numerical s o l u t i o n s o f domain w a l l motj-on ( 3 ) ) . Combining equ. 13 and 14 one can however s a y t h a t t h i s f o r c e is i n excess of

which is s u f f i c e f o r t h e present d i s c u s s i o n . Materi a 1 Considerations

Comparing t h e t h r e e f o r c e s equations 8,

and

t h e requirements t o be met a r e minimise f a t t so t h a t t h e p o t e n t i a l w e l l magnitude H , is minimised reducing t h e required g y r o t r o p i c f o r c e while maintaining t h e b i t period r low. Of t h e v a r i a b l e s i n question

can b e t a k e n as

and t h e exchange c o n s t a n t

can be assumed constant at

x erg/cm. Immediately obvious is t h a t a low h

3 - 4 ~ ) high Q ( = 4 ) m a t e r i a l reduces t h e r e l a t i v e e f f e c t of Fat- and t h a t a h i g h a

-

i n c r e a s e s F m n . T h i s latter one would i n t u i t i v e l y expect a s l a r g e damping decreases domain d i s t o r t i o n and l a r g e VBL osci.3latory e f f e c t s during propagation ( 3 ) . The upper l i m i t on a can be seen from t h e r a t i o of VBL v e l o c i t y t o w a l l v e l o c i t y

i. e. , a l a r g e damping i n c r e a s e s t h e r a t i o *=11/wL increaqing t h e required w a l l displacement and t h u s H~ (remembering t h a t Hz is an e f f e c t i v e f i e l d r e s u l t a n t from d r i v e , demagnetising and w a l l bending e f f e c t s ) . The c r i t e r i o n high Q is s t r a i g h t forward as one would r e q u i r e l a r g e Q t o maintain VBL s t a b i l i t y .

p o s s i b l e disadvantage of a t h i n m a t e r i a l ( h = 3 - 4 ~ ) might be i n d e t e c t i o n (magnetoresistive d e t e c t i o n of bubble domains) although as it is only t h e d e t e c t o r t h a t is t o be of permalloy a t h i n f i l m d e t e c t o r with high signa1:noise c o ~ l l d b e employed.

I n conclusion t o e s t i m a t e d e n s i t y ( r ) we consider a m a t e r i a l with a

Q

4, h

4 ~ ,

sW/e and

2 x 10-7 erg/cm.

Density

I t would c l e a r l y be d e s i r a b l e t o have F w

F a t t not only t o ensure s t a b i l i t y of

t h e b i t p o s i t i o n s , b u t a l s o f o r s u c c e s s f u l propagation of a random b i t p a t t e r n .

Taking t h e r e f o r e F -

F a t t and combining equations 8 and 12, a rel.ationship

between p o t e n t i a l w e l l depth (%) and b i t p e r i o d : s t r i p e width r a t i o ( r / + ) can b e

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Bit period/Stripe width (r/Sw)

Pig. 3. Density as a function of Fig. 2 . P o t e n t i a l w e l l depth as a function of 1/+2 f o r ( a ) t h e simple b i t p e r i o d : s t r i p e width r a t i o f o r s t r i p charge model, ( b ) t h e width of ( a ) 5 ~ ( b ) 2.5f.m. ( c ) 1 . 0 ~ 1 , ,

revised charge model ( d ) 0 . 5 ~ .

accounting f o r charge variati.on through

t h i c k n e s s and ( c ) r

0.8%.

obtained as shown f o r v a r i o u s s t r i p e widths i n Pig. 2 . The expected p o t e n t i a l w e l l depth f o r decreasing s t r i p e width is shown approximately as an arrow from which an e s t i m a t i o n of t h e b i t period can be made. (The p o s i t i o n i n g of t h e arrow is derived from previous numerical c a l c u l a t i o n s of

propagation ( 3 ) and tlnpublished r e s u l t s , b u t it can be seen t o correspond approximately t o t h e 'knee' of t h e curves.

is expected p o t e n t i a l w e l l depth i n c r e a s e s s h a r p l y with decreasing b i t period and t h e corresponding d e n s i t i e s

l/r x

bits/cm2) vari.es approximately as a function o f l/Sw2. (See Fig. 3.

A

more d e t a i l e d e s t i m a t i o n of

magnetic charge accol1nt.ing f o r v a r i a t i o n of w a l l magnetisation angle through t h e m a t e r i a l t h i c k n e s s , g i v e s a smaller VBL charge and (following t h e same argtment as above) as shown on Pig. 3 a d e n s i t y which approaches 500Ubits/cm2 f o r 0 . 5 ~ s t r i p e width. We must t h e r e f o r e concltde t h a t t h e above model is conservative and t h a t i n p r a c t i c e some gain i n d e n s i t y would be achieved.

However as a r u l e of thumb, h i t spacing of t h e order 0.8% ( f o u r t i m e s t h e VBL p a i r width) could

used. This

considered somewhat analogous t o t h e required bit spacing o f four times t h e bubble diameter i n a conventional bubble device although i n t h a t c a s e t h e r e l e v a n t force i s r e p u l s i v e .

Conclusion

Design r u l e s have been proposed f o r VBL memory using a simple p o i n t charge model.

Although t h e model is considered conservative two important conclusions can be drawn from it. F i r s t l y a low t h i c k n e s s ( h = 4 ~ ) m a t e r i a l reduces t h e r e l a t i v e e f f e c t of t h e mutual a t t r a c t i o n between VBL p a i r s and secondly t h e b i t period should be of t h e o r d e r 0.8 &. I n comparison t o a conventional bubble m e m o r y c e l l a r e a (16 h 2 ) an i n c r e a s e i n d e n s i t y o f twenty f o l d is expected. Considerations o f f a b r i c a t i o n requirements f o r such a device while beyond t h e scope o f t h i s p r e s e n t work a r e t o be covered a t a l a t e r d a t e .

References

1. Konishi, S., I.E.E.E. Trans. Magn., Mag-19, p. 1838, L983.

2. Malozemof f and Slonczewski, "Magnetic Domain Walls i n Bubble Mat.erialS", Applied S o l i d S t a t e Science, p. 162, Academic Prt?Ss InC., 1979.

3. ~ a t s u y a m a , K. and Konishi,

Trans. Magn., Mag-20, p. 1141, 1984.

4. Konishi, S . , Matsuyama, K . , Chida, I . , Kubota, S . , Kawahara, H., and Ohbo, M.,

I.E.E.E. Trans. Magn., Mag-20, p. 1129, 1984.