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48 IEEE TRANSACTIONS ON MAGNETICS, VOL. 26, NO. 1, JANUARY 1990

S T R I P E DOMAINS I N N i C o O B L I Q U E L Y D E P O S I T E D F I L M S

H . A I T L A M I N E , M. LABRUNE and 1.B.PUCHALSKA

P O T E N T I A L CANDIDATES FOR INFORMATION STORAGE

C N R S - L a b o r a t o i re de M a g n e t i sme e t d I Opt i que des Sol ides 92190 MEUDON (FRANCE).

ABSTRACT

We have i n v e s t i g a t e d i n d e t a i l s t rong and weak s t r i p e domains which appear i n o b l i q u e l y deposi ted NiCo f i l m s above a c r i t i c a l th ickness f o r va r ious composit ions and angles o f depos i t i on i n o rde r t o t e s t t h e a b i l i t y of such s t r u c t u r e s f o r i n fo rma t ion storage.

I - INTRODUCTION

The magnetic p r o p e r t i e s o f o b l i q u e l y deposi ted NiCo f i l m s have received a l a r g e i n t e r e s t as h i g h d e n s i t y media f o r l o n g i t u d i n a l record ing. Recent ly an a l t e r n a t i v e has been proposed by Puchalska e t a l l us ing a modulat ion o f t h e s t rong s t r i p e domains t o s t o r e t h e i n fo rma t ion i n such media. Above a c r i t i c a l t h i ckness s t rong or /and weak s t r i p e s , which a r e c h a r a c t e r i s t i c of f i l m s w i t h an o b l i q u e l y induced anisot ropy, occur. The behaviour o f such s t r u c t u r e s and t h e a b i l i t y t o use them as i n fo rma t ion s torage a re analysed. Weak* and s t rong3-4 s t r i p e s a re w e l l known s t r u c t u r e s a l ready observed on s o f t NiFe a l l o y s . Magnetic models were presented by Hara5 (see a l s o Malozemoff e t a16) and a re shown on f i g . 1.

4-d w

Fig.1: a) Geometry, b)Model o f t h e s t rong s t r i p e domains. I n t h e b u l k t h e bas i c domains a re a n t i p a r a l l e l and magnetized along a t i l t e d a x i s near t h e columnar ax i s . c) Weak s t r i p e s t r u c t u r e , i n t h e b u l k t h e magnet izat ion osc i 1 l a t e s

around t h e sample p1ane;meanwhile t h e sur face domains, t h e magnetic f l u x i s n o t c losed along t h e s t r i p e d i r e c t i o n .

El

I I-EXPERIMENTAL

Ni,Co,-, a l l o y s ( w i t h x=0.8, 0.5 and 0.2) have been prepared by thermal evaporat ion f o r d i f f e r e n t th icknesses D ranging from 0.1 t o 2pm and f o r va r ious angles o f inc idence a o f t h e vapour beam w i t h respect t o t h e normal t o the subst rate, a being from 0" t o 60". The specimen were deposi ted on to rock s a l t o r g lass subs t ra tes a t room temperature i n a vacuum o f

Manuscript received September 5 , 1989.

t o

Torr . The magnetic s t r u c t u r e s have been i n v e s t i g a t e d by means o f e l e c t r o n microscopy, Ke r r e f f e c t and B i t t e r p a t t e r n . Hysteres is loops have been p l o t t e d by VSM and an iso t ropy constants measured by torque magnetometer. The p r o p e r t i e s o f t h e f i l m s discussed i n t h i s paper a re as fo l l ows :

0.2 60 0.96 0.5 28 4.3 0.2 60 1.6 0.7 1100 0.5 50 1.3 1.1 850

0.8 40 0.94 0.35 600 14 0.53 .2

P roper t i es o f Ni,Co,-, f i l m s : X a t% o f Ni,a angle o f inc idence o f t h e vapour beam, D th ickness (pm), W s t rong s t r i p e p e r i o d i c i t y (pn), MS s a t u r a t i o n magnet izat ion (emu/cc), P an iso t ropy a x i s w i t h respec t t o t h e normal t o t h e subs t ra te , KO ob l i que an iso t ropy ( l o 6 ergs/cc) and Q=Ko/2i;MS2.

11-1. Ni70@,0 system:

For t h i s composit ion i t was shown',' t h a t s t r o n g s t r i p e s o n l y occur f o r angles o f depos i t i on &60° and th ickness Dr1000A. P e r i o d i c i t y o f t h e s t r u c t u r e W, f o r va r ious f i l m th ickness D , i s shown on f i g . 2 . I n t h i s composi t ion weak s t r i p e s have n o t been observed what may be due t o magne tos t r i c t i ve e f f e c t as proposed by Pat terson and Mul ler* .

i w= 0 /

-0 O O , 'P

Fig.2: St rong s t r i p e s w i d t h W versus f i l m t h i ckness D (Ni20C080 , a=60°).

Ker r e f f e c t observat ions o f t h e s t rong s t r i p e s t r u c t u r e have been made on t h i c k samples ( A 1 and A2 i n Table) . The magnetic s t r u c t u r e i s v i s i b l e by l o n g i t u d i n a l , t ransve rse o r p o l a r Ke r r e f f e c t , w i t h a weak c o n t r a s t i n t h e l a s t case. These observat ions c l e a r l y i n d i c a t e t h a t a weak perpendicu lar component o f magnet izat ion M s t i l l remains i n t h e v i c i n i t y o f t h e sur face. Moreover t h e i n -p lane component d i r e c t i o n i s no t f i x e d as i n t h e Hara model ( f i g . l b ) b u t r a t h e r r o t a t e s cont inuously . We s h a l l d iscuss t h i s mechanism i n more d e t a i l l a t e r i n t h e case o f specimen B .

OO18-9464/90/01OO-0048$01 .OO 0 1990 IEEE

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49

11-2. Ni,,@,, system:

The magnetic behaviour of this composition is similar to that described previously. The shape of the hysteresis loops is equivalent except than for the values of the critical fields (coercivity, saturation field) as well as the induced oblique anisotropy which are lower here. The strong stripe domains occur for angles of deposition ~ 4 0 " and above a critical thickness of the sample D=2700A. Morever the Kerr effect observations reveal the same basic characteristics already described for Ni2,CoaO. Figs.3a and b show a regular strong stripe structure obtained after demagnetizing the sample in a field along the OX axis (see Fig.la) by longitudinal Kerr effect. For the microscopique observations, the sample is placed so that the stripes lie either perpendicular (fig.3a) or parallel (fig. 3b) to the incident plane of light.

a

PI I 5 P U

b

Fig.3 : Stripe domains in the demagnetized state observed by longitudinal Kerr effect. a) the incident plane of light ( P I ) is perpendicular to the stripes , b) PI is parallel -(sample B 1 ) - .

The formation of the Kerr contrast due to the distribution of the magnetization on the surface is qualitatively explained and depicted in Fig.4 . Figs.4a b show the magnetization distribution and the expected contrast respectively in the case of the Hara's model (fig.lb).

L

C

WI

Fig.4: a) in-plane magnetization distribution and b) expected Kerr contrast c) proposed model and d) resulting contrast. P I : direction of the incident plane of light, the strong stripes lie along the OX axis.

from the Hara model.

We see (fig.4b) that no contrast is expected when the stripe lies parallel to the incident plane of light. This is not what we observe (fig.3b). Therefore we think that Hara's model is not complete . I n fig. 4c is depicted our proposal model of the distribution of the in-plane magnetization component near the surface. This distribution is the first step for the understanding of a more general and completely continuous transition from the basis domains to the surface,as already proposed by Hubertq. Now, in this new situation, we can explain what is observed on the Kerr effect pattern: alternate black and white lines over a grey background. The contrast of the lines originates from areas where M rotates and the grey contrast from areas where M is perpendicular to the direction of elongation of the stripes. We have made the first attemps to check whether a modulation o f this magnetic pattern is possible or not. For this reason we have applied a magnetic field perpendicular and next parallel to the stripes

Field applied perpendicular to the stripes:

Modulation of the stripe domains has been observed (see Fig.5a and b, sample Bl): Every alternate domain has been observed t o shrink while the other to widen. For larger values of the field the structure is broken and some isolated small domains (bubble-like) occur (Fig;5c and d) .Simultaneously stripes parallel to the field are nucleated similar to a weak stripe domain pattern. This last structure gives no contrast when we rotate the sample of 90" under the microscope. This behaviour is quite different to the evolution of the Hara model where no modulation of the pattern was assumed.

PI

5 C"m n Fig.5: Modulation of the stripe pattern under a field applied perpendicular to the stripe direction a) H=+100 Oe. , b) H=-100 Oe. In c) and d) the stripes are placed parallel to P I . c) H=100 Oe. , d) H=140 Oe. -sample B1-

Field applied parallel to the stripes:

Under this condition, no modulation is observed (Fig.6a

50

_

-

sample 82) .Black and wh i te s t r i p e s ma in l y keep t h e same w id th under an app l i ed f i e l d . Wi th i nc reas ing f i e l d t h e p a t t e r n i s broken g i v i n g r i s e t o b u b b l e - l i k e domains. s t a r t i n g from t h e sa tu ra ted s t a t e a decrease of t h e app l i ed f i e l d induces t h e n u c l e a t i o n o f an a r r a y of bubb le - l i ke domains which coalesce i n lower f i e l d l ead ing t o a s t rong s t r i p e s t r u c t u r e i n zero f i e l d .

-7 -~ ~ ~~ 1 ................ % ................ s... . !

_ . ..................................... . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I I r---- -7 - 1 R I

Fig.6: E f f e c t o f an app l i ed f i e l d p a r a l l e l t o t h e s t r i p e p a t t e r n . a) H=210 Oe., b) H=250 Oe. (sample B2)

11-3. Ni..&, system

The magnetic behaviour o f t h i s composi t ion s t r o n g l y d i f f e r s f rom t h a t o f t h e formers and r e c a l l s t h e s t r u c t u r e which was observed i n Permalloy f i l m s . St rong s t r i p e s appear f o r &20" and D=lOOOA. I n t h i s case a s t rong s t r i p e s t r u c t u r e (main ly p a r a l l e l t o OX) and weak s t r i p e s ( e longated p a r a l l e l t o t h e app l i ed f i e l d ) have been c l e a r l y observed e i t h e r by l o n g i t u d i n a l Ke r r e f f e c t o r by d i r e c t i nspec t i on o f t h e h y s t e r e s i s loops (Fig.7 sample C).

,f I

- 1 - 2

Fig .7 : Hysteres is loops obta ined f rom specimen C w i t h t h e app l i ed f i e l d i n t h e p lane o f t h e sample, a) p a r a l l e l and b) pe rpend icu la r t o t h e p lane o f inc idence o f t h e vapour beam.

F i e l d app l i ed pe rpend icu la r t o t h e s t r i p e s :

I n t h i s s i t u a t i o n o n l y t h e weak s t r i p e s t r u c t u r e i s observed. Two d i f f e r e n t s t a t e s may occur accord ing t o t h e d i r e c t i o n o f t h e app l i ed f i e l d a long t h e OY ax i s . The t r a n s i t i o n between these two s t a t e s i s r a t h e r s o f t ( see t h e change o f t h e s lope near Hc i n t h e corresponding h y s t e r e s i s loop) so t h a t f o r moderate app l i ed f i e l d s we can get a m i x t u r e o f these two s ta tes .

F i e l d app l i ed p a r a l l e l t o t h e s t r i p e s

If now t h e f i e l d i s app l i ed a long t h e OX a x i s we can c rea te weak or/and s t rong s t r i p e s . From t h e sa tu ra ted s t a t e , a decrease o f t h e f i e l d induces t h e n u c l e a t i o n of weak s t r i p e s over a l l t h e sample. For lower f i e l d s t rong s t r i p e s occur f i r s t as l ong w i res which f i n a l l y cover t h e whole sur face near zero f i e l d . The beginning of t h e t r a n s i t i o n from weak t o s t rong , which correspond t o t h e opening o f t h e h y s t e r e s i s loop, i s i l l u s t r a t e d i n f i g . 8 .

PI t H I H 5C"m

Fig.8: Weak t o s t rong s t r i p e s t r a n s i t i o n observed by Ker r e f f e c t on sample C (Ni,.Co,. , a=40").

We use here t h e image s u b t r a c t i o n technique. The s to red reference image corresponds t o t h e weak s t r u c t u r e alone, t h e second image t o t h e mixed weak/strong state.The image s u b t r a c t i o n corresponds t o t h e Ker r micrograph of f i g . 8 where t h e s t rong s t r i p e s appear as l ong b lack w i res w h i l e t h e weak s t r u c t u r e (not v i s i b l e on t h e micrograph due t o t h e image s u b t r a c t i o n ) occupies t h e un i fo rm grey area.

I I I CONCLUSION

Ob l ique ly deposi ted NiCo f i l m s have been inves t i ga ted , and a c o n t r i b u t i o n t o a more general s t rong s t r i p e model has been presented. We have shown t h a t a modulat ion o f t h i s s t r u c t u r e i s p o s s i b l e e s p e c i a l l y f o r app l i ed f i e l d pe rpend icu la r t o t h e s t r i p e d i r e c t i o n . M a t e r i a l s w i t h h ighe r c o e r c i v i t y a re needed f o r app l i ca t i on . For t h i s purpose i t i s d e s i r a b l e t o modify t h e p repara t i on cond i t i ons (e.g. use o f C r sublayer) what i n consequence cou ld r e s u l t a l ess cont inuous media. The quest ion i s whether i n such a media t h e s t r i p e domains would be s t i l l e x i s t i n g ? I n t h e course of t h i s work new ideas came t o l i g h t such as: ( i ) t h e oppor tun i t y t o use weak s t r i p e domains of a l t e r n a t e p o l a r i t y f o r N i r i c h a l l o y s (par.11-3A) o r (ii) t h e a b i l i t y o f t h e i s o l a t e d b u b b l e - l i k e domains obta ined a t h igh f i e l d f o r c o b a l t r i c h f i l m s (par.11-2) t o be used as i n fo rma t ion s torage.

ACKNOWLEDGEMENTS

The authors would l i k e t o thank B.Mirecki f o r t e c h n i c a l ass is tance and Profs . A.Fert and I.Campbel1 ( f rom Labora to i re de Physique des So l i des -0rsay- ) t o g i v e access t o t h e i r torque balance.

REFERENCES

(1) I.B. Puchalska, A.Hubert, S.Winckler and B . M i r e c k i : 1EEE.Trans.Mag. MAG24 (1988) 1787. (2) R.J.Spain:Appl. Phys. L e t t e r s 3 (1963) 208 ( 3 ) 1.B.Puchalska and R.P.Ferrier: Th in S o l i d F i lms 1 (1967) 437. (4) R.P.Ferrier and 1.B.Puchalska: Phys. S ta t . (1968) 335. ( 5 ) K.Hara:J.Sci.Hiroshima Univ.Ser. A I I , 34 ( (6) A.P.Malozemoff, W.Fernenge1 and A. Brunsch 12 (1979) 201. (7 ) H.Alt lamine, M.Labrune and I .B. Puchalska: o f t h e 12-ICMFS,Le Creusot( l988) TP22 (8) R.W.Patterson and M.W.Muller: Int.J.Mag 3( (9) A.Hubert :I.E.E.E. Trans. Mag. MAG21 (1985

Sol . 28

970) 139 J.M.M.M.

Proceed.

972) 293 1604.