IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-22, NO. 5 , SEPTEMBER 1986 337

MAGNETIC PROPERTIES OF EVAPORATED CoCr FILMS Shigeo Honda* and Jonathan Storer

&IT Sector Laborator ies 3M Center, St. Paul PIN 55144

Abs t rac t - Perpend icu la r magnetic CoCr f i l m s were prepared on glass s u b s t r a t e s us ing electron beam evaporation, The magnetic proper t i es depend s t rong ly on the sgpf t ra te temperature, T * The a n i s o t r o p y f ie ld , H and t h e perpendicufar c g e r c i v i t y , H,(l) show max mum values a t T around 25Q C. In t h i s case, t h e a1 ignment o f c 2 a x i s i s optimal and t h e g r a i n s i r e i s smallest, The l a t t i c e spacing o f the c-planes increases w i t h 8 u n t i l 300'C. When t h e films prepared below 200 were annealed around 300% i n high vacuum, the saturat ion magnetization, M increased. The Val tfe o f Ms, however, decreases'iy anneal i n g above 400 C, I n s p i t e 8 f thgf+ecrease o f M due t o the annealing above 480 C, Hk and H (I) jacrease by annealing above 400 C , I f the f i l d were bombarded by argon i o n s d u r i n g film growth, M decreased and t h e internal stress changed f r o 8 tensile t o compressived For,)pw subst rate temperature (below 150 C), tik decreased due t o i o n bombardment

r

IN7RODUCTXON The magnetic proper t i es l'n CoCr films vary

considerably depending on t h e d e p o s i t i o n c o n d i t i o n s [1,2,3] and post-deposition heat t reatments [4,5,6,716 If the films a r e annealed i n dry a i r a t 300 - 450 C [7], t h e Cr atoms a r e ox id ized p r e f e r e n t i a l l y and t h i s o x i d a t i o n b r i n g s on an r'ncrease i n the s a t u r a t i o n magnet izat ion Ms, the perpendicular coe rc i v i t y , H (1) and the perpendicul ar an iso t ropy enbgy , K

condi t ions; the subs t ra te temperature and ion bombardment onto t h e films dur ing d e p o s i t i o n , and a l so the e f f e c t s o f the heat t r e a t r w n t in high vacuum on the magnetic p r o p e r t i e s .

This paper repo r t s the ef fect ! o f the deposition

EXPERIMENTAL PROCEDURE CoCr films were prepared on glass subst ra tes

wi th the electron-beam evaporation technique. In the vacuum chamber used, s i x glass subs t ra tes were introduced and the films were sequentially deposited. Subst ra te temperature was measured us ing a chrornel-alumel thermocouple in contact w i t h the back surface o f the substrates. The d e p o s i t i o n r a t e was held i n the range u f 100 - 150 R/sec, Thy pressure i n t he vacuum chamber was 0.8 - 1.2 x 10- To r r7 be fore evaporat ion, and ruse t o 2.5 - 5,U x 10- Tor r d u r i n g evapora t ion .

The heat treatment o f deposited fi1ms7was c a r r i e d out i n a vacuum o f 0,8 - 1,O x 10- Tgrr. The annealing temperature was v a r i e d f rom 200 C tu 5 d C and t h e anneal i n g t ime was 2 hours

during f i lm growth, a broad beam i o n source was installed i n t h e vacuum chamber, While the CoCr depos i t i on occurred a t normal inc idencg t h e bombarding Ar ions were incident a t 30 t o the s u b s t r a t e normal. Argon gas was f lowed into t h e ion gun a t t h e r a t e of 5 sccm which resulted i n an increase of chamber pressure t o 1-1.8 x l ou4 Torr. The ion beam acceleration voltage was 500 5 and the ion beam current density was about 1 mA/cm For making c l e a r t h e i o n e f f e c t s , non-bombarded f i l m s and bombarded f i l m s were a1 te rna te l y prepared. Argon gas f lowed i n t o the chamber du r ing a l l o f the s i x depositions.

* Permanent Address: Hiroshima University, Higashi-Hiroshima, 724 Japan

For the study of t h e e f f e c t s o f ion bombardment

RESULTS AND DISCUSSION EFFECTS OF ,SUBSTRATE TEMPERATUREeff

The perpendicular an i so t rop i c f i e l d , Hk and the perpendicular coe rc i v i t y , ti (l), depend strongly on both substrate temperature, f I d u r i n g d e p o s i t i o n and f i l m composition as shown inSFigs. 1 an9 2&r the compositions w i t h M, = 250 - 350 emu/cm Hk and H (1) show maximum values a t T near 300 C, On the other gand, f o r the compositio8Sfpf M = 100 - 230 emufcm the maximum values o f H~ an8 H (1) are obtained a t T around 200 C. C q .

6' I I I I 1 0

* V

2 I

W ul

* Y W

X

3

2 : 0 100 200 300 400 SO@

TS ('C1

FIG.1, Anisotropy f i e l d , Hk e f f f o r the f i l m s 9 prepared a t several subst rate temperatures, Ts

1000 I 0 100 200 300 4OQ 500

T, ("C)

FIG.2, Perpendicular coercivity, H & l ) f o r several substrate temperatures, T

S

The gra in s ize, D, can be estimated using the Scherrer ' s q u a t i on ; where X i s the wavelength 09 x-ray, 28 the peak diffraction angle, and A28 the h a l f - w i d t h o f the

D = O . ~ ~ A / A H p 0 , (1)

d l f f rac t ion peak. The val& o f 628 o f t h e (002) spectrum are shown i n F ig . 3 as a fun o f the s u b s t r a t e temperature. Using h = K,) and 28 = 44 5' for ( O W ) , t h g gra in sf e i s estimated t8 be 320 a f o r A28 = 0.3 and 240 a f o r A28 = 0.4 e 300'C. The h ighes t values o f W (1) around 300 C may be a result o f this smallest g r h n size. Fig, 4 shows the spacing of (002) planes versus substrate temperature. The l a t t l ' ce constant increasgs w i t h T until a maximum o f d = 2,031 1 a t 300 C, The S saturated value o f d (*02) = 2.031 is s l ight ly smalle t h a n the val bg06k bulk Go crystal. d c 2.035 I These small values o f t he lattice ' C8Rikit

The m a l 1 es? g r a i n s grow on substrates5Re&d a t

a 0018-9464/86/0900-0337$0~,00O1986 IEEE

338

correspond t o the s t r o n g i n t e r n a l tensile s t r e s s @,9 f o r f i lys deposibed a t low temperature; cr'= 3.8 x 10 dyne/cm for 100 C. Due t o t h i s s t r o n g i n t e r n a l s t r e s s , the th ick f i l m s depos i ted a t low t e m p e r a t u r e a r e eas i ly peeled o f f . The s t r e s s , however, decreases w i th Ts cor respond ing w q t h t h e i n c r e a s e o f d(oo2) +

I I f I I I

r 1

*

0 100 200 300 400

Ts ( " C )

FIG.3, H a l f - w i d t h , o f the X-ray d i f f r a c t i o n spectrum around the (002) peak for t h e severa l

temperatures, Ts

Y v

2

2 t 1 I 1 0 1ou 200 300 4UO 500

Tb ( 'C1

F I L 4 , Lattice spacing between (002) planes, d(oo2), versus s u b s t r a t e temperature, T s o

ANNEALING EFFECTS The magnetic p r o p e r t i e s a r e al tered considerably

by anneal ing . Typical d a t a a r e shown i n F i g . 5 for f i l gs depos i ted a t 120%. For the films prepared a t 120 C, M i nc reases s l i g h t l y I++ annealing be low 300'C. bn t h e o t h e r hand, He and H (1) va ry i n a para1 1 el fashLrlrg, independen1 o f the Ehange i n M by anneal i ng. H and H c ( l ) i n c r e a s e w i t h the anneal i n g ternberatwe, Ta, up t o 450C and then decrease. Th is b e h a v i o r was observed for a1 1 T,,

S

1moc t o 3UO0C.

4

..-. a!

3 s w 1c1 W

Y

X X

.OOi l o o t

0 I i I 1 I - 0

0 100 200 300 400 500

Ta ( * C )

e f f FIG.5, Changes i n M Hk 9 and H (1) by anneal i n g a t seve ra l t&pgra tu res , T,, f o r the f i 1 ms prepared a t 120 C.

C

sol u b i 1 i t y of Cr i n the E( hcp) phase (jncreases f r o m 13 a t % Cr a t 300C t o 16 a t % Cr a t 500 C - Therefore, t h e Cr atoms which segregated t o the col umn boundar ies [9] w i l l m i g r a t e i n t o t h e b u l k o f the columns by thermal d i f f u s i o n du r ing anneal i n g above 300 C. I f the films wgre prepared on s u b s t r a t e s a t temperatures below 200 C, t h e excess Cr atoms wi l l be quenched i n t h e &-phase w i t h nonequil i b r i um stobhi ametry d u r i ng deposi t i on 5y anneal i ng near 300 C, t h e excess Cr atoms will segregate t o the col umn boundaries f r om the i n t e r i o r s o f t h e &-phase co't umns. Thus, the Ca- r i ch phase i s r e a l i z e d i n t h e columns, wh-ich causes the increment o f M as shown i n F i g . 5,

The o r i g i n s o f the changes i n H and H (1) a r e no t clear a t present , Considerab 'E e change5 I n e i t h e r x-ray d i f f r a c t i o n or apparent s t r a i n s cannot be observed w i th anneakfyg. I t m i g h t be thought t h a t the increase of b o t h H and H (4) may be due t o t h e release o f i n n e r defec k s o r t h e c r e 7 a x a t i o n o f local i n t e r n a l s t rg+ fes by anneal ing. Here, i t shou ld be noted tha toHk and H ( ) i n c r e a s e by anneal i n g around 400 C i n s p i t e .+ o t h e decrease o f M . T h i s f a c t i m p l i e s t h a t t h e segregation o f non-rnsgnetic Cr atoms t o column boundar ies i s not e s s e n t i a l f o r t h e o r i g i n o f t h e an i so t ropy and t h e coercive fo rce ,

S e f f

EFFECTS OF ION-BOMBARDMENT Sputtered f i l m s g e n e r a l l y show f i lm s t r u c t u r e

which i s s t r o n g l y dependent on Ar pressu re [ l o , 111 and b i a s v o l t a g e appl ied t o the s u b s t r a t e [12]. When a f i l m i s sputter-deposited on a ze ro -b ias s u b s t r a t e i n h i g h Ar pressure , columnar s t r u c t u r e i s observed. However, by s p u t t e r i n g i n low Ar pressure [13, 141 or' by a p p l y i n g a b i a s voltage [E, 151, t h e columns become f i n e r and t h e n f i n a l l y become unobservable. I n correspondence w i th the change i n t h e film s t r u c t u r e , the i n t e r n a l s t r e k changes f r o m tensile t o compressive wi th t h e decrease o f A T pressure o r t h e I n c r e a s e o f b i a s v o l t a g e [13 , 151, I n low pressure, t h e film i s bombarded by t h e spu t te red p a r t i c l e s w i t h h i g h energy, and a l s o the p a r t i c l e s w i t h h i g h energy wil l .be implanted i n t o i n t e r s t i t i a l s i tes , Therefore, t h e l a t t i c e cons tan t becomes l a r g e r and compressive stresses appear. An inc rease i n t h e l a t t i c e cons tan t a l s o takes p lace by t h e same mechanism which operates when an i o n g u n prov ides i o n bombardment, namely, f o r w a r d s p u t t e r i n g or II i on-peeni ng" .

I n t h i s exper iment, the s t r e s s changes t o compress ive from t e n s i l e by t h e peening e f f e c t as the fi lm was bombarded by i o n s d u r i n g film growth. The internal s t r e s s s h i f t s t o compressive by ion-bombardment as shown i n Fig , 6, In f i g u r e s 6-10 t h e open c i r c l e s represent i o n gun on and t h e sol i d c i r c l e s , gun o f f . In Fig.6 the o r d i n a t e i s t h e bow i n t h e sample normal ized t o t h e film thickness , AI so , t h e d i s t a n c e d becomes 7 arger as sho n i n F i g . 7. Some val ues (!?$)larger than t h e 2,035 1 o f bul k Co.

1 ' 'Pb = 2UOQC I 1 0 x 1 0 9 l o - - 6

c 0 0 -4 m C O m p K G S S l V c - r, ro H

0 z

-LO + *

I L I I I I 0 100 200 300 4 OD

M~ (emu/crn) 3

Fig .6. Normal i zgd s t r a i n (sampl e bow) v s , M, f o r

P I

The v a r i a t i o n o f M can be expla ined by the phase diagram o f CoCr a h o y system [8], The T = 20OUCr Gun on, off. s s

339

2 051 1 I

0 0 0 0

0

a e/*'*- 0 e -

1 1 2.02

I 0 100 200 300

TS ( * C )

FIG,7, L a t t i c e spacing between (002) planes as a f u n c t i o n o f the s u b s t r a t e tempera ture T, f o r the films prepared w i t h (0 ) and wi thout (@) i on-bombardment .

The change i n M, w i t h t h e t ime a t d e p o s i t i o n u s i n g one Co-Cr a1 loy ingot f o r T = 100 C i s shown i n Fig, 8, Since, when i n the moften s t a t e , t h e vapor pressure o f Cr i s h i g h e r t h a n t h a t of Co, t he Cr content i n the a l l o y i n g o t decreases w i t h evapora t i on time. Therefore, the Cr concentration i n t h e deposi ted f f I m decreases w i th t h e d e p o s i t i o n t ime, which results i n the increase o f M . Th is f i g u r e a l so shows t h a t t he M f o r the id-bombarded f i l m s i s smaller than t h a t f b t h e non-bombarded films having the same composition. Sirnil ar data were obta ined f o r other substrate temperatures. The decrease o f M could i n d i c a t e an increment o f Cr-solution ib t h e -phase by the i o n - m i x i n g e f f e c t .

.,I 0 -I

Evaporatlan T t m e

FIG.8, Sa tu ra t ion magnetization, M for f i l m s prepared on subs t ra tes a t 16% as a f unc t i on o f the evaporat ion time, Gun on, o f f ,

The coercive force, H (L), i s not a f f e c t e d by ion-bombardment and i t d e p h d s on the Cr conten t in the &-phase o r t h e s a t u r a t i o n magnetization, M s 3 a s shown i n Fig. 9, The ma3imum val ues o f Hc (1) were obtained near 300 emu/crn Sirnil ar da t a were obtained f o r other substrate temperatures

On t h e o ther hand, t h e an iso t ropy f i e l d , HFff , i s 8f fec ted st rongly by ion-bombardment f o r T

decrease drastically for a l l composi t ions ion-bombardment. For filrps deposited above 200 C, however, t h e values o f He are lj&f{e changed by ion-bombardment. The rekson why H decreases with ion-bombardment on ly fur T, below If 5OoC i s not clear a t present. Since t h e value o f A0 f o r (002) peak

cannot be a t t r i b u t e d t o crystal 1 ographic c HA anges. I t could be a t t r i b u t a b l e t o local internal stress due t o implanted Ar i o n s f o r low s u b s t r a t e temperature, I f t h e films were deposited on t h e higher temperature substrates, t h e implanted A r ions cou ld be released by an anneal ing e f f e c t dur ing deposi t i and ion bombardment would not be seen t o change Hk

b&+? Hk

w 150 C as shown in Fig. 10. The values o f S

'2f-F hanged l i t t l e by ion-bombardrd?, t h e decrease o f

v

X 20D 1

Hs cemu/cnh

FIG.9. Perpend icu la r c o e r c i v i t y H (1) ve rsus sa tura t ion magngti r a t ion / is f o r f i 1 ms prepared a t 100 C. Gun on, o f f I

1 I

Ms (ernu/cnh

FIE.10. Anisotropy f i e l d , H 5 versus s a t u r a t i o n magnet izat ion, M 'eor films prepared a t 100'~. GU~I on, i3ff.

e f f

cmcwsrm ' In conclus ion i t can be s a i d t h a t anneal i n g can

i nc rease t h e magnetic a n h o t r o p y and c o e r c i v i t y of evaporated CoCr fiIms.[l6] But i f stresses need t o be modi f ied then ion bombardment i n con junc t ion w i t h elevated s u b s t r a t e temperature i s necessary.

He wish t o thank Joel Gerber, Dave Koskenmaki and Chuck Bruzzune f o r t h e i r he1 p.

ACKNOWLEDGEMENTS

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[ Z ] T M e l i n g a and J.C.Lodder, IEEE Trans, Magn,

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[ 5 ] M.Ishizuka, TJomoda, LTsuchimoto, M.Yoshikawa, S.Tshio and MJakahashi, J , Magn. Magn. Mat, 35, 286 (1983).

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McGraw-Hi 1 1 Book Company, New York, T r o n t and London p469 (1958) a

[9] R,Sugita, IEEE Trans, Magn. MAG-20, 687 (1984) , [lo] J,A,Thornton, 3 , Vac, Sci . Tech, 12, 830 (1975) Ell] M.Sagoi, R.Nishikawa and T.Suzuk-i, I E E E Trans.

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Appl. Phys, 52, 2298 (1981). [13] R A B l a n d , G,J.Kominiak and D,M.Mattox, J , Vac,

Sci . Tech, 11, 671 (1974) [I41 LKusuda , S,Honda and M.Ohkoshi 3. Appl a Phys,

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