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CORRELATION BETWEEN CRYSTALLOGRAPHIC
AND MAGNETIC ANISOTROPY OF CoCr THIN
FILMS
P. Heilmann, U. Kullmann, H. Haberkorn
To cite this version:
P. Heilmann, U. Kullmann, H. Haberkorn. CORRELATION BETWEEN CRYSTALLOGRAPHIC
AND MAGNETIC ANISOTROPY OF CoCr THIN FILMS. Journal de Physique Colloques, 1985, 46
(C6), pp.C6-97-C6-100. �10.1051/jphyscol:1985617�. �jpa-00224855�
JOURNAL
DE PHYSIQUEColloque C6, supplbrnent au n09, Tome 46, septembre 1985 page C6-97
CORRELATION BETWEEN CRYSTALLOGRAPHIC AND MAGNETIC ANISOTROPY OF CoCr THIN FILMS
P. Heilmann, U. Kullmann and H. Haberkorn
BASF AG VIT/EM and
ZKM/S,
0-6700 Luduigshafen, F. R. G.Resume
-
Des f i l m s de CoCr o n t e t 6 dGpos6s p a r p u l v e r i s a t i o n cathodique r a d i o - frequence sur des s u b s t r a t s de Si, Al, v e r r e e t polyimide. P l u s i e u r s degres d ' o r i e n t a t i o n p e r p e n d i c u l a i r e des f i l m s o n te t e
obtenus en f a i s a n t v a r i e r l e s paramPtres experimentaux. L ' o r i e n t a t i o n c r i s t a l l o g r a p h i q u e a e t & c a r a c t @ r i s @ e par ~ 0 5 0 , ta n d i s que, dans l e p l a n e t perpendiculairement, l e s aimantations rivnanentes r e l a t i v e s m; e t m# e t a i e n t p r i s e s comme des mesures de l ' a n i s o - t r o p i e magnetique. On montrera que m; e s t l e m e i l l e u r c h o i x pour c a r a c t e r i s e r l ' a n i s o t r o p i e magnetique. Avec des v a l e u r s de m; a l l a n t de 0,05ii
0,2 une c o r r e l a t i o n e t r o i t e avec ~ 0 5 0 a l l a n t delo
ii
lZO
a pu E t r e f a i t e dans l a p l u p a r t des cas. A l o r s que do50 ne c a r a c t e r i s e que l a d i r e c t i o n (002), m; e s t p l u s s e n s i b l e aux c r i s t a u x d e s o r i e n t e s du f i l m de CoCr. Par consequent, l e s deux v a l e u r s donnent des i n f o r m a t i o n s compl&entaires pour c a r a c t e r i s e r t o t a l m e n t 1 'anisotropic p e r p e n d i c u l a i r e des f i l m s CoCr.A b s t r a c t : CoCr f i l m s were d e p o s i t e d by RF s p u t t e r i n g on S i , A l , g l a s s and p o l y i m i d e substrates. Various degrees o f perpendicular o r i e n t a t i o n o f t h e f i l m s w e r e o b t a i n e d by v a r y i n g t h e e x p e r i m e n t a l p a r a m e t e r s . The c r y s t a l l o g r a p h i c o r i e n t a t i o n was ch;racterized by A0 O, w h i l e i n plane and perpendicular r e l a t i v e remanences m and m: were eva7uated as a measure f o r t h e m a g n e t i c a n i s o t r o p y . I t w i l l Ke shown t h a t mr" i s t h e b e s t c h o i c e t o c h a r a c t e r i z e t h e m a g n e t i c a n i s o t r o p y . W i t h mr" r a n g i n g f r o m 0.05 t o 0.2 a c l o s e c o r r e l a t i o n t o r a n g i n g f r o m lo t o 12O c o u l d be f o u n d i n m o s t cases. W h i l e o n l y c h a r a c t e r i z e s t h e ( 0 0 2 ) o r i e n t a t i o n ,
"
i s more s e n s i t i v e t o m i s o r i e n t a t e d c r y s t a l s o f t h e CoCr f i l m . Therefore, m60th values give complementary i n f o r m a t i o n t o f u l l y c h a r a c t e r i z e t h e perpendicular anisotropy o f CoCr f i l m s .I n t r o d u c t i o n
CoCr f i l m s f o r p e r p e n d i c u l a r r e c o r d i n g a r e commonly c h a r a c t e r i z e d by t h e i r c r y s t a l l o g r a p h i c anisotropy. I n most cases the h a l f w i d t h o f t h e r o c k i n g curve of t h e (002) d i f f r a c t i o n peak i s taken as a measure f o r t h e c r y s t a l l o g r a p h i c anisotropy /I/. F o r magnetic recording, t h e magnetic anisotropy i s o f i n t e r e s t , too. Several features o f a h y s t e r e s i s l o o p /2/ can be used t o q u a n t i f y t h e magnetic anisotropy. These are discussed and the b e s t one i s selected. The c o r r e l a t i o n between c r y s t a l l o g r a p h i c and magnetic anisotropy i s shown i n a s e r i e s o f experiments were s p u t t e r c o n d i t i o n s and substrates are varied. The d e v i a t i o n s from the c o r r e l a t i o n , which were found i n some cases, w i l l be discussed.
Experiments
The CoCr f i l m s were deposited by RF s p u t t e r i n g . A water cooled substrate t a b l e was used which r o t a t e d w i t h an a d j u s t a b l e speed o f 0
-
10 rpm under t h r e e t a r g e t s and a h e a t i n g s t a t i o n . A maximum o f 2 kW RF power c o u l d be a p p l i e d t o t h e CoCr t a r g e t ( 8 " d i a m e t e r , 8 1 % Co, 19 % Cr). I n some e x p e r i m e n t s a b i a s v o l t a g e o f up t o 200 V was used. Due t o the r o t a t i n g s u b s t r a t e t a b l e , a d e p o s i t i o n r a t e o f about 6 nm/min c o u l d be achieved. The b a c k g r o u n d p r e s s u r e was b e t t e r t h a n 2 0 1 0 - ' T o r r and t h e a r g o n pressure d u r i n g s p u t t e r i n g was k e p t constant a t 2-
3 mTorr.J O U R N A L DE PHYSIQUE
Glass, A1
,
S i s i n g l e c r y s t a l s and polyimide ( P I ) f i l m s were used as substrates. A l l s u b s t r a t e s were s p u t t e r etched f o r 10 min w i t h 1 kW RF and then a CoCr f i l m o f 0.5 pm thickness was deposited. The s u b s t r a t e temperature was between 150' and 200'~. Determination o f perpendicular anisotropyThe c r y s t a l l o g r a p h i c anisotropy o f t h e CoCr f i l m s was determined by x-ray counter d i f f r a c t o m e t r y u s i n g Cu k, r a d i a t i o n . Scans were u s u a l l y taken i n t h e range o f 38O
<
28 (50' t o cover t h e ( l o o ) , (002) and (101) B$agg peaks o f Co. The (100) and (101) Bragg peaks which u s u a l l y were as weak as 10- o f t h e (002) peak were r e s o l v e d by i n c r e a s i n g the recorder s e n s i t i v i t y . Thus even t r a c e s o f m i s o r i e n t e d c r y s t a l s c o u l d be detected. As a q u a n t i t a t i v e measure f o r t h e c r y s t a l l o g r a p h i c o r i e n t a t i o n o f t h e CoCr f i l m s t h e h a l f w i d t h A 0 o f t h e r o c k i n g curve o f the (002) peak was taken. T h i s curve was measured w i t h angu?ar increments o f 0.25'.I n order t o determine the magnetic anisotropy o f t h e f i l m s , h y s t e r e s i s loops were measured w i t h a v i b r a t i n g sample magnetometer. T y p i c a l h y s t e r e s i s 1 oops perpendicular and p a r a l l e l t o the CoCr f i l m plane are shown i n F i g . 1. While t h e l a t t e r should have no hysteresis, t h e f i r s t should i d e a l l y have
a
r e c t a n g u l a r shape. Consequently t h e c o e r c i v i t y Hc and t h e r e l a t i v e remanence mr" = MrM/MS i n plane should be zero o r as small as p o s s i b l e (Fig. 1 b). To e l i m i n a t e t h e demagnetizing e f f e c t s perpendicular t o t h e f i l m p l ane, t h e remanence:
M
i s d e f i n e d by t h e i n t e r s e c t i o n o f the h y s t e r e s i s l o o p w i t h a s t r a i g h t l i n e through t h e o r i g i n p a r a l l e l t o t h e h y s t e r e s i s l o o p a t M = 0 ( F i g . 1 a). Thus m,? = M;/M, w i l l approach 1 f o r f i l m s w i t h h i g h magnetic anisotropy.F i g . 1: Typical h y s t e r e s i s loops perpendicular ( a ) and i n plane ( b ) o f t h e CoCr f i l m . With b o t h measures o f the magnetic a n i s o t r o p y j m< and ml, problems may occur. The r e l a t i v e remanence m
"
can o n l y be determined w ~ t h singFe l a y e r media since t h e magnetization o f a f i i ~ e under1 ayer o v e r r i d e s the magnetization o f the CoCr f i l m p a r a l l e l t o t h e f i l m plane. T h i s i s n o t t h e case f o r w y l o r i e n t e d NiFe f i l m s measured perpendicular t o the f i l m plane. However, m, has t o be determined g r a p h i c a l l y and, consequently, i s n o t very accurate i n p a r t i c u l a r f o r loops w i t h shoulders /3/ o r which are otherwise d i s t o r t e d . For these reasons, mrU i s p r e f e r r e d as a measure f o r the magnetic a n i s o t r o p y w i t h s i n g l e l a y e r media whereas m,? has t o be used f o r double l a y e r media.C o r r e l a t i o n between c r y s t a l l o g r a p h i c and magnetic a n i s o t r o p y
To f i n d a c o r r e l a t i o n between and m
",
CoCr f i l m s were prepared under a v a r i e t y o f c o n d i t i o n s . I n one s e r i e s o f experimenFs t h e b i a s v o l t a g e Ub d u r i n g s p u t t e r i n g was varied. Fig. 2 shows Af150 and mr" as a f u n c t i o n o f Ub f o r two d i f f e r e n t substrates. For both substrates m and increase w i t h i n c r e a s i n g b i a s voltage. T h i s c o r r e ? a t i o n between bofh a n i s o t r o p i e s can be shown more c l e a r 1 y by p l o t t i n g mrU versus 5 0 (Fig. 31. W i t h i n t h e accuracy o f t h e measurements t h e r e s u l t s show a good correl!?ion.CoCr f i l m s on g l a s s substrates have a poorer anisotropy than on S i s u b s t r a t e s (Fig. 3). This c o u l d be found i n o t h e r experiments, too, where A1 substrates w i t h amorphous Nip c o a t i n g and polyimide f i l m s were a1 so used. F i g . 4 gives t h e r e s u l t s o f a l a r g e
0 25 50 75 100
bias voltage, u b , V
0 5 10 15
half width. Ae5,. degr. F i g . 2: P e r p e n d i c u l a r a n i s o t r o p y as a F i g . 3: C o r r e l a t i o n between mr" and
f u n c t i o n o f t h e b i a s v o l t a g e Ub A850 ( d a t a o f F i g . 2 )
glass
number o f e x p e r i m e n t s w i t h these s u b s t r a t e s where t h e s p u t t e r c o n d i t i o n s were k e p t n e a r l y c o n s t a n t . The b o x e s i n d i c a t e t h e s c a t t e r o f t h e r e s u l t s . Again, t h e r e s u l t s a r e c l o s e a t o r above t h e c u r v e w h i c h i s taken f r o m Fig. 3.
O t h e r p a r a m e t e r s l i k e t h e d e p o s i t i o n speed a n d t h e RF p o w e r , w h i c h a f f e c t t h e s u b s t r a t e t e m p e r a t u r e a n d t h e f i l m t h i c k n e s s w e r e v a r i e d , t o o . No s i g n i f i c a n t and s y s t e m a t i c changes i n 6 8 5 0 and mr" c o u l d be found. A l l r e s u l t s o b t a i n e d i n t h i s i n v e s t i g a t i o n a r e shown i n F i g . 5, t o g e t h e r w i t h t h e c u r v e o f F i g . 3. A1 t h o u g h a general c o r r e l a t i o n between magnetic and c r y s t a l 1 o g r a p h i c a n i s o t r o p y i s c o n f i r m e d , t h e r e e x i s t some samples w h i c h show m a g n e t i c a l l y a l o w e r a n i s o t r o p y as expected b y t h e i r c r y s t a l 1 o g r a p h i c o r i e n t a t i o n .
I n s t e a d o f my", m,? was a1 so c o r r e l a t e d w i t h As d i s c u s s e d above, p r o b l e m s emerge f o r samples w i t h m! c l o s e t o 1. O t h e r w i s e a s i m i l a r c o r r e l a t i o n i s found.
z , 0.2
E
0.5 a,=
L U c 0.4-
a, c a, 0g
0.1 Ce
2
0.3-
S i A14
o glass I I1
2
._
-
a-
a,0
0 2 4 6 8half width, A @ 5 o , degr.
F i g . 4: C o r r e l a t i o n between mr" andAB50. 0 d' I
f o r CoCr f i l m s on S i , A l , p o l y i m i d e 0 5 10 15 ( P I ) and g l a s s s u b s t r a t e s . half widfh, A degr. F i g . 5: C o r r e l a t i o n between mr" and r e s u l t s f r o m d i f f e r e n t experiments. D i s c u s s i o n
The obvious assumption t h a t a c r y s t a l lo g r a p h i c a l l y we1 1 o r i e n t e d CoCr 1 ayer i s a1 so w e l l o r i e n t e d m a g n e t i c a l l y i s c l e a r l y supported by t h e r e s u l t s p r e s e n t e d i n Fig. 5. However i n some c a s e s t h e m a g n e t i c o r i e n t a t i o n i s much l o w e r t h a n e x p e c t e d b y t h e c r y s t a l l o g r a p h i c o r i e n t a t i o n . To e x p l a i n t h i s b e h a v i o u r i t i s necessary t o c o n s i d e r t h e d i f f e r e n t m e a n i n g o f mr" w i t h r e s p e c t t o t h e a n i s o t r o p y o f t h e f i l m . A850 i s a m e a s u r e f o r t h e deg!!! o!et~e o r i e n t a t i o n o f t h e c r y s t a l l o g r a p h i c c - a x i s n o r m a l t o t h e f i l m p l a n e . I t i s n o t a f f e c t e d b y c r y s t a l s w h i c h a r e t o t a l l y o u t o f t h i s o r i e n t a t i o n , e.g. w h i c h show a (100) o r (101) o r i e n t a t i o n .
C6-100
JOURNAL DE
PHYSIQUE
perpendicular o r i e n t a t i o n . Therefore, f i l m s which have a
poi'+
higher mr" value than expected from the A 8 value maym: Ae50, c o n t a i n a number o f misaligned c r y s t a l s wgich do n o t
c o n t r i b u t e t o the h a l f w i d t h o f t h e r o c k i n g curve, b u t have a magnetic moment i n t h e plane o f t h e f i l m and increase mrl'. To
0.40 2.4 prove t h i s assumption, x-ray d i f f r a c t i o n scans o f such f i l m s
were made w;#th t h e h i g h e s t p o s s i b l e s e n s i t i v i t y . I n cases w i t h h i g h mr values, small (100) and (101) peaks c o u l d be detected. F i g . 6 shows f o u r o f such scans which correspond t o
0.16 2.6 t h e f o u r values o f mr" which a r e connected by a broken l i n e
i n F i g . 5. I n a l l Itfour cases, i s approximately 2.5'. With i n c r e a s i n g m (100) o r i e n t a t i o n s can be detected. I n o t h e r cases, (1015 o r i e n t a t i o n s c o u l d be found as w e l l , what
0.09 2.5 makes i t d i f f i c u l t t o q u a n t i f y t h e c o r r e l a t i o n between mr" and t h e m i s o r i e n t a t i o n .
A review o f a l l t h e f i l m s evaluated shows t h a t w i t h "<0.12, no m i s o r i e n t e d c r y s t a l s c o u l d be detected. T h i s i s
0.06 2.4 %dependent o f the ha1 f w i d t h A 9 b u t the exact l i m i t
c e r t a i n l y depends on t h e :ensi tivi?!'of t h e x-ray equipment.
50 45 40 There e x i s t s a minimum mr value a t any value o f be5
.
I t i s28, degr. given by t h e component o f t h e magnetization i n t h e p?ane o f
the f i l m o f those c r y s t a l s which a r e o r i e n t e d s l i g h t l y o f f Fig. 6: X-ray d i f - t h e perpendicular axis. T h i s component can be approximated f r a c t i o n scans o f very r o u g h l y by s i n A 0 which i s i n agreement w i t h th! f i l m s w i t h t h e same minimum curve found i n
pig.
5. Any higher values o f m b u t d i f f e r e n t i n d i c a t e g r o s s l y m i s a l i g n e d c r y s t a l s which are thought t o Le magnetic anisotropy l o c a t e d i n t h e i n i t i a l l a y e r s o f t h e CoCr f i l m .I n /4/ t h e existence o f an i n i t i a l l a y e r i s shown by SEM photos, whereas i n /5/ e l e c t r o n d i f f r a c t i o n was used t o d e t e c t m i s o r i e n t e d c r y s t a l s i n t h e i n i t i a l 1 ayer. A d d i t i o n a l l y , t h e experimental c o n d i t i o n s which l e d t o h i g h m
"
values i n d i c a t e t h a t a misaligned i n i t i a l l a y e r may be involved. I n some cases tge s u r f a c e was h e a v i l y etched b e f o r e s p u t t e r i n g which may have caused r e s p u t t e r i n g from t h e s u b s t r a t e t a b l e and contamination o f t h e substrates. The f a c t t h a t d i f f e r e n t types o f s u b s t r a t e s have very s p e c i f i c values f o r mrl' and A 9 (Fig. 4 ) a l s o i n d i c a t e s t h a t t h e i n i t i a l growth o f t h e f i l m i s r e s p o n s i b l e f o r tgese d i f f e r e n c e s . The w i d e s t range o f r e s u l t s was observed w i t h A1 substrates which m i g h t r e f l e c t problems w i t h t h e surface qua1 i t y , too.Concl usion
The perpendicular anisotropy o f CoCr f i l m s can be c h a r a c t e r i z e d i n two ways. The c r y s t a l l o g r a p h i c anisotropy, measured by t h e h a l f w i d t h A 9 o f t h e r o c k i n g curve o f the (002) Bragg peak shows t h e degree o f o r i e n t a t i o n o f t i e c-axis, whereas t h e magnetic anisotropy, measured by t h e r e l a t i v e remanence inr" i n t h e f i l m plane i s s e n s i t i v e t o m i s a l i g n e d c r y s t a l s . Therefore both measures g i v e complementary i n f o r m a t i o n t o f u l l y c h a r a c t e r i z e the perpendicular o r i e n t a t i o n . A c o r r e l a t i o n between both i s found and can be explained. D e v i a t i o n s from t h i s c o r r e l a t i o n are due t o a d i s t o r t e d i n i t i a l l a y e r which shows up o n l y i n t h e magnetic measurements. A low value o f mr" (
<
0.12) i s t h e r e f o r e necessary t o guarantee a h i g h perpendicular magnetic o r i e n t a t i o n .Acknowledgment
The authors would l i k e t o thank Dr. E. Koster f o r h e l p f u l discussions. References
-saki, K. Ouchi, IEEE Trans.Magn.,Vol.MAG 14, No. 5 (1978) p. 849 (2) R. Sugita, T. Kunieda, F. Kobayashi, IEEE Trans.Magn. ,Vol .MAG 17, No. 6
(1981) D. 3172
( 3 ) T. ~ i e l j n g a , J.C. Lodder, J. Worst, IEEE Trans.Magn., Vol. MAG 18, No. 6, (1982) p . 1107
( 4 ) E. Wuori, J.H. Judy, IEEE Trans.Magn., Vol. MAG 20, No. 5, (1984) p. 774
