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SPUTTERED NdFe FILMS WITH PERPENDICULAR ANISOTROPY
R. Carey, P. Crapper, B. Thomas
To cite this version:
R. Carey, P. Crapper, B. Thomas. SPUTTERED NdFe FILMS WITH PERPENDIC- ULAR ANISOTROPY. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-111-C6-115.
�10.1051/jphyscol:1985620�. �jpa-00224867�
J O U R N A L DE PHYSIQUE
Colloque C6, suppl6ment au n09, Tome 46, septembre 1985 page C 6 - I l l
SPUTTERED N d F e FILMS WITH PERPENDICULAR ANISOTROPY
R. Carey, P. Crapper and B.W.J. Thomas
Coventry (Lanchester) PoZytechnic, Priory S t r e e t , Coventry CV1 5FB, U . K .
RQsumd
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Des f i l m s minces de Nd-Fe avec une c o n c e n t r a t i o n en Nd d ' e n v i r o n 60 a t %, prdpards p a r p u l v 6 r i s a t i o n r . f . s u r des s u b s t r a t s e n v e r r e 1 tempdrature ambiante, o n t 6td d t u d i d s par e f f e t Kerr p o l a i r e . Tous l e s f i l m s~ r d s e n t e n t des c y c l e s d t h y s t 6 r d s i s r e l a t i v e m e n t c a r r d s , l e champ d t a n t i n v e r s 6 s e l o n l a normale aux f i l m s , avec des r e c t a n g u l a r i t 6 s proches de 1 , e t une r o t a t i o n Kerr p o l a i r e 3 s a t u r a t i o n d ' e n v i r o n 0,4'. Quelques f i l m s obtenus avec une grande v i t e s s e de p u l v 6 r i s a t i o n p r 6 s e n t e n t de fason i n h a b i t u e l l e de f o r t s champs c o e r c i t i f s , 1 B 4 kOe, e t une n e t t e diminution du pouvoir r 6 f l e c t e u r . I1 a & t d montrb, au microscope d l e c t r o n i q u e 1 balayage, que c e t t e d e r n i P r e p r o p r i d t 6 e s t a s s o c i d e 1 l a formation de f i n e s
~ a r t i c u l e s 3 l a s u r f a c e .
A b s t r a c t - Thin f i l m s o f NdFe w i t h composition i n t h e r e g i o n o f 60 a t . % Nd have been prepared by r . f . s p u t t e r i n g on t o g l a s s s u b s t r a t e s a t room
t e m p e r a t u r e and i n v e s t i g a t e d u s i n g t h e p o l a r Kerr, e f f e c t . A l l f i l m s e x h i b i t r e l a t i v e l y square h y s t e r e s i s loops when t h e f i e l d is r e v e r s e d a l o n g t h e f i l m normal, many with squareness r a t i o s c l o s e t o 1 . 0 , and s a t u r a t i o n p o l a r Kerr r o t a t i o n s about O . Q O . Some f i l m s prepared a t very high d e p o s i t i o n r a t e s e x h i b i t unusually l a r g e c o e r c i v i t i e s , 1-4kOe, and show a marked r e d u c t i o n i n s u r f a c e r e f l e c t i v i t y which a Scanning E l e c t r o n Microscope i n v e s t i g a t i o n showed t o be a s s o c i a t e d with t h e o n s e t o f f i n e p a r t i c l e formation a t t h e s u r f a c e .
INTRODUCTION
S i n c e t h e f i r s t r e p o r t on melt-spun NdFe a l l o y s by Croat"), c o n s i d e r a b l e e f f o r t has been d i r e c t e d a t t h e development o f a new c l a s s o f high-performance r a r e e a r t h - t r a n s i t i o n metal (RE-TM) permanent magnet m a t e r i a l s based on NdFe and PrFe.
Concurrent w i t h t h i s development h a s been a growth o f i n t e r e s t i n t h e p o s s i b i l i t y o f t h e a p p l i c a t i o n o f l i g h t RE-TM a l l o y f i l m s t o high d e n s i t y d a t a s t o r a g e . Compared w i t h t h e e f f o r t expended on t h e s t u d y o f heavy RE-TM a l l o y t h i n f i l m s such a s Gd-Co and Tb-Fe, t h a t d i r e c t e d towards t h e s e f e r r o m a g n e t i c a l l y coupled a l l o y s has been almost i n s i g n i f i c a n t . Gronau ( 2 ) however h a xamined t h e in-plane magnetization p r o p e r t i e s o f Sm-Co f i l m s w h i l s t Lee e t a 1
T37
have r e p o r t e d evidence o f perpendic- u l a r a n i s o t r o p y i n SmxCo,oo-x f i l m s when 20 < x < 30. Although Tsutsumi andSughara ( 4 ) r e p o r t l i t t l e s u c c e s s i n developing p e r p e n d i c u l a r a n i s o t r o p y i n s p u t t e r e d NdxFe w i t h x = 0.34, t h e y p o i n t o u t t h a t t h e a d d i t i o n o f a s m a l l q u a n t i t y o f T i
1 -x
does l e a d t o t h e production o f f i l m s w i t h p e r p e n d i c u l a r a n i s o t r o p y .
Recently ( 5 ) we have r e p o r t e d t h e p r o d u c t i o n o f high c o e r c i v i t y Ndo .gFeo .s f i l m s
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985620
JOURNAL
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PHYSIQUEe x h i b i t i n g a s i g n i f i c a n t p o l a r Kerr r o t a t i o n and a squareness r a t i o
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1 . 0 f o r hyste-.r e s i s l o o p s measured p e r p e n d i c u l a r t o t h e f i l m p l a n e . I n t h i s paper we r e p o r t r e c e n t developments i n t h i s work on r . f . s p u t t e r e d NdFe f i l m s .
EXPERIMENTAL
Thin f i l m s i n t h e t h i c k n e s s range 1 - 3 D have been p r e p a r e d by r . f . s p u t t e r i n g i n a Nordiko NM2000 s p u t t e r i n g module o n t o w a t e r cooled g l a s s s u b s t r a t e s from a
4
i n c hNdFe a l l o y t a r g e t . A p r o t e c t i v e l a y e r o f St02 i s s p u t t e r e d on t o t h e s u r f a c e o f t h e f i l m s b e f o r e t h e y a r e removed from t h e system. Film d e p o s i t i o n r a t e s ranged from 27 nm/min t o 44 nm/min with background argon p r e s s u r e s maintained a t -. 1 5 x Torr from a n u l t i m a t e background p r e s s u r e of 8 x T o r r . Film composition and s u r f a c e s t r u c t u r e i n v e s t i g a t i o n s were performed i n a P h i l i p s SEM505 scanning , e l e c t r o n microscope w i t h a n energy d i s p e r s i v e X-ray a n a l y s i s f a c i l i t y . The
m a w e t i c p r o p e r t i e s of t h e f i l m s were i n v e s t i g a t e d u s i n g t h e p o l a r Kerr e f f e c t with d . c . d r i v e f i e l d s
'
lOkOe and a He-Ne l a s e r s o u r c e (1 = 0.6328 urn).RESULTS
A l l t h e f i l m s r e p o r t e d h e r e were measured t o b e 60/40 a t . % Nd-Fe t o w i t h i n f 2%.
The p o l a r Kerr e f f e c t h y s t e r e s i s l o o p f o r a
1.5
f i l m prepared a t t h e lower d e p o s i t i o n r a t e (27 nm/min) i s shown i n F i g u r e 1. The f i l m i s o f uniform r e f l e c t - i v i t y over t h e whole s u r f a c e .Fig.1. P o l a r Kerr h y s t e r e s i s l o o p f o r 1 . 2 p f i l m prepared a t 27 nm/min.
Films prepared a t t h e h i g h e r d e p o s i t i o n r a t e a l l showed l a r g e s u r f a c e a r e a s o f reduced r e f l e c t i v i t y . Although no s i g n i f i c a n t change i n composition can b e
d e t e c t e d a c r o s s t h e whole o f t h e f i l m , t h e p o l a r h y s t e r e s i s l o o p s f o r t h e s e samples showed marked d i f f e r e n c e s from p o i n t t o p o i n t a c r o s s t h e f i l m s u r f a c e . F i g u r e 2 shows such v a r i a t i o n i n t h e h y s t e r e t i c behaviour of a 2.6 u m t h i c k f i l m a s t h e i n t e r r o g a t i n g l a s e r beam i s moved from ( a ) a h i g h r e f l e c t i v i t y r e g i o n t o ( b ) one of lower r e f l e c t i v i t y . The c o e r c i v i t y ( H ~ ) o f t h e l a t t e r r e g i o n i s very l a r g e 3.9 kOe and t h e s a t u r a t i o n p o l a r Kerr r o t a t i o n (
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0.41°) i s c l o s e t o t h a t of Fe. The squareness r a t i o i s e s s e n t i a l l y 1 . 0 .Figure 2 ( a ) . Polar Kerr h y s t e r e s i s loop f o r high r e f l e c t i v i t y region o f
2.6
um f i b prepared a t44
nm/min.Figure 2 ( b ) . Polar Kerr h y s t e r e s i s loop f o r low r e f l e c t i v i t y region of 2.6 yrn f i l m prepared a t
44
nm/min.Detailed examination of t h e r a p i d l y deposited f i l m s i n t h e SEN revealed marked d i f f e r e n c e s i n t h e s u r f a c e s t r u c t u r e between t h e regions o f high and low r e f l e c t - i v i t y . Figure 3 shows t h e progressive growth o f p a r t i c l e s a t t h e f i l m s u r f a c e t o g e t h e r w i t h t h e changing h y s t e r e t i c p r o p e r t i e s f o r a 1 ym t h i c k f i l m a s t h e l a s e r spot i s moved from ( a ) t h e highly r e f l e c t i n g region through t o ( c ) where t h e r e f l e c t i v i t y i s a t i t s lowest. The p a r t i c l e s i n ( a ) a r e q u i t e evenly d i s t r i b u t e d with estimated diameters
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0.2 pm, w h i l s t t h o s e i n ( c ) a r e again evenly d i s t r i b u t e d w i t h diameters-
0.5 ym. No major change i n composition can be d e t e c t e d between t h e s u r f a c e o f t h e highly r e f l e c t i n g region and t h e i n d i v i d u a l p a r t i c l e s i n 3 ( c ) . Highly r e f l e c t i n g f i l m s prepared a t low d e p o s i t i o n r a t e s have subsequently been subjected t o a range o f annealing t r e a t m e n t s w i t h temperatures up t o " 6 0 0 ~ ~ . Although t h e h y s t e r e t i c behaviour o f t h e f i l m s has been s i g n i f i c a n t l y changed bysuch annealing c y c l e s - t h e h y s t e r e s i s loops becoming g e n e r a l l y m e & e & w i t h r e b e d c o e r c i v i t y
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no evidence of p a r t i c l e growth has y e t been observed.C6-114 JOURNAL
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PHYSIQUEFigure 3 . C o r r e l a t i o n between p a r t i c l e s i z e and h y s t e r e t i c p r o p e r t i e s o f NdFe f i l m s . The d i r e c t i o n o f t h e magnetic f i e l d i s i n t h e o p p o s i t e d i r e c t i o n t o f i g u r e s 1 and 2 .
DISCUSSION
The o n s e t o f h i g h c o e r c i v i t y and squareness r a t i o s c l o s e t o u n i t y i n r a p i d l y d e p o s i t e d Ndo.6Feo.4 f i l m s h a s been shown t o c o r r e l a t e with t h e formation o f s p h e r i c a l p a r t i c l e s o f diameter
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0 . 5 p n .Croat'') h a s p r e v i o u s l y r e p o r t e d a l a r g e i n c r e a s e i n Hc t o g e t h e r w i t h a d e c r e a s e i n Ms f o r NdxFeLpx melt spun a l l o y s a s x is i n c r e a s e d . He a s s o c i a t e d t h e i n c r e a s e d
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c o e r c i v i t y i n Ndo.sFeo.6 w i t h t h e f o r m a t i o n o f f i n e c r y s t a l l i t e s o r c l u s t e r s w i t h s i z e s c l o s e t o t h e s i n g l e domain p a r t i c l e optimum. I n Ndo. 3 5 ( ~ e ~ ) o . e 6 , t h e optimum c h a r a c t e r i s t i c s were me&zyed when t h e c r y s t a l l i t e s were roughly s p h e r i c a l with w i t h diameters 20-80 nm
.
Bearing i n mind t h e g r e a t l y i n c r e a s e d Nd c o n t e n t o f t h e f i l m s o f Ndo.6Feo.4 s t u d i e d h e r e and t h e a s s o c i a t e d r e d u c t i o n i n t h e s a t u r a t i o n m a g n e t i z a t i o n , it seems l i k e l y t h a t t h e p a r t i c l e s observed a r e c l o s e t o t h e c r i t i c a l s i z e f o r s i n g l e domain behaviour f o r t h i s a l l o y and t h a t t h e o r i g i n o f t h e h i g h c o e r c i v i t y i s s i m i l a r t o t h a t f o r t h e melt spun Nd-Fe a l l o y s .
The degree o f p a r t i c l e growth observed i s thought t o depend c r i t i c a l l y on l o c a l t e m p e r a t u r e v a r i a t i o n s a t t h e s u b s t r a t e s u r f a c e d u r i n g t h e d e p o s i t i o n p r o c e s s . To d a t e we have been unable t o c r e a t e a s i m i l a r t e m p e r a t u r e environment t h a t w i l l i n i t i a t e such p a r t i c l e growth o u t s i d e t h e s p u t t e r i n g system.
REFERENCES
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,
App.Phys.Letters 3 9 ( 4 ) , 357, 1981.( 2 ) Gronau, M . , S c h u f f l e r , D. and Sprenger, S., I.E.E.E. Trans Mag., MAG-20, 66, 1984.
( 3 ) Lee, Z. Y . , Numata, T., Inokuchi, S. and S u k a r a i , Y . , I.G.E.E. Trans Mag., MAG-20, 1335, 1984.
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Tsutsumi, K., and Sugahara, H . , Jap. J. Appl. Phys.,3,
~ 1 6 9 , 1984.( 5 ) Carey, R., O'Rourke, P.A., and Thomas, B.W. J., J. Mag. & Magn. Mat.,
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1985 ( i n p r e s s ) .( 6 ) C r o a t , J. J . , H e r b s t , J. F., Lee, R. W . , and P i n k e r t o n , F.E., J. Appl.Phys., 55, 2078, 1984.
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