Long-range order and ordering kinetics in CoPt 3
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COHEN
The C o - P t s y s t e m is well s u i t e d for s t u d i e s of o r d e r - d i s o r d e r by f i e l d - i o n m i c r o s c o p y . B e c a u s e of t h i s , and the fact that detailed i n f o r m a t i o n on o r d e r i n g is a v a i l a b l e for only one L12 alloy (Cu3Au), an e x t e n s i v e X - r a y study was m a d e of CoPt3. L o n g - r a n g e o r d e r f a l l s to lower v a l u e s (S ~ 0.64) in this alloy than in Cu3Au. T h e r e is a t w o - p h a s e field n e a r T c (685~ extending for about 20~ The o r d e r e d p h a s e has a s m a l l e r l a t t i c e p a r a m e t e r than the d i s o r d e r e d phase close to T c , but at r o o m t e m p e r a t u r e the r e v e r s e is t r u e . O r d e r i n g (when the d o m a i n size i s l a r g e ) follows R o t h s t e i n ' s k i n e t i c t h e o r y , with an a c t i v a t i o n e n ergy of 74.0(9) k c a l p e r g - a t o m . Domain growth is s i m i l a r to g r a i n growth, with a t i m e exponent of about 0.45 a n d an a v e r a g e a c t i v a t i o n e n e r g y of about 62 kcal p e r g - a t o m . The a n t i p h a s e d o m a i n s t r u c t u r e is i s o t r o p i c , in c o n t r a s t to Cu3Au. T h e r e is little d i f f e r e n c e in a t o m i c v o l u m e of cobalt and p l a t i n u m in the o r d e r e d s t a t e a s c o m p a r e d to the p u r e e l e m e n t s .
THE Pt-Co system exhibits complete solubility at high temperatures with at least two ordered phases occurring at low temperatures,I Fig. I. A tetragonal superlattice, well known for its good magnetic properties, forms near the composition CoPt. 2 Near the composition CoPts, an Lie-type superlattice (Cu3Au)was first reported by Geisler and Martin.3 They determined the critical temperature (Tc) for an alloy with 30 at. pct Co to lie between 700~ to 800~ However, a temperature of 630~ to 650~ was reported by Menzinger and Paoletti4 in a study, with neutron diffraction, on an alloy of 24.9 at. pct Co. Detailed studies of ordering kinetics and order vs temperature are really only available for one alloy of the Lle type--namely Cu3Au. Yet the Co-Pt alloys are widely employed in studies of ordering by field-ion microscopy,s'6 This investigation was undertaken to provide such detailed information for proper heat treatment, to resolve the uncertainty in Tc, and to provide information on a superlattice isomorphous to Cu3Au. I)
PROCEDURES
X - r a y s t u d i e s (see next section). The fine g r a i n size was n e e d e d to m i n i m i z e s c a t t e r in the m e a s u r e d X - r a y intensities. F o r s t u d i e s of e q u i l i b r i u m o r d e r vs t e m p e r a t u r e , single c r y s t a l s w e r e grown f r o m the s t r i p by M e t a l s R e s e a r c h L i m i t e d : a B r i d g e m a n technique was e m ployed u n d e r a p r e s s u r e of i n e r t gas to r e d u c e v a p o r i zation of cobalt. C o m p a r i s o n s w e r e made of the l a t t i c e p a r a m e t e r s of a n n e a l e d f i l i n g s f r o m the c r y s t a l and the s
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