Magnetic moments and bonding in Co-Nb-B alloys

Journal of Magnetism and Magnetic Matermls 31-34 (1983) 1537-1539 MAGNETIC

AND B O N D I N G

MOMENTS

B.W. C O R B , R . C . O ' H A N D L E Y ,

1537

IN Co-Nb-B

ALLOYS

N.J. GRANT

MIT, CambrMge. MA 20139, USA a n d V. M O R U Z Z I I B M Watson Research Center, Yorktown Heights, N Y 10598, USA

Several new CoNbB amorphous and nucrocrystalhne alloys have been melt quenched and their magnetic moments measured Compamson is made w~th cobalt moment suppression m CoB, CoNb, CoV and other CoNbB alloys Two ~ssues are addressed 1) the different rates at which Nb and V (same number of valence electrons) suppress the cobalt moment and 2) the combmed effect of boron and nlobmm together m suppressmg the cobalt moment It is suggested that the characteristic moment variations can be accounted for by d - d hybridization m the first case and by p - d and d - d hybn&zatlon m the second case

1. Introduction Interpretation of m o m e n t trends in strongly ferromagnetic Co-base alloys is less difficult than in Fe-base alloys where both spin bands contribute to the m o m e n t The effects of early transition metal (TE) a d d m o n s to Co are well handled to a first approximation by Fmedel's wrtual-bound-state (VBS) model [1], or its generahzatlon to more concentrated systems, the spht-band model [2]. Friedel's model predicts that ff the T E d b a n d is completely above the Fermi energy E F the cobalt mom e n t suppression as given by

OlZco/C3CTF = -- (10 + Z ) f t . , where Z is the difference m n u m b e r of valence electrons between the TE species and Co, CTE as the concentration of early transition metal, and #B IS the Bohr magneton This expression predicts that both V and N b ( Z = - 4 ) should lower the m o m e n t of Co by 6#B per atom. The m o m e n t suppression is - 6 / ~ B for V [3] but is only - 4 5/~ B for N b [4] This discrepancy is explained below to be a consequence of d - d hybridization [5] The presence of glass-formers (M) m m a n y a m o r p h o u s transition (T) metal alloy systems has heightened mterest in the old question of the role of n o n - m a g n e h c elements on m o m e n t formation The apphcation of new theoretical techmques to this problem has improved our understanding and led to a more realistic p~cture of magnetism in metals Metalloid atoms are understood to suppress the T metal m o m e n t not by charge transfer to the d-band but rather by p - d bondlng which decreases the n u m b e r of polarizable d states [6-9] We have melt spun several C o N b B glassy and mlcrocrystalhne alloys and measured their magnetic m o m e n t s [10,11] These new data are compared with earher data on CoB [12], C o N b [4] and C o N b B [13] glasses, as well as CoV crystalhne alloys [3]. Trends m m o m e n t suppresslon with B and N b or V additions reveal 1) effects 0304-8853/83/0000-0000/$03

of p - d hybridization (COB), 2) effects of d - d hybridization ( C o N b vs. CoV), and 3) effects due to the presence of both TE and M species together (CoNbB).

2. Results and discussion Fig 1 displays the variation of the saturation magn e h c m o m e n t s of a variety of Co-base alloys with N b additions. The datum points for CoB alloys (lying along the ordinate below the m o m e n t for Co which is taken as 1 7/~B) are extrapolations from Hasegawa and Ray [12] ( 3 ~ c o / 0 c B = - 3 3/xB). Note first that the m o m e n t suppresston for C o , _ x N b ~ ( - 4 . 5 # a ) [4] is less than that reported for Co I xVx(-6~B) [3] To explain this &fference requires that one go beyond the VBS or sphtb a n d models and examine the electromc structure in more detail Watson and Bennett [5] have developed an electronegatlvlty scale for T alloys which differs significantly from Pauhng's covalent scale because of d - d hybridization between the two T species d - d hybridization causes the electronegatlvlty of N b to be greater 20

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Fig 2 Fotal denstty of states as functions of energy (E~ 0) for CoaV and ('o~Nb m the C u a A u structure from augmented spherical wave calculations t h a n that of V, r e v e r s i n g the relattve o r d e r of their c o v a l e n t e l e c t r o n e g a t t v l t l e s T h i s l m p h e s that the m e a n e n e r g y of the N b d - s t a t e s is lower t h a n t h a t for V d-states Augmented sphertcal wave band structure c a l c u l a t t o n s [14] for C o 3 N b a n d C o N in t h e C u ~ A u s t r u c t u r e (fig 2) s h o w a g r e a t e r d e n s i t y of s t a t e s at the F e r t m e n e r g y in t h e f o r m e r c a s e (3 95 vs 3 55 s t a t e s / e V u n t t cell at Ef ) T h u s t h e c h a r g e d i s p l a c e d to the C o d - b a n d f r o m t h e T E s t a t e s a b o v e E v is l e t s for N b t h a n for V b e c a u s e of t h e g r e a t e r c o n t r l b u t t o n of t h e f o r m e r to t h e spectral d e n s t t y at a n d b e l o w E T h i s e x p l a i n s t h e w e a k e r Co m o m e n t s u p p r e s s i o n d u e to N b t h a n d u e to V Fig 1 also s h o w s that the a d d i t i o n o f b o r o n m a k e s t h e C o m o m e n t m o r e s u s c e p t i b l e to s u p p r e s s i o n b y N b a d d t t l o n s T h i s ts i l l u s t r a t e d in fig 3 w h m h p l o t s the s l o p e O ~ / & - r l of t h e lines m fig 1 vs B c o n t e n t ( T h e VBS m o d e l p r e d i c t s a s l o p e of - 6 / * 8 for C o N b ) B o r o n by ttself is u n d e r s t o o d to s u p p r e s s the c o b a l t m o m e n t b e c a u s e o f its c h e n n c a l l n t e r a c u o n (p d hyb r t d l z a t t o n ) w h m h d e l o c a h z e s s o m e of the m a g n e t m d s t a t e s a n d r e d u c e s their m a g n e t i c polarizability [ 6 - 9 ] T h e effect i l l u s t r a t e d m fig 3 c a n be e x p l a i n e d by a s s u m i n g p d b o n d m g b e t w e e n B a n d N b as well as b e t w e e n B a n d Co. ( C l u s t e r c a l c u l a t t o n s o n C o M n B altoy~ [15] t n d l c a t e a s t r o n g M n B i n t e r a c t i o n for o r b l t a l s n e a r E~ ) N b B b o n d i n g lowers the energy of t h e N b d s t a t e s f r o m their p o s l t l o n m a C o - N b alloy well a b o v e t h e F e r m i e n e r g y ( T h e a d d m o n of N b t e n d s to s t r e n g t h e n C o B glassy alloys [13] a n d m c r e a s e t h m r c r y s t a l h z a t l o n t e m p e r a t u r e s [10]) M o r e t m p o r t a n t l y for o u r p r e s e n t p u r p o s e s , the B - N b p - d s t a t e s n o l o n g e r h y b n d t z e as effectively wtth the C o d - s t a t e s as dtd the p u r e N b d - s t a t e s T h u s the effect of d d h y b r i d i z a t i o n m w e a k e n i n g the m o m e n t s u p p r e s s t o n d u e to t h e TIE s p e c m s is d l m m t s h e d b e c a u s e of t h e s t r o n g p d h y b r t d l zatJon of T E d s t a t e s ,alth B p s t a t e s T h t s h y p o t h e s i s c o u l d be tested by c o m p a r i n g t h e effect of d i f f e r e n t

m e t a l l o t d s o n (,~/*/d(Nb T h o s e b o n d , n g m o r e strongly wtth N b w o u l d c a u s e the m a g m t u d e o[ the m o m e n t s u p p r e s s i o n to i n c r e a s e m o r e t h a n t h o s e w h i c h b o n d e d onl\' weakl,~

3. Summary T h e s t a r t i n g p o i n t for u n d e r s t a n d i n g t h e m a g n e t i c p r o p e r t t e s o f C o N b B alloys ts t h e s p i l t - b a n d m o d e l N t o b m m gtves rtse to a c l u s t e r of ~trtual b o u n d s t a t e s a b o v e the d b a n d o f the m a t r t x M o s t of t h e h,,e N b s + d e l e c t r o n s e n t e r t h e l o w - l y i n g d b a n d ol t h e ( ' o m a m x , s u p p r e s s m g t h e m o m e n t relative to that of the parent CoB composmon This cobalt moment suppressw, n IS w e a k e r t h a n that d u e to x a n a d m t n b e c a u s e ol stronger d d hybr,&zatton between Co and Nb B o r o n T m e t a l p d h y b n d l z a t t o n d e c r e a s e s the m a g netic m o m e n t of t h e C o m a t r i x a n d e n h a n c e s the C o m o m e n t r e d u c t t o n d u e to N b (by d e c r e a s i n g d d hx.br, d l z a t , o n b e t w e e n N b a n d Co) T h e d t h g e n t a s s t s t a n c e of S t e p h e n P a r a d l s In melt s p i n n i n g m a n y of t h e alloys ts g r e a t l y a p p r e c i a t e d W o r k at M I 1 ~ w a s s u p p o r t e d m part b y t h e U S A r m y Res e a r c h Office u n d e r c o n t r a c t No. D A A G - 2 9 - 8 0 - K - 0 0 8 8 a n d t h r o u g h t h e C e n t e r for M a t e r i a l s S o e n c e a n d Eng m e e r m g b v N S F G r a n t N o 78-24185 D M R

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