Hyperfine Interactions 15/16 (1983) 669-672 9 J.C. Baltzer Scientific Publishing Company
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NMR ENHANCEMENT FACTOR IN CuMn(Fe) SPIN GLASSES
CH. SCHLOSSER and H. BR~MER I n s t i t u t f~r M e t a l l p h y s i k und N u k l e a r e F e s t k ~ r p e r p h y s i k , Technische Universit~t Braunschweig, D-3300 Braunschweig, Fed. Rep. G e r m a n y
The spin glass magnetic enhancement f a c t o r ~ has been measured a t T = 1.3 K w i t h 63/65Cu NMR spin echo technique. A l l o y i n g i r o n to CuMn, n(H n) shows a l a r g e range of constancy a g a i n s t an e x t e r n a l ~i'eld Ho, a p p l i e d in and opposite to the d i r e c t i o n of the f r e e z i n g f i e l d Hc. I t is supposed t h a t the s t i f f n e s s of the frozen spin glass is s t a b i l i z e d in t h i s range by a subsystem of f e r romagnetic i r o n c l u s t e r s . Bound by i t s own c r y s t a l l i n e a n i s o t r o p y energy, the subsystem has i t s own s t i f f n e s s . I t probably behaves as a powder system c o n s i s t i n g of f e r r o m a g n e t i c s i n g l e domain p a r t i c l e spheres, having a l so an i s o t r o p i c d i s t r i b u t i o n of domain m a g n e t i z a t i o n d i r e c t i o n s and g i v i n g r i s e to a mean f e r r o m a g n e t i c NMR enhancement f a c t o r .
NMR measurements of 63/65copper s a t e l l i t e l i n e s in frozen CuMn spin glasses at a temperature T = To/5 in zero and in low extern--al magnetic f i e l d s Ho have been performed. Ho was a p p l i e d in and oppos i t e to the d i r e c t i o n of the remanent m a g n e t i z a t i o n . The signal a r i s e s from 63/65copper n u c l e i being the 4th neighbours of Mn spins. They t h e r e f o r e sense a l o c a l h y p e r f i n e f i e l d a r i s i n g from the RKKY i n t e r a c t i o n w i t h the nearby Mn spins. The r o f . f i e l d is enhanced by the spin glass magnetic enhancement f a c t o r G. In two e a r l i e r papers E I , 2 ] n has been i n t e r p r e t a t e d by a two component model of a spin g l a s s . One component being a "system of s i n g l e Mn s p i n s " , which i n t e r a c t s v i a RKKY i n t e r a c t i o n , w h i l e the o t h e r one, being a "subsystem of Mn c l u s t e r s ~ i n t e r a c t s v i a d i p o l e i n t e r a c t i o n to the s u r r o u n d i n g . In order to study the i n f l u e n c e of the subsystem of c l u s t e r spins on the behaviour of the enhancement f a c t o r , new measurements w i t h a l l o y s c o n t a i n i n g a d d i t i o n a l l y i r o n atoms in c o n c e n t r a t i o n s ranging from 0.05 at% to 0.5 at% Fe have been performed. In f i g . I n(H o) curves measured with a pure CuMn spin glass are shown. A l l curves, depending on the f r e e z i n g fie-Td Hc, behave s i m i l a r . They t y p i c a l l y r i s e to a cusp and then decrease w i t h f u r t h e r decreasing HO. In an e a r l i e r paper [ 2 ] the curves have been i n t e r p r e t e d by the e x i s t e n c e of two a n i s o t r o p y f i e l d s Hi and Hc a r i s i n g from the system of s i n g l e Mn spins and a subsystem of M~ c l u s t e r spins, r e s p e c t i v e l y . Both these f i e l d s are phenomenological magnitudes. T h e i r d i f f e r e n c e d e s c r i b e s , as a mean v a l u e , the movements of the s i n g l e spin system as a whole under i n c l u s i o n of movements r e l a t i v e to the subsystem of c l u s t e r s . D i f f e r e n t from these f i e l d s each s i n g l e spin and each c l u s t e r spin is governed by i t s own a n i s o t r o p y f i e l d , which d i r e c t i o n and value is unknown. Iron modifies the curves n(H o) in a remarkable manner. This is
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Ch. Schlosser, H. BrOmer, NMR Enhancement in CuMn(Fe)
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shown in f i g . 2. With low c o o l i n g f i e l d s Hc, ~(Ho) f i r s t r i s e s w i t h d e c r e a s i n g f i e l d , becomes c o n s t a n t and then decreases going f u r t h e r i n t o n e g a t i v e f i e l d d i r e c t i o n . The e x t e n t i o n of the c o n s t a n t p a r t is a s t o n i s h i n g l y l a r g e . This e x t e n t i o n decreases w i t h i n c r e a s i n g Hc and f i n a l l y ceases. With high f i e l d s Hc i r o n only i n f l u e n c e s the f r e e z i n g p r o c e s s , s h i f t i n g the cusp i n t o n e g a t i v e f i e l d d i r e c t i o n . In o r d e r to e x p l a i n the h o r i z o n t a l p a r t s of ~(Ho), connected w i t h lower f i e l d values of Hc, we propose the f o l l o w i n g model: In s p i t e of quenching the a l l o y , t h e r e are many small f e r r o m a g n e t ic i r o n c l u s t e r s of about ten a t o m s [ 3 ] . These c l u s t e r s , though bound t o the s i n g l e spin system, possess a c r y s t a l l i n e a n i s o t r o p y energy w i t h d i f f e r e n t easy d i r e c t i o n s w i t h r e s p e c t to the i r o n c y r s t a l l a t t i c e . To these d i r e c t i o n s the i r o n m a g n e t i z a t i o n is c o u p l e d . By d e c r e a s i n g Ho, as in the pure copper manganese case, ~(H o) i n c r e a s e s and the s i n g l e spin system becomes weaker and weaker bound. At a c e r t a i n v a l u e , the slope of n(H O) v a r i e s smoothless ? ~ t h i s p o i n t the subsystem of i r o n c l u s t e r s begins to govern t h - - ~ t i f f n e s s of the s i n g l e spin system. This i s , because the subsystem o f i r o n c l u s t e r s has i t s own s t i f f n e s s a g a i n s t the e x c i t i n g r . f . f i e l d by i t s c r y s t a l l i n e a n i s o t r o p y energy. With f u r t h e r d e c r e a s i n g Ho, at the end of the c o n s t a n t r e g i o n of n(Ho), the system of s i n g l e spins becomes more and more s t i f f again
Ch. Schlosser, H. Br6mer, NMR Enhancement in CuMn(Fe)
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Fig. 2. ~ vs Ho of a CuMn (3 at% Mn; 0.1 at% Fe) spin glass with Hc a s ~ a r a m e t e r : o Hc = 9.76 kOe; * Hc = 7.5 kOe; Hc = 5.0 kOe.
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and the subsystem of i r o n c l u s t e r s loses i t s s t a b i l i z i n g f u n c t i o n . Going to a curve w i t h a h i g h e r Hc the s t i f f n e s s of the subsystem is weaker and t h e r e f o r e i t is o n l y able to s t a b i l i z e the s i n g l e spin system at a h i g h e r l e v e l of ~. The s t i f f n e s s of the subsystem of i r o n c l u s t e r s is w e l l described by the behaviour of the mean f e r r o m a g n e t i c
Ch. Schlosser, H. BrOmer, NMR Enhancement in CuMn(Fe)
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enhancement f a c t o r nF~ of iron powder, consisting of single domain p a r t i c l e spheres. Because of the large demagnetizing f i e l d , nFe, r e s u l t i n g from a d i s t r i b u t i o n of magnetization d i r e c t i o n s , is constant in the f i e l d range of Ho used. Just this behaviour is supposed f o r the subsystem of iron c l u s t e r s . Fig. 3 shown the enhancement f a c t o r of an annealed CuMn(Fe) sample. The annealing leads to the segregation of large a-6tiferromagnetic iron clusters [3,41. In t h i s case ~(HO) behaves nearly as in a pure CuMn spin glass. The iron segregations are obviously of only small i~-fluence. I t should be marked, that the cusp is shifted into the p o s i t i v e f i e l d d i r e c t i o n by annealing.
Acknowledgements We g r a t e f u l l y acknowledge the f i n a n c i a l support by the Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg.
References
[I] [2] [3] [4]
Ch. Schlosser and H. Br~mer, J.Magn.Magn. Mater. 28(1982)209. Ch. Schlosser and H. Br~mer, Z.Phys.B 50(1983)247. B. window, Phil.Mag. 26t1972)681. H. Wendt and R. Wagner, Acta Metal1. 30(1982)1561.
