Fe alloys from “NMR spin-echo spectra”

Volume 11, number 3

PHYSICS

DISTRIBUTIONS OF FROM "NMR

LETTERS

1 August 1964

Her f A T Co IN C o / F e ALLOYS SPIN-ECHO SPECTRA"

R. F. JACKSON, R. G. SCURLOCK, D. B. UTTON and T. H. WILMSHURST

Departments of Physics and Electronics, University of Southampton Received 11 July 1964

Recent NMR 1, 2) and M S s s b a u e r 3) m e a s u r e m e n t s have d e m o n s t r a t e d the e x i s t e n c e of a r e l a t i v e l y n a r r o w s p e c t r u m of well defined effective n u c l e a r fields at Co and F e , r e s p e c t i v e l y , in dilute f e r r o m a g n e t i c alloys. By u s i n g a v a r i a b l e f r e q u e n c y s p i n - e c h o NMR s p e c t r o m e t e r , a " s p i n - e c h o s p e c t r u m 't of the effective fields can be obtained when the " l i n e width", or field d i s t r i b u t i o n , is too b r o a d for n o r m a l f r e q u e n c y modulation methods to be effective. We wish to r e p o r t some p r e l i m i n a r y r e s u l t s u s i n g this technique in which the e x i s t e n c e of wide r a n g e s of effective fields at Co in C o / F e a l loys is d e m o n s t r a t e d . The c o r r e c t e d s p i n - e c h o s p e c t r a for Co 59 in a s e r i e s of C o / F e a l l o y s a r e shown in fig. 1. The s p e c t r u m for each alloy i s built up in the following way. The echo height following a p a i r of 1 ~ s e c p u l s e s s e p a r a t e d by a fixed t i m e t, of 25 ~ s e c , is d e t e r m i n e d at 1 or 2 M c / s i n t e r v a l s a c r o s s the whole f r e q u e n c y r a n g e . At each f r e quency, the s p i n - s p i n r e l a x a t i o n t i m e T 2 (which v a r i e s between 80 and 600 ~ s e c ) i s d e t e r m i n e d , so that the s p e c t r a can be " n o r m a l i s e d " to a

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fixed t / T 2. The i n t e r p r e t a t i o n of the s p i n - e c h o s p e c t r a in t e r m s of d i s t r i b u t i o n s of H etf depends on a knowledge of the e n h a n c e m e n t f a c t o r s a c r o s s the s p e c t r u m . T h e s e have not been d e t e r m i n e d yet, but it i s r e a s o n a b l e to a s s u m e that the e n h a n c e m e n t i s p r o p o r t i o n a l to (Heft) 2 a c r o s s the whole s p e c t r u m of a single alloy, provided the o b s e r v ed r e s o n a n c e a r i s e s f r o m m o t i o n of the s a m e type of wall, and the a l l c y i s t r u l y homogeneous. T h i s a s s u m p t i o n has been applied to c o r r e c t the o b s e r v e d s p e c t r a and give the d i s t r i b u t i o n s shown in fig. 1 independent of a v a r i a b l e e n h a n c e m e n t factor. All s a m p l e s , except the 90% Co alloy, w e r e in powder f o r m . They w e r e p r e p a r e d by f i l l i n g alloy ingots which had been h o m o g e n i s e d at 1000oc for a week following a r c - c a s t i n g . The pow-

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Fig. 1. Spin echo spectra, corrected f o r enha-cement factor cc (Heft)2, for Co59 in the Co/Fe alloy s y s t e m . The centre of gravity of each spectrum is indicated by dots. d e r s w e r e then a n n e a l e d for 2 h o u r s at 750°C followed by a slow cool to r e m o v e s t r a i n s . The 90% Co ~tlloy was in the f o r m of a foil. A foil s a m p l e of the 50% alloy was slowly cooled f r o m 900°C over a p e r i o d of 3 weeks in an a t t e m p t to f o r m a 197

Volume 11, number 3

PHYSICS LETTERS

c o m p l e t e l y o r d e r e d s y s t e m ; the r e s u l t i n g s p e c t r u m (not shown) was ~lmost i d e n t i c a l to that in fig. 1 for the s o - c a l l e d " o r d e r e d " 5~o powder alloy p r e p a r e d with the s t a n d a r d a n n e a l i n g t r e a t m e n t . In a f u r t h e r e x p e r i m e n t , the 50% and 60% Co s a m p l e s w e r e d i s o r d e r e d by quenching in liquid n i t r o g e n f r o m 900°C. The s p e c t r u m ob.tained f r o m the d i s o r d e r e d 50% powder p r e p a r e d in this way i s shown in fig. 1. The s p e c t r u m f r o m the 60% Co s a m p l e was s i m i l a r to that obtained f r o m the sample before quenching. The m o s t s t r i k i n g f e a t u r e of the r e s u l t s shown in fig. 1 is the e x i s t e n c e of a giant s t r u c t u r e in the d i s t r i b u t i o n of hyperfine f i e l d s , with strong peaks o c c u r r i n g at 200 M c / s , 250 M c / s and 280 M c / s over the composition r a n g e 50% to 80% Co. F u r t h e r , t h e r e is a qualitative change in the s p e c t r a with the a p p e a r a n c e of the bcc phase. We can find no s i m p l e explanation for the giant s t r u c t u r e . It does, however, a p p e a r to be quite d i f f e r e n t in o r i g i n to the much s m a l l e r s t r u c t u r e r e p o r t e d in r e f s . 1,2 and 3, which has been explained in t e r m s of d i p o l a r effects, and v a r i a t i o n s in the spin d e n s i t y of the conduction e l e c t r o n s around i m p u r i t y sites. To explain

THE

1 August 1964

the giant s t r u c t u r e , much l a r g e r effects a r e r e q u i r e d , p r o b a b l y a s s o c i a t e d with changes of core p o l a r i s a t i o n a n d / o r conduction e l e c t r o n p o l a r i s a t i o n at the Co n u c l e i with d i f f e r e n t neighbour configurations. We a r e deeply indebted to Dr. C. E. Johnson of the Solid State P h y s i c s D i v i s i o n , United Kingdom Atomic E n e r g y R e s e a r c h E s t a b l i s h m e n t , Harwell, for the loan of s e v e r a l alloy spec i m e n s , and to the United Kingdom Atomic E n e r gy R e s e a r c h E s t a b l i s h m e n t for a r e s e a r c h grant. One of us (RFJ) i s g r a t e f u l to the D. S. I.R. for a m a i n t e n a n c e grant.

References 1) R.C. La Force, S.F.Ravitz and G.F.Day. Phys. Rev. Letters 6 (1961) 226. 2) Y. Koi, T. Hihara and T. Kushida, J. Phys. Soc. Japan 16 (1961) 574. 3) G. K. Wertheim, V. Jaccarino, J.H. Wernick and D.N.E. Buchanan, Phys. Rev. Letters 12 (1964 24.

LOW TEMPERATURE MAGNETIC M n ( H C O 2 ) 2. 2 H 2 0

SUSCEPTIBILITY *

OF

A. FONER COHEN, S.A. FRIEDBERG and G. R. WAGNER ** Carnegie Institute of Technology, Pittsburgh, Pennsylvania Received 2 July 1964

The powder s u s c e p t i b i l i t y of Mn(HCO2) 2. 2H 2 has been found to obey one C u r i e - W e i s s law 1) between 14 ° and 20OK and a n o t h e r 2) with significantly s m a l l e r C u r i e and W e i s s c o n s t a n t s below 1.5°K. It has been pointed out 2, 3) that this behaviour is c o n s i s t e n t with the fact that Mn ++ * Work supported by the National Science Foundation and the Office of Naval Research. ** On leave from the Westinghouse Research Laboratories, Pittsburgh, Pennsylvania.

198

ions in this salt occupy equal n u m b e r s of two kinds of i n e q u i v a l e n t lattice s i t e s 4). In the c o u r s e of a c a r e f u l study of the single c r y s t a l s u s c e p t i b i l i t i e s of Mn(HCO2) 2. 2H2, however, we have found evidence at 1.7 ° and 3.~OK of the e x i s tence of two u n u s u a l cooperative t r a n s i t i o n s t

t During the preparation of this report we learned that similar results have recently been obtained independently by H. Abe.