STIMULATED SPIN ECHO STUDIES OF 60Co NUCLEI ORIENTED IN IRON

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STIMULATED SPIN ECHO STUDIES OF 60Co

NUCLEI ORIENTED IN IRON

H. Foster, P. Cooke, D. Chaplin, P. Lynam, D. Swan, G. Wilson

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque

C6, supplkment au no 8, Tome 39, aoiit 1978, page C6-1182

6 0

STIMULATED SPIN ECHO STUDIES

OF

Co NUCLEI ORIENTED IN IRON

H.R. F o s t e r , P. Cooke, D.H. Chaplin, P. Lynam, D.E. Swan and G.V.H. Wilson Department o f Physics, University of

New

South Wales, Royal MiZitary CoZZege, Ihmtroon, A. C. T . 2600, AustraZia

RBsum6.- Une technique u t i l i s a n t q u a t r e impulsions r6sonnantes e s t uti1is.de a f i n de p r o d u i r e des Bchos de s p i n stimulBs q u i s o n t d d t e c t d s

1

l ' a i d e du rayonnement nuclBaire. Ces Bchos p r 6 s e n t e n t une pdriode d ' e x t i n c t i o n , f o n c t i o n de l a r e l a x a t i o n spin-rgseau, au c o n t r a i r e de l ' e f f e t observ6 dans l e s experiences

P

t r o i s impulsions seulement.

A b s t r a t c . - A four-pulse resonance technique has been developed t o produce s t i m u l a t e d s p i n echoes which a r e t h e n observed by r a d i a t i v e d e t e c t i o n . These echoes show a decay time which depends on t h e s p i n - l a t t i c e r e l a x a t i o n ; t h i s i s n o t t h e c a s e f o r t h e echoes produced by t h e t r i p l e - p u l s e sequence.

INTRODUCTION AND THEORY.- Recently we r e p o r t e d / ] / o b s e r v a t i o n s by r a d i a t i v e d e t e c t i o n of s p i n echoes, produced by t r i p l e - p u l s e sequences, f o r 'CO n u c l e i o r i e n t e d i n i r o n . Studies121 of t h e dependence of t h e echo amplitude upon p u l s e i n t e r v a l showed t h a t , a f t e r an r f p u l s e , t h e n u c l e a r s p i n s i r r e v e r s i b l y l o s e t h e i r coherence i n t h e t r a n s v e r s e plane w i t h a decay time T2 'L 1-2 ms. This i s a very s h o r t time f o r a d i l u t e a l l o y when compared t o t h e spin- l a t t i c e r e l a x a t i o n time TI 75 s / 3 / f o r t h e con- d i t i o n s of r e f e r e n c e / 2 a . To s t u d y f u r t h e r t h e ap- p l i c a t i o n of pulsed resonance techniques i n nu- c l e a r o r i e n t a t i o n , we have a p p l i e d a four-pulse sequence, with i n t e r v a l s T ' ,T" and T"'

,

t o t h e same sample. Whereas t h e echo decay time f o r a t r i p l e - p u l s e sequence should r e l a t e t o Tp, t h e decay time f o r a four-pulse ( s t i m u l a t e d echo) sequence, should, under t h e c o r r e c t c o n d i t i o n s , r e l a t e t o T I . I n n u c l e a r o r i e n t a t i o n experiments, t h e f o u r t h p u l s e i s used t o r e t u r n a component of t h e t r a n s - v e r s e magnetization t o t h e z-axis and t h e y-aniso- t r o p y i s t h e n measured as a f u n c t i o n of T"'. The s t i m u l a t e d echo w i l l appear a s a change i n t h i s a n i s o t r o p y when T"' = T ' .

To c a l c u l a t e t h e amplitude of the expected echo s i g n a l i t i s necessary t o i n t e g r a t e over a l l s p i n s s i n c e r f f i e l d s HI which a r e comparable i n magnitude only w i t h t h e inhomogeneous broadening have been used. For T '

,

T " < < T ~ , T2 and a system i n which t h e major term i n t h e y-anisotropy i s

P2(cosO), the f r a c t i o n a l change i n t h a t a n i s o t r o p y a f t e r t h e p u l s e sequence i s given by

m

s2

= 1

-

P ~ ( C O S ~ ) = 1

-

P ( ( * I ~ - W ~ ) P ~ ( C O S ~ ~ )

f

d(wo-wo)

where

&

is t h e c e n t r e of t h e d i s t r i b u t i o n (assu- med t o b e Gaussian) cf resonant a-gular f r e q u e n c i e s O f i s

t h e f i n a l a n g l e between a s p i n and t h e z-axis, af- t e r allowing f o r r o t a t i o n s d u r i n g each p u l s e and r o t a t i o n about t h e z-axis d u r i n g each p u l s e i n t e r - v a l . Also we d e f i n e a parameter R as

w l / A w

where

Aw

is t h e h a l f - w i d t h a t half-maximum of t h e inhomo- geneously broadened resonant frequency and w l i s t h e r f a n g u l a r frequency.

C a l c u l a t e d s p i n echo l i n e shapes f r o e q u a l 90' p u l s e s a t v a l u e s of R of and 0.45 a r e given i n f i g u r e 1 : R = 0.45 corresponds t o our experi-

F i g . 1 : T h e o r e t i c a l echo responses f o r a dominant P2(cos8) a n g u l a r d i s t r i b u t i o n f o r R = and R=0.45. The p u l s e i n t e r v a l s , a r b i t r a r i l y chosen,' a r e

,

-c1=2/Aw=4ps and ~"=8/Aw=16l~s. The arrows i n d i c a t e t h e p r i n c i p l e s t i m u l a t e d echo.

mental c o n d i t i o n s . For a t r u e 90° p u l s e sequence (R = m), harmonic sub echoes a r e a b s e n t . For more g e n e r a l t u r n a n g l e s , S2 has twenty echoes a t times

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later than the third rf pulse/4/. Even for

R = 0.45 the stimulated echo remains prominent des- pite the additional competing structure due to the harmonic sub echoes and their broadening due to other effective angles of rotation centred on 90°.

EXPERIMENTAL DETAILS AND RESULTS.- The sample used was that of references/l/ and /2/ with a calcula-

ted fractional concentration of 6 0 ~ o atoms %

5

x

in the diffusion layer. A 0.1 T polarizing field was used, corresponding to a resonant frequency of 165.5 MHz ; the linewidth, Aw/21r, was 0.5 MHz. No evidence for 6 0 ~ o z satellite structure or for a 60Co(& resonance could be found in this heavily doped sample.

After each four-pulse sequence, the time dependence of the y-anisotropy was determined for

250s. The "signal" was obtained by computer extra- polation of the anisotropy back to a time immedia- tely after the fourth pulse. No signals were- obser- ved when rf frequencies a few MHz either side of resonance were used, indicating that off-resonant heating was negligible.

For T' = 5 ys stimulated echoes were observed for values of T" of

1 ,

10, 25 and 40 s (figure

2).

For these values of T ' and T" and the establi-

echo. Gomparison.of the calculated and observed lineshapes for T" = 1 s shows the same general beha- viour though the amplitude of the observed shape is somewhat lower, as in/l/. This reduction is proba- bly associated with inhomogeneous broadenhg of the rf field HI.

A simple spin-temperature analysis of the time dependence of they-anisotropy after removal of a frequency modulated C.W. rf field yielded a value TI = (80 f 5) s at 12 mK for a 0.1 T applied field. This is comparable with the decay time of the amplitude of the .stimulated echo. Hence we can conclude that the four-pulse nuclear orientation stimulated echo technique does lead to a relaxation time which is related to

TI,

rather than the much faster Tp. The previously observed short value for TP appears to validate the use of the spin temperature concept in the analysis of spin-lattice relaxation. Further experiments on 6 0 ~samples of o ~ varied concentration are in progress. These should establish whether a grain boundary diffusion mecha- nism exists, causing an anomalously high local concentration in this sample, or whether the results presented here indicate a general property of the very dilute 6 0 ~system. o ~

References

/I/ Foster,H.R., Cooke,P., Chaplin,D.H., Lynam,P. and Wilson,G.V.H., Phys. Rev. Letters (1977) 1546

/2/ Foster,H.R., Cooke,P., Chaplin,D.H., Lynam,P., Swan,D.E. and Wilson,G.V.H., Hyperfine Inter- actions (1978) 357

/ 3 / Klein,E., Hyperfine Interactions

2

(1977) 389 /4/ Cooke,P., Chaplin,D.H. and Wilson,G.V.H., to be

published

Fig. 2 : Observed stimulated echo lineshapes for

T ' =

5

ys and various values of T".

shed short Tp, the baseline of the stimulated echo