Nuclear magnetic acoustic resonance in KTaO3

Volume 24A, number 12

PHYSICS LETTERS

NUCLEAR

MAGNETIC

ACOUSTIC

RESONANCE

5 June 1967

IN K T a O 3 *

R . W . M E B S , L.H. BENNETT Institute for materials Research, National Bureau of Standards and J. R. LEIBOWIT Z University of Maryland Received 2 May 1967

Direct acoustic excitation of the 181Ta nuclear spin is observed at room temperature in KTaO3.

We r e p o r t the f i r s t o b s e r v a t i o n of the d i r e c t acoustic excitation of the 181Ta at room t e m p e r a t u r e in a cubic p e r o v s k i t e s t r u c t u r e KTaO 3 c r y s tal. I n d i r e c t excitation as it affects a c o n v e n tional NMR line, has been o b s e r v e d by P r o c t o r [1] in an oxygen deficient s a m p l e . In the d i r e c t NMAR technique p i o n e e r e d by Bolef and Menes [2], the r.f. coil of conventional NMR is r e p l a c e d by a p i e z o e l e c t r i c t r a n s d u c e r applied to one of a p a i r of plane and p a r a l l e l s a m p l e faces. The spin i n t e r a c t i o n i s o b s e r v e d as a consequence of coupling between the n u c l e a r quadrupole m o m e n t and the dynamic inhomogeneous e l e c t r i c field a s s o c i a t e d with the injected phonons r a t h e r than the n u c l e a r dipole m o m e n t and an applied r.f. m a g n e t i c field. Since the changes in u l t r a s o n i c attenuation a s s o c i a t e d with n u c l e a r spin coupling a r e of the o r d e r of 10-6, high s e n s i t i v i t y detection methods a r e r e q u i r e d

[2]. The KTaO 3 single c r y s t a l was grown by Linde Company in an oxygen a t m o s p h e r e to give an i m p u r i t y c o n c e n t r a t i o n of l e s s than 10 p.p.m, and a r e s i s t i v i t y g r e a t e r than 1010f~cm. A ¼ inch diameter x-cut "wrap-around" quartz tranducer was bonded to the ¼ inch cube s p e c i m e n and a typical m e c h a n i c a l r e s o n a n c e impedance peak of the combination is displayed in fig. 1. With 10 MHz phonons d i r e c t e d along the [100] s p e c i m e n axis, the NMAR peak to peak a b s o r p t i o n d e r i v a t i v e i n t e n s i t y for 181Ta Am = + 2 t r a n s i tions was r e c o r d e d for v a r i o u s angles of rotation * Support in part by Advanced Research Projects Agency.

Fig. 1. Oscillogram pattern obtained with sweep frequency generator connected through matching inverting network to test samule resonator consisting of a 10 MHz quartz crystal transducer cemented to the KTaO3 crystal. of the applied field with r e s p e c t to the [100] axis. In c o m p a r i n g NMAR o b s e r v a t i o n on 181Ta in the p r e s e n t KTaO3 i n v e s t i g a t i o n with those by G r e g o r y and BOmmel [3] on Ta m e t a l at 77OK, the p r e s e n t line width is 6 g a u s s for Am = ~-2, w h e r e a s that for T a m e t a l was 45 gauss. They were unable to o b s e r v e NMAR on T a at room t e m p e r a t u r e , evidently due to the l a r g e density of point defects found in m e t a l s . The v a r i a t i o n of the peak to peak i n t e n s i t y of the a b s o r p t i o n d e r i v a t i v e line with angle e b e tween sound axis and m a g n e t i c field is shown in fig. 2. T h e o r e t i c a l l y , this should v a r y as sin40 m u l t i p l i e d by a factor due to any d i r e c t i o n a l effect of d i s l o c a t i o n s and other defects [2]. The lat~ t e r factor is p r o p o r t i o n a l to the s a t e l l i t e i n t e n s i t y 665

Volume 24A, n u m b e r 24

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ANGLE BETWEEN SOUNDAXIS [IOO] AND MAGNETICFIELD,DEGREES

Fig. 2. Absorption d e r i v a t i v e peak to peak amplitude for n u c l e a r magnetic acoustic r e s o n a n c e of 181Ta, A m= = ± 2 t r a n s i t i o n s , in the KTaO3 c r y s t a l as a function of angle 0 between magnetic field and sound axis [ 100]. of n o r m a l N M R on t h e s a m e c r y s t a l in i t s d e p e n d e n c e on a n g l e b e t w e e n t h e [100] a x i s a n d t h e m a g n e t i c f i e l d [2]; t h e s t r o n g c e n t r a l c o m p o n e n t w h i c h i s o b s e r v e d o n l y in N M R i s i n d e p e n d e n t of rotation angle. Thus, NMAR becomes potentially a n i m p o r t a n t t o o l in c r y s t a l c h a r a c t e r i z a t i o n . N M R t e s t s on t h e K T a O 3 s a m p l e (not s h o w n ) i n d i c a t e d l e s s t h a n 10 p e r c e n t s i g n a l a m p l i t u d e v a r i a t i o n w i t h r e s p e c t to r o t a t i o n a n g l e .

TEMPERATURE NICKEL

DEPENDENT ABOVE THE

LETTERS

5 June 1967

The deviation from sin40 behaviour cannot be a t t r i b u t e d p r i m a r i l y to a c r y s t a l d e f e c t s t r u c t u r e . G r e g o r y a n d B 0 m m e l [3] f o u n d s u c h d e v i a t i o n s in t h e i r r o t a t i o n a l p a t t e r n s on T a to b e u n a f f e c t e d by annealing or uniaxial straining. Moreover, where they employed solely longitudinal or shear mode transducers; they found evidence for both m o d e s in e a c h of t h e i r r o t a t i o n a l p a t t e r n s . T h i s m o d e m i x i n g m a y b e a t t r i b u t e d in p a r t [4] to t h e s m a l l r a t i o of t r a n s d u c e r a n d s a m p l e d i a m e t e r D * to s o u n d w a v e l e n g t h ~. D/~ w a s a b o u t 14 for their Ta samples and about 7 for the present t e s t s . In B o l e f a n d M e n e s t e s t s on Kr~[2] w h e r e D/~ w a s a b o u t 34, m o d e m i x i n g w a s m i n i m i z e d a n d c o n f o r m a n c e to t h e o r y w a s e v i d e n t . A l t h o u g h a m u c h h i g h e r D/X r a t i o i s n o t p o s s i b l e w i t h t h e present small size crystal, further tests should be directed toward a sufficiently high ratio and t o w a r d s t h e s t u d y of t h e r o t a t i o n p a t t e r n f o r t h e Am = ± 1 t r a n s i t i o n , w h i c h i s m o r e s u b j e c t to quadrupole broadening.

References 1. w. G. P r o c t o r , private communication. 2. D . I . B o l e f a n d M. Menes, Phys. Rev. 114(1959) 1441; M. Menes and D.I. Bolef, Phys. Rev. 109 {1958) 218. 3. E.It. Gregory and If. E. Bbmmel , Phys. Rev. Lett e r s 15 {1965) 404; E.H. Gregory, Ph. D. thesis, UCLA (1965) (unpublished). 4. H.J. McSkimin. J. Aeoust. Soc. Am. 28 (1956)484. * In the c a s e s cited, the t r a n s d u c e r d i a m e t e r D and that of the sample are approximately the same.

STONER ENHANCEMENT CURIE TEMPERATURE

FOR

E. P. WOHLFARTH

Departn~ent ,Of Mathematics, Imperial College, London, England Received 2 May 1967

The enhancement is estimated on the b a s i s of static susceptibility m e a s u r e m e n t s ; r e f e r e n c e is made to r e cent neutron s c a t t e r i n g m e a s u r e m e n t s which a r c also influenced by this enhaeement.

C a b l e e t al. [1] h a v e r e c e n t l y m e a s u r e d t h e n e u t r o n s c a t t e r i n g c r o s s - s e c t i o n in p a r a m a g ~ e t i c 666

nickel at temperatures the Curie temperature.

n e a r 1.6 Tc, w h e r e Tc i s This cross-section may,