HAL Id: jpa-00221344
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Submitted on 1 Jan 1981
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STATIC POLARIZATION ECHOES IN METAL
POWDERS
F. Tsuruoka, K. Kajimura
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C6, suppZ&ment au n o 12, Tome 42, de'cembre 1981 page C6-864
STATIC POLARIZATION ECHOES IN METAL POWDERS
F. ~suruoka* and K. Kaj imura
EZectrotechnicaZ Luboratory, S a k u r a - m a , Ibaraki 305, Japan
Abstract.- Static polarization echoes with long time storage in metal powders placed in steady magnetic field have been studied. The echo amplitudes were found to be unstable when the surrounding He gas pressures were less than 10 Pa, whereas they were stabilized by introducing a small amountofHe gas of 2 100 Pa at 4.2 K. The phenomenon can be understood if the process involves the macroscopic reorientation of individual particles and if the mechanical rotation of the individual particles is stabilized by the increase in inter- particle friction due to the presence of He gas molecules on the particles. A particular base axis of the rotation is proposed to be specified by an anisotropic magnetic dipole caused by electric currents associated with the mechanical oscillation of particles.
1. Introduction.- In powder samples consisting of a large number of particles (>lo5) of piezoelectric, magnetoelastic, normal metallic, and superconducting materials,
two-pulse polarization echoes occur at t = m ~ (m=2,3,4,
...
) and stimulated three-pulseechoes at t=pT+rT (p,r=1,2,3
...
) following applied rf pulses at times t=O, T, andT > T . The primary pulse at t=O excites resonant acoustic oscillations in the parti- cles. Only the powder particles having sizes of an order of the acoustic wavelength excited in the particles are responsible for the echo formation. Powder echoes are separated into two general types. Dynamic echoes1 have relaxation timesT associated
2
with acoustic oscillations of the particles. The relaxation time T2 coincides with the decay time of free oscillation of the constituting particles. Static or memory echoes2 are those for which the relaxation time TI of the stimulated three-pulse echo at t=T+T exceeds the lifetime of any dynamic process. The static polarization
echoes were observed in powders of piezoelectric,2 ferr~rna~netic,~ and normal metal-
lic
material^.^
The echo formation mechanism and long time storage proposed byMelcher and shirenz for piezoelectric powders assume mechanical reorientation of particles caused by the torque exerted on an oscillating dipole by an external rf pulse. This model, referred to as the torque-rotation model, has been successful in
explaining the properties of the static echoes in piezoelectric powders.' The model
is readily applicable to the static echoes of magnetoelastic powders in which the magnetostrictive axis behaves as a base axis for an oscillating dipole. In metal powders, however, it has not been clear so far that what types of information are stored in the individual particles. In this paper we report experimental results on the static polarization echoes of metal powders of Al, Sn, and Nb and of type 11
"permanent address : Faculty of Science, Tokyo Institute of Technology, Oh-okayama,
Meguro-ku, Tokyo 152, Japan
superconducting powderofv Si which suggest mechanical reorientation of individual 3
particles. We also propose that a particular base axis for the torque rotation can be specified by the anisotropic magnetic dipole caused by the electric current asso- ciated with the acoustic oscillations of the particles.
2. Experimental results.- A detailed description of experimental procedure and
results were published in Refs. 6and 7. We sumarize only the essential experimental
result indicating the macroscopic reorientation of metal particles in the following: (i) The echo amplitude took a maximum when the resonance condition was met for the acoustic vibration of individual particles. (ii) Integration of the static echo amplitude,e3,to its maximum value by repetitive application of a two-pulse sequence with the relative phase strictly fixed was required to observe e3 with reasonable signal to noise ratio under He-gas pressures>100 Pa. (iii) In a vacuum of lo-' Pa e3 was not detected even for repetitive application of a two-pulse sequence. Under He gas pressures less than 10 Pa e3 was unstable in a sense that the integration by the two-pulse sequence and the destruction by the third-pulse sequence occurred irregularly in time. Actually, when a third probing pulse was applied after turning off the repetitive two-pulse sequence, e3 died out immediately. Above 100 Pa inte- gration of e3 to its saturation was observed although the integration rate was very low. The storage was able to be read out nondestructively by the third-pulse sequence after turning off the two-pulse sequence and erased only when the surround- ing gas pressure was reduced and an rf pulse was applied.
3. Discussion.- In the presence of surrounding gases the oscillation energyistrans- ferred from a particle to them through its surface and consequently the oscillation amplitude is damped as has been investigated in the study of dynamic echoes.'96 When the surrounding gas is pumped out toavacuum, the oscillation amplitude becomes large enoughforparticles jumping around their unstable sitting positions, resulting in irreversible disturbance of the stored pattern. When He gas is introduced, both the damping of oscillation, r, and the interparticle friction against the mechanical
rotation, ra, in the torque-rotation model increase. The measured increase in
r
wasat most 50 % when the He pressure was increased from to 10' The striking
effect of the presence of surrounding gases on the static echoes comes from the
sudden increase in
r
,
which prevents the irreversible disturbances of the storedpattern. In fact the integration rate became much lower than that expected from only
the increase in
r .
It is noted that the effect of surrounding gases takes placeoutside the metal powders but not inside the particles similar in nature to holo- graphic processese or dislocation motions.
C6-866 JOURNAL DE PHYSIQUE
m e c h a n i c a l o s c i l l a t i o n i s e x c i t e d by L o r e n t z f o r c e a c t i n g o n c o n d u c t i o n e l e c t r o n s a n d p o s i t i v e i o n s d u e t o t h e r f a n d s t e a d y m a g n e t i c f i e l d a s w a s i n v e s t i g a t e d e a r l i e r by M e r e d i t h e t a l . l o T h i s mechanism was a p p l i e d t o powders by t h e p r e s e n t
author^.^
An e l e c t r i c c u r r e n t i s i n d u c e d a t t h e s u r f a c e o f a n o s c i l l a t i n g p a r t i c l e p l a c e d i n t h e s t e a d y m a g n e t i c f i e l d d u e t o t h e a c o u s t o e l e c t r o m a g n e t i c r e s p o n s e o f e l e c t r o n s a n d i o n s i n t h e d i r e c t i o n n o r m a l t o b o t h t h e s u r f a c e a n d t h e d i r e c t i o n o f p r o j e c t i o n o f t h e r f m a g n e t i c f i e l d o n t h e m e t a l s u r f a c e . The i n d u c e d c u r r e n t t h e n c a u s e s a m a g n e t i c d i p o l e moment a l o n g t h e l a t t e r d i r e c t i o n . The t h e o r e t i c a l c a l c u l a t i o n b a s e d o n t h e t o r q u e - r o t a t i o n model1 w i t h t h e a b o v e m e n t i o n e d d i p o l e moment g i v e s a r e s u l t i n s a t i s f a c t o r y a g r e e m e n t w i t h t h e e x p e r i m e n t a l r e s u l t s o n t h e d e p e n d e n c e s o f t h e s t a t i c e c h o a m p l i t u d e , e3, o n t h e r f p u l s e a m p l i t u d e s a n d t h e s t r e n g t h o f t h e s t e a d y m a g n e t i c f i e l d .'
I n c o n c l u s i o n we f o u n d t h a t s u r r o u n d i n g He g a s s t r o n g l y a f f e c t e d t h e s t a b i l i t y o f t h e s t a t i c e c h o e s a t 4.2 K. Based o n t h i s e x p e r i m e n t we c o n c l u d e t h a t t h e s t a t i c e c h o e s i n m e t a l powders a r e u n d e r s t o o d by t h e t o r q u e - r o t a t i o n model i n which t h e b a s e a x i s f o r r o t a t i o n is s p e c i f i e d t o b e a l o n g t h e a n i s o t r o p i c d i p o l e moment c a u s e d by t h e e l e c t r i c c u r r e n t a s s o c i a t e d w i t h t h e m e c h a n i c a l v i b r a t i o n o f i n d i v i d u a l p a r t i c l e s . The a u t h o r s would l i k e t o a c k n o w l e d g e P r o f e s s o r T. F u k a s e f o r h i s s u p p l y w i t h V S i s a m p l e s . One o f t h e a u t h o r s (FT) i s g r a t e f u l l y i n d e b t e d t o Dr. T. I s h i g u r o a n d 3 P r o f e s s o r Y . H i k i f o r t h e i r g i v i n g him a n o p p o r t u n i t y t o s t a y a t E l e c t r o t e c h n i c a l L a b o r a t o r y . 1. K. F o s s h e i m , K. K a j i m u r a , T.G. Kazyaka, R.L. M e l c h e r , a n d N.S. S h i r e n , P h y s . R e v . B17
964 (1978) a n d r e f e r e n c e s q u o t e d t h e r e i n . 2. R.L. M e l c h e r and N.S. S h i r e n , P h y s . Rev. L e t t .2,
8 8 8 (1976) a n d r e f e r e n c e s t h e r e i n . 3. R.L. M e l c h e r a n d N.S. S h i r e n , P h y s . L e t t .a,
377 ( 1 9 7 6 ) a n d r e f e r e n c e s t h e r e i n . 4. S. Kupca, I. M a a r t e n s e , H.P. Kunkel, a n d C.W. S e a r l e , Appl. P h y s . L e t t .a
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