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Observation of the flux line lattice in MPMG-processed YBCO using a decoration technique of ferromagnetic particles
Physiea C 185-i89 (i991) 2653--2654 North-Holland
OBSERVATION DECORATION
OF T H E F L U X L I N E L A T T I C E TECHNIQUE OF F E R R O M A G N E T I C
IN M P M G - P R O C E S S E D PARTICLES
YBCO
USING
Yutaka HIGASHIDA, Yukio KUBO, Masato MURAKAMI*a Hiroyuki FUJIMOTO e, YAMAGUCHI*, Tsutomu TAKATA*, Akihiro KONDOH*, and Naoki KOSHIZUKA*,
A
Kojl
Research and Development L a b o r a t o r y , J a p a n Fine C e r a m i c s C e n t e r , 2-4-I Mutsuno-cho, Atsuta-ku, 456 Nagoya, Japan, *Superconductivity Research Laboratory, International SupercOnductivity T e c h n o l o g y Center, 10-13 S h i n o n o m e 1-chome, Koto-ku, 135 Tokyo, Japan O b s e r v a t i o n of the flux line d i s t r i b u t i o n in M P M G - p r o c e s s e d YBa2Cu307 has been c o n d u c t e d using a d e c o r a t i o n t e c h n i q u e of f e r r o m a g n e t i c particles, it has been found that the flux lines are t r a p p e d mainly at Y2BaCuO5 inclusions or the interface between the inclusion and the s u p e r c o n d u c t i n g matrix. 1. I N T R O D U C T I O N
In order to obtain a s u p e r c o n d u c t o r having high critical current density, it is e s s e n t i a l to introduce pinning c e n t e r s w h i c h p r e v e n t the m a g n e t i c flux motion. The m e l t - p r o c e s s is s u i t a b l e for producing a bulk s u p e r c o n d u c t o r with no w e a k l i n k s a n d r e l a t i v e l y large critical c u r r e n t density. The Y2BaCu05
(a)
D = 2urn I
inclusions in t h e m e l t - p r o c e s s e d YBa2Cu307 have been proposed as candidates for the flux pinning centers. It has been considered that the strong pinning force of t h e MPMG(Melt Powder Melt Growth)processed Y B a 2 C u 3 0 7 is attributed to the fine Y2BaCuO5 inclusions dispersed in the m a t r i x . I V a r i o u s k i n d s of pinning centers have also been
I 10Mm
Ib) FIGURE
D < lum I
~
~OMm
I
The p o l a r i z a t i o n m i c r o g r a p h s for the d e c o r a t e d pattern of Ni particles on the M P M G - p r o c e s s e d YBa2Cu307 c o n t a i n i n g Y2BaCuO5 with different sizes. D denotes the mean grain size of Y2BaCuO5 inclusions. The decoration conditions :T = 10 K, H = 10 Oe.
09214534/91/$03.50 O 1991 -Elscvicr Scicnce Publishers B.V. All rights reservcd.
2654
Y. Higashida et a£ / The flux line lattice in MPMG~processed YBCO
proposed, s u c h as p o i n t d e f e c t s , dislocations, and twin boundaries. It has been found that the absolute value of the pinning potential d e t e r m i n e d by the resistive transition b e h a v i o r in an applied m a g n e t i c field is oneorder smaller than the value d e t e r m i n e d by the time degradation in the magnetization. Further, it is also pointed out that the pinning potential obtained experimentally has s t r o n g temperature dependence. These results suggest the p o s s i b i l i t y that the high temperature superconductor have several kinds of pinning centers, each of w h i c h has different pinning potential energies. I n o r d e r to v e r i f y whether the Y2BaCuO5 inclusions can act as pinning centers, the fluxoids trapped by the M P M G - p r o c e s s e d YBa2Cu307 were o b s e r v e d using the Bitter p a t t e r n technique . 2. EXPERIMENTAL
The sample size of M P M G - p r o c e s s e d YBa2Cu307 was 1x7x0.05 mm3. The surface was polished using I pm d i a m o n d paste. The sample was fieldcooled at I00e d o w n to 10 K a n d subjected to decoration. The ferromagnetic particles were g e n e r a t e d by e v a p o r a t i n g a small amount of nickel wire in a helium gas atmosphere. The h e l i u m gas pessure was a b o u t 0.1 torr, w h i c h was e n o u g h to g e n e r a t e fine n i c k e l p a r t i c l e s . T h e helium carrier gas, which was flushed through the chamber, drove the "smoke" of the nickel particles in the v i c i n i t y of the s a m p l e s u r f a c e . T h e d e c o r a t e d pattern was observed using a polarized microscope. 3. RESULTS AND D I S C U S S I O N S Fig.1 shows the d e c o r a t e d B i t t e r patterns for M P M G - p r o c e s s e d YBa2Cu307 c o n t a i n i n g fine Y 2 B a C u O 5 i n c l u s i o n s w i t h d i f f e r e n t s i z e s . The Y 2 B a C u O 5 i n c l u s i o n s a p p e a r as w h i t e i s l a n d s . The black dots are the deposited nickel particles. The nickel p a r t i c l e s are mainly present on and around the Y2BaCuO5 inclusions, indicating that the Y2BaCu05 inclusions are d o m i n a n t pinning centers. H o w e v e r , t h e r e e x i s t some n i c k e l p a r t i c l e s w h i c h are not on Y 2 B a C u O 5 inclusions. This may be because some flux lines w h i c h a r e p i n n e d by the Y2BaCuO5 inclusions present inside are o b s e r v e d at the s u r f a c e area w h e r e
"fi'i"i77/
:l!i}ii;oiti) FIGURE
2
Cross sectional view of the magnetic f l u x l i n e s t r a p p e d by t h e d i s p e r s e d Y2BaCuO5 inclusions. there are no Y2BaCuO5 inclusions ( F i g u r e 2). This a m b i g u i t y w i l l be removed by thinning the sample down to the size of Y2BaCuO5 inclusions . The very disturbed and irregular d i s t r i b u t i o n of nickel particles also suggests that the Y 2 B a C u O 5 inclusions provide strong flux pinning.
4. C O N C L U S I O N S The first and direct observation of pinning sites of the m a g n e t i c flux in M P M G - p r o c e s s e d YBa2Cu307 is presented using the Bitter pattern technique. The r e s u l t s indicate that the Y2BaCuO5 inclusions actually operate as pinning centers. ACKNOWLEDGMENTS A part of this by NEDO.
work
was
supported
2EFERENCES I. H. Fujimoto, M. Murakami, S. Gotoh, ~ . Koshizuka • ~ T . Oyama , Y . Shiohara~ and S. Tanakat Advances in Superconductivityt Proc. international Conf. on Superconductivity~ Tsukuba, Japan, 1989 ed. Ko X a ~ i m u r a and T. Ishigurot Springer~ 1989.
OBSERVATION DECORATION
OF T H E F L U X L I N E L A T T I C E TECHNIQUE OF F E R R O M A G N E T I C
IN M P M G - P R O C E S S E D PARTICLES
YBCO
USING
Yutaka HIGASHIDA, Yukio KUBO, Masato MURAKAMI*a Hiroyuki FUJIMOTO e, YAMAGUCHI*, Tsutomu TAKATA*, Akihiro KONDOH*, and Naoki KOSHIZUKA*,
A
Kojl
Research and Development L a b o r a t o r y , J a p a n Fine C e r a m i c s C e n t e r , 2-4-I Mutsuno-cho, Atsuta-ku, 456 Nagoya, Japan, *Superconductivity Research Laboratory, International SupercOnductivity T e c h n o l o g y Center, 10-13 S h i n o n o m e 1-chome, Koto-ku, 135 Tokyo, Japan O b s e r v a t i o n of the flux line d i s t r i b u t i o n in M P M G - p r o c e s s e d YBa2Cu307 has been c o n d u c t e d using a d e c o r a t i o n t e c h n i q u e of f e r r o m a g n e t i c particles, it has been found that the flux lines are t r a p p e d mainly at Y2BaCuO5 inclusions or the interface between the inclusion and the s u p e r c o n d u c t i n g matrix. 1. I N T R O D U C T I O N
In order to obtain a s u p e r c o n d u c t o r having high critical current density, it is e s s e n t i a l to introduce pinning c e n t e r s w h i c h p r e v e n t the m a g n e t i c flux motion. The m e l t - p r o c e s s is s u i t a b l e for producing a bulk s u p e r c o n d u c t o r with no w e a k l i n k s a n d r e l a t i v e l y large critical c u r r e n t density. The Y2BaCu05
(a)
D = 2urn I
inclusions in t h e m e l t - p r o c e s s e d YBa2Cu307 have been proposed as candidates for the flux pinning centers. It has been considered that the strong pinning force of t h e MPMG(Melt Powder Melt Growth)processed Y B a 2 C u 3 0 7 is attributed to the fine Y2BaCuO5 inclusions dispersed in the m a t r i x . I V a r i o u s k i n d s of pinning centers have also been
I 10Mm
Ib) FIGURE
D < lum I
~
~OMm
I
The p o l a r i z a t i o n m i c r o g r a p h s for the d e c o r a t e d pattern of Ni particles on the M P M G - p r o c e s s e d YBa2Cu307 c o n t a i n i n g Y2BaCuO5 with different sizes. D denotes the mean grain size of Y2BaCuO5 inclusions. The decoration conditions :T = 10 K, H = 10 Oe.
09214534/91/$03.50 O 1991 -Elscvicr Scicnce Publishers B.V. All rights reservcd.
2654
Y. Higashida et a£ / The flux line lattice in MPMG~processed YBCO
proposed, s u c h as p o i n t d e f e c t s , dislocations, and twin boundaries. It has been found that the absolute value of the pinning potential d e t e r m i n e d by the resistive transition b e h a v i o r in an applied m a g n e t i c field is oneorder smaller than the value d e t e r m i n e d by the time degradation in the magnetization. Further, it is also pointed out that the pinning potential obtained experimentally has s t r o n g temperature dependence. These results suggest the p o s s i b i l i t y that the high temperature superconductor have several kinds of pinning centers, each of w h i c h has different pinning potential energies. I n o r d e r to v e r i f y whether the Y2BaCuO5 inclusions can act as pinning centers, the fluxoids trapped by the M P M G - p r o c e s s e d YBa2Cu307 were o b s e r v e d using the Bitter p a t t e r n technique . 2. EXPERIMENTAL
The sample size of M P M G - p r o c e s s e d YBa2Cu307 was 1x7x0.05 mm3. The surface was polished using I pm d i a m o n d paste. The sample was fieldcooled at I00e d o w n to 10 K a n d subjected to decoration. The ferromagnetic particles were g e n e r a t e d by e v a p o r a t i n g a small amount of nickel wire in a helium gas atmosphere. The h e l i u m gas pessure was a b o u t 0.1 torr, w h i c h was e n o u g h to g e n e r a t e fine n i c k e l p a r t i c l e s . T h e helium carrier gas, which was flushed through the chamber, drove the "smoke" of the nickel particles in the v i c i n i t y of the s a m p l e s u r f a c e . T h e d e c o r a t e d pattern was observed using a polarized microscope. 3. RESULTS AND D I S C U S S I O N S Fig.1 shows the d e c o r a t e d B i t t e r patterns for M P M G - p r o c e s s e d YBa2Cu307 c o n t a i n i n g fine Y 2 B a C u O 5 i n c l u s i o n s w i t h d i f f e r e n t s i z e s . The Y 2 B a C u O 5 i n c l u s i o n s a p p e a r as w h i t e i s l a n d s . The black dots are the deposited nickel particles. The nickel p a r t i c l e s are mainly present on and around the Y2BaCuO5 inclusions, indicating that the Y2BaCu05 inclusions are d o m i n a n t pinning centers. H o w e v e r , t h e r e e x i s t some n i c k e l p a r t i c l e s w h i c h are not on Y 2 B a C u O 5 inclusions. This may be because some flux lines w h i c h a r e p i n n e d by the Y2BaCuO5 inclusions present inside are o b s e r v e d at the s u r f a c e area w h e r e
"fi'i"i77/
:l!i}ii;oiti) FIGURE
2
Cross sectional view of the magnetic f l u x l i n e s t r a p p e d by t h e d i s p e r s e d Y2BaCuO5 inclusions. there are no Y2BaCuO5 inclusions ( F i g u r e 2). This a m b i g u i t y w i l l be removed by thinning the sample down to the size of Y2BaCuO5 inclusions . The very disturbed and irregular d i s t r i b u t i o n of nickel particles also suggests that the Y 2 B a C u O 5 inclusions provide strong flux pinning.
4. C O N C L U S I O N S The first and direct observation of pinning sites of the m a g n e t i c flux in M P M G - p r o c e s s e d YBa2Cu307 is presented using the Bitter pattern technique. The r e s u l t s indicate that the Y2BaCuO5 inclusions actually operate as pinning centers. ACKNOWLEDGMENTS A part of this by NEDO.
work
was
supported
2EFERENCES I. H. Fujimoto, M. Murakami, S. Gotoh, ~ . Koshizuka • ~ T . Oyama , Y . Shiohara~ and S. Tanakat Advances in Superconductivityt Proc. international Conf. on Superconductivity~ Tsukuba, Japan, 1989 ed. Ko X a ~ i m u r a and T. Ishigurot Springer~ 1989.