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Selected area measurements of magnetization in high-Tc YBCO superconductor
Physica C 185-189 (1991) 2299-2300 North-Holland
SELECTED AREA MEASUREMENTS OF MAGNETIZATION IN HiGH-Tc YBCO SUPERCONDUCTOR Toshio NAGASHIMA and Yuh FUKAI Physics Dept., Chuo University, Bunkyo-ku, Tokyo 112, Japan Selected area measurements of magnetization and remanent magnetization fields in sintered specimens of highTc YBCO superconductors are reported. The measurements show that the field dependence of the intergranular currents can be expressed in the exponenti~d form, while the intra-granular currents at higher fields by a power-law. Temperature dependence of the critical currents, both of inter- and intra-graln currents, shows the behavior expected for the Josephson junctions in the dirty-limit of the BCS theory.
It is commonly speculated, without direct pr~,~f, that low critical current densities in the sintered high-Tc
the applied field is kept below about i00 Oe. The contributions from the inter- and intra-granular currents
superconductors are determined by Josephson junctions formed at grain boundaries 1 or inside crystal grains 2.
were identified by comparing the results obtained with a
In order to examine this problem, we performed separate determination of inter- and intra-granular critical
of the macroscopic current were bisected by a slit. The shapes of the induced magnetization observed at different
currents in sintered YBCO samples by selected-area
temperatures appear yew similar and in fac~,proved to be
measurements of induced magnetization and remanent
scaled linearly by appropriateny choosing scaling factors for
magnetization and examined their dependence on
the horizonta] and vertica] axes. Based on the critical s~ate
temperature and magnetic field strength.
mode], we can fit these curves ~.othe ~xponentiaI form
The measurements were made by using a miniature Hall probe with an active area of =100x200 gm 2, having a
Jc=Jcoexp(-B/B0), with only two parame~er:~ Jc(}'~D and B0, where D is the
resolution of about 8 % of a single flux quantum, ¢0=2.07x10 -7 Gcm 2. Scanning rates adopted were about 3
width of the current path. As shown in Fig.2, the
sample of shape A with results obtained when circular paths
magnetization, and 10 Oe/s for the remanent magnetization.
parameters fitted to the observed data at H20, B-H_~.0can repreduce excellently the observed data over the whole range of the applied field. The parameter JcO*D thus
In all cases the measurements were started with zero-field-
obtained is plotted in Fig.3 as a function of temperatm'e.
Oe/s and 0.3 Oe/s for measurements of
the induced
cooled samples. The temperature was controlled to within 85K
:L0.1K during the measurements. Sintered samples were prepared by a conventional solid
2
°
state reaction, i.e. firing at 920 °C for 5h, followed by
\
e~ o A
r~orlndlno a n d nollotiTin, th~ nr*gr*rl ~ ....~ a ~ "2 .";~o at
°.
920 *C f c 24h. Samples were prepared in two different shapes; a disc of 08mmx4mm with ~] cemral hole of q~3mm(sample A) and a disc of ~6mmxlmm ( sample B). Figure 1 shows an example of the field d,zpendence of the induced magnetization (4toM=B-H) at tt~e center of a hole of sample A, where M can be regarded to be essenlially determined by inter-granular ctuzents, as long as
-2
f
-3o
~
~
o
-z0
-xe H
0 le (Oe)
2e
3~
F~g. 1. Typical fie~d dependence of induced ma~eaza~e~ curve measured at a center of a hole of .~a~p]e A a'. ~57<
0921-4534/91/$03.50 © 1991 - Elscvicr Scicncc PuNishers B.V. All fights ~,,:scrvcd
2300
T. Nagashima, Y. Fukai
/ Selected
area measurements of magnetization
-O=-6
10"
@ - ~ . ~
lye-=--
:
4 ©
% 5 ¢O
¢O
0 78
80
82
84
86
88
90
92
T (K) 11/6.8Oe
Fig.2. Field dependence of induced magnetization for sample A at 85K(dots) compared with a simulated curve by using an exponential form for Jc. The fact that the inter-granular critical current depends linearly on temperature suggests, according to the theory of Ambegaokar and Baratoff3, that it is determined by weaklink Josephson junctions. On the other hand, in order to determine the intragranular critical currents, we performed measurements of both induced and remanent magnetizations. The field dependence of the remanent magnetization manifests contributions of inter- and intra-granular currents. These contributions can be separated fairly easily by comparing powder and sintered samples, and also from their dependence on the magnetic-field strength. The contribution of the inter-granular currents was found to decrease with increase of the applied field, and become undetectable above 60 Oe. There, the remanent magnetization measured at the center showed no dependence on sample shapes. As there is sufficient allowance between this value(6lK)e) and the applied field which allows full penetxation into the grains, H*(-250Oe at 77K), we can determine H* from the remanent magnetization and from the peak to peak value of the induced magnetization based on the Bean's model4. As H* is proportional to the intra-granular critical current, the
Fig.3. Temperature dependence of inter- and intra-granular critical currents. For sample A, inter-granular current times the width of the current path, Jc0xD, is plotted (®), and for sample B, intra-granular currents (in arbitrary units) estimated from H* determined by remanent magnetization(-') and peak to peak height of induced magnetiztion (0) are plotted. current was fitted by the power law Jc=Jc0 H-n, with two parameters, n=0.92, Jc0=2xl06 AJcm 2 above 2H*, and Jc=constant below 2H*. It is worth while to note that the transition temperature for the inter-grain superconductivity is about 2K lower than that of the intra-grains. This is consistent with the behaviors commonly observed in X"-measurements or multiple steps in resistance measurements5. Identification of the Josephson junctions that determine inter- and intragranular critical currents is left for future studies. Acknowledgments This work was p~a fly suppo.,'t.edby a Grant-in-Aid for Scientific Research on Priority Areas, "Chemistry of New Superconductors", from the Ministry of Education, Science and Culture of Japan. References
1. J.W.Ekin et al., J. App]. Phys. 62(1987) 4821.
values of H* obtained for sample B are plotted in Fig.3 as Jc in arbitrary scale. Here again, the temperature
2. G.Deutscher and K.A.Muller, Phys. Rev. Lett. 51(1987)15.
dependence is linear, which indicates that the intra-granu!ar critical current is also determined by Josephson junctions.
3. V.Ambegaokar and A.Baratoff, Phys. Rev. Lett. 110(1963)486;1 I(1963)104.
From the remanent magnetization measurements on sample
4. C.P.Bean, Phys. Rev. Left. 8(1962),250.
B (77K), the field dependence of the intra-granular critical
5. D.Goldshmidt, Ph5;. Rev. B39(1989)2372.
SELECTED AREA MEASUREMENTS OF MAGNETIZATION IN HiGH-Tc YBCO SUPERCONDUCTOR Toshio NAGASHIMA and Yuh FUKAI Physics Dept., Chuo University, Bunkyo-ku, Tokyo 112, Japan Selected area measurements of magnetization and remanent magnetization fields in sintered specimens of highTc YBCO superconductors are reported. The measurements show that the field dependence of the intergranular currents can be expressed in the exponenti~d form, while the intra-granular currents at higher fields by a power-law. Temperature dependence of the critical currents, both of inter- and intra-graln currents, shows the behavior expected for the Josephson junctions in the dirty-limit of the BCS theory.
It is commonly speculated, without direct pr~,~f, that low critical current densities in the sintered high-Tc
the applied field is kept below about i00 Oe. The contributions from the inter- and intra-granular currents
superconductors are determined by Josephson junctions formed at grain boundaries 1 or inside crystal grains 2.
were identified by comparing the results obtained with a
In order to examine this problem, we performed separate determination of inter- and intra-granular critical
of the macroscopic current were bisected by a slit. The shapes of the induced magnetization observed at different
currents in sintered YBCO samples by selected-area
temperatures appear yew similar and in fac~,proved to be
measurements of induced magnetization and remanent
scaled linearly by appropriateny choosing scaling factors for
magnetization and examined their dependence on
the horizonta] and vertica] axes. Based on the critical s~ate
temperature and magnetic field strength.
mode], we can fit these curves ~.othe ~xponentiaI form
The measurements were made by using a miniature Hall probe with an active area of =100x200 gm 2, having a
Jc=Jcoexp(-B/B0), with only two parame~er:~ Jc(}'~D and B0, where D is the
resolution of about 8 % of a single flux quantum, ¢0=2.07x10 -7 Gcm 2. Scanning rates adopted were about 3
width of the current path. As shown in Fig.2, the
sample of shape A with results obtained when circular paths
magnetization, and 10 Oe/s for the remanent magnetization.
parameters fitted to the observed data at H20, B-H_~.0can repreduce excellently the observed data over the whole range of the applied field. The parameter JcO*D thus
In all cases the measurements were started with zero-field-
obtained is plotted in Fig.3 as a function of temperatm'e.
Oe/s and 0.3 Oe/s for measurements of
the induced
cooled samples. The temperature was controlled to within 85K
:L0.1K during the measurements. Sintered samples were prepared by a conventional solid
2
°
state reaction, i.e. firing at 920 °C for 5h, followed by
\
e~ o A
r~orlndlno a n d nollotiTin, th~ nr*gr*rl ~ ....~ a ~ "2 .";~o at
°.
920 *C f c 24h. Samples were prepared in two different shapes; a disc of 08mmx4mm with ~] cemral hole of q~3mm(sample A) and a disc of ~6mmxlmm ( sample B). Figure 1 shows an example of the field d,zpendence of the induced magnetization (4toM=B-H) at tt~e center of a hole of sample A, where M can be regarded to be essenlially determined by inter-granular ctuzents, as long as
-2
f
-3o
~
~
o
-z0
-xe H
0 le (Oe)
2e
3~
F~g. 1. Typical fie~d dependence of induced ma~eaza~e~ curve measured at a center of a hole of .~a~p]e A a'. ~57<
0921-4534/91/$03.50 © 1991 - Elscvicr Scicncc PuNishers B.V. All fights ~,,:scrvcd
2300
T. Nagashima, Y. Fukai
/ Selected
area measurements of magnetization
-O=-6
10"
@ - ~ . ~
lye-=--
:
4 ©
% 5 ¢O
¢O
0 78
80
82
84
86
88
90
92
T (K) 11/6.8Oe
Fig.2. Field dependence of induced magnetization for sample A at 85K(dots) compared with a simulated curve by using an exponential form for Jc. The fact that the inter-granular critical current depends linearly on temperature suggests, according to the theory of Ambegaokar and Baratoff3, that it is determined by weaklink Josephson junctions. On the other hand, in order to determine the intragranular critical currents, we performed measurements of both induced and remanent magnetizations. The field dependence of the remanent magnetization manifests contributions of inter- and intra-granular currents. These contributions can be separated fairly easily by comparing powder and sintered samples, and also from their dependence on the magnetic-field strength. The contribution of the inter-granular currents was found to decrease with increase of the applied field, and become undetectable above 60 Oe. There, the remanent magnetization measured at the center showed no dependence on sample shapes. As there is sufficient allowance between this value(6lK)e) and the applied field which allows full penetxation into the grains, H*(-250Oe at 77K), we can determine H* from the remanent magnetization and from the peak to peak value of the induced magnetization based on the Bean's model4. As H* is proportional to the intra-granular critical current, the
Fig.3. Temperature dependence of inter- and intra-granular critical currents. For sample A, inter-granular current times the width of the current path, Jc0xD, is plotted (®), and for sample B, intra-granular currents (in arbitrary units) estimated from H* determined by remanent magnetization(-') and peak to peak height of induced magnetiztion (0) are plotted. current was fitted by the power law Jc=Jc0 H-n, with two parameters, n=0.92, Jc0=2xl06 AJcm 2 above 2H*, and Jc=constant below 2H*. It is worth while to note that the transition temperature for the inter-grain superconductivity is about 2K lower than that of the intra-grains. This is consistent with the behaviors commonly observed in X"-measurements or multiple steps in resistance measurements5. Identification of the Josephson junctions that determine inter- and intragranular critical currents is left for future studies. Acknowledgments This work was p~a fly suppo.,'t.edby a Grant-in-Aid for Scientific Research on Priority Areas, "Chemistry of New Superconductors", from the Ministry of Education, Science and Culture of Japan. References
1. J.W.Ekin et al., J. App]. Phys. 62(1987) 4821.
values of H* obtained for sample B are plotted in Fig.3 as Jc in arbitrary scale. Here again, the temperature
2. G.Deutscher and K.A.Muller, Phys. Rev. Lett. 51(1987)15.
dependence is linear, which indicates that the intra-granu!ar critical current is also determined by Josephson junctions.
3. V.Ambegaokar and A.Baratoff, Phys. Rev. Lett. 110(1963)486;1 I(1963)104.
From the remanent magnetization measurements on sample
4. C.P.Bean, Phys. Rev. Left. 8(1962),250.
B (77K), the field dependence of the intra-granular critical
5. D.Goldshmidt, Ph5;. Rev. B39(1989)2372.