- Email: [email protected]
Mixed state of sol-gel YBaCuO type high-temperature superconductors
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1311-1312
Journalof magnetism ~ H and magnetic J H materials
ELSEVIER
Mixed state of sol-gel Y-Ba-Cu-O type high-temperature superconductors W.M. Woch a,*, T. Sci~2or a, A. Kotodziejczyk a, j. Chmist a, Z. Trybuta b, J. Stankowski b, T. Brylewski c, K. Przybylski c Solid State Physics Dept., Uniuersity of Mining and Metallurgy, 30-059 Krak6w, Poland h Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznah, Poland c Physical Chemistry of Solids Dept., Unil~'ersityof Mining and Metallurgy, 30-059 Krak6w, Poland a
Abstract The microwave absorption and ac susceptibility vs. magnetic field and temperature were measured for Y - B a - C u - O type high-Tc superconductors which were prepared by the sol-gel method. Some characteristic fields were found and their temperature dependences were measured. The grain size and porosity effects on the penetration of a magnetic field into the sample were also studied.
The granular structure of high-temperature superconductors (HTS) plays an important role in determining their transport and magnetic properties [1]. The ceramic samples are basically modeled as consisting of homogeneous superconducting grains connected by both Josephson junctions and weak intergrain links [2]. The superconductivity is achieved in two steps, that is grains become superconducting first and then the whole sample, through intergrain junctions. Therefore, most of the magnetic properties show intergranular and intragranular characteristics at very low fields and higher fields, respectively. Thus, the intergranular critical state exists at low fields beyond which the grains are decoupled and the intragranular critical state appears at higher fields. The microwave absorption and ac susceptibility measurements of HTS are widely used to characterize their critical state. The bulk ceramic samples of YBazCu30 x were prepared by the sol-gel method [3]. These samples were pressurized under a pressure from 5 to 800 MPa and sintered in oxygen in the temperature interval from 905°C to 985°C. As a result, a series of samples with grain diameters of 3.0, 4.7, 5.6, 6.5, 7.1, 9.0 and 14.2 p~m and critical temperatures ranging from 90.3 to 91.4 K was obtained. The field modulation microwave absorption (FMMA) was measured using a standard X-band spectrometer in the
* Corresponding author. [email protected].
Fax:
+48-12-341247;
email:
following set up: amplitude of field modulation, microwave power, and sweep rate of applied magnetic field were less than 0 . 2 0 e , 2 mW and 10 O e / m i n , respectively. After each measurement step a sample was heated up above Tc and cooled down using the zero field cooling (ZFC) procedure. Measurements of the imaginary (absorption) X" part of the ac susceptibility were made using a standard mutual inductance bridge operating at an ac field of about 100 mOe and a frequency of 133 Hz. The measurements were carried out both as a function of the temperature and as a function of the applied dc magnetic field. The characteristic low field maximum (LFM) on FMMA curves was measured as a function of temperature in the magnetic field from 0.2 to about 200 Oe. The field in which the LFM has occurred is always a threshold field below which the remanency and hysteresis of the microwave absorption signal are not observed and the signal is fully reversible [4]. Therefore, this field was called reversibility field Hrev. The reversibility field as a function of temperature for the sample of 6.5 ~ m grain size is shown in Fig. 1. One can notice that the temperature dependence of the reversibility field could be divided into two regions: the first, low-temperature region from 5 to about 50 K and the second, high-temperature region from 50 K to the critical temperature. In the first region, Hrev(T) is well approximated by the glassy-like de Almeida-Thouless line ( A - T line) [5] in the form:
mrev(T ) = C1(1 - r / T c ) 3/2.
(1)
In the second region, however, experimental data of //rev
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01432-9
1312
W.M. Woch et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1311-I312
80
250-
60 • ' ~ )
-"
6.56m
200 '~
ft
A
•
6.5 ~tn
Hc/j
150.
O 40
100-
-r- 20
50 .
•
,
n
AA u : n ma " n ~
° o
muZ~l~u.
~
0
OI
( ' ' 2'0'
40 6'0 T [K]
20
'8'0~" 100
Fig. 1. The reversibility field as a function of temperature. Curve (1) was obtained using Eq. (1). The constant Cj is 80 Oe. Curve (2) was obtained using Eq. (2). The constant C 2 is 37 Oe. deviate from the A - T line. Therefore, in this region the classical formula for the critical field, Hc(T ) = 62(1 - ( T / T c ) 2 ) ,
(2)
was used for the fit of Hr~v(T). The agreement is quite good. This behavior is observed for the first time only for the sol-gel samples, most likely due to a narrow and more homogenous distribution of the intergrain junctions which are fairly decoupled close to T~. The reversibility field Hrev(T) of ceramic samples obtained by the standard solid state reaction procedure, as well as sol-gel samples but with large grains and wide grain size distribution, is well described by the A - T line within the whole temperature range below T~. Therefore, within the temperature range close to T~, reversibility fields correspond to the critical field of the intergrain junctions. At lower temperature Hr~ can be connected to the flux penetration field. The field H~ v decreases as the average grain size increases and it is equal to 15, 12 and 9 0 e at liquid nitrogen temperature for 4.7, 6.5 and 7.1 Fm samples, respectively. Some characteristic fields can be defined on the imaginary part of ac susceptibility ( X " ) vs applied magnetic field plots (Fig. 2). In this paper only two of them (H(1) and H(2)) have been discussed. H(1) is the field for which X" starts to increase from the initial near-zero value, and H(2) is the field of the first maximum. The temperature dependence of H(1)(T) (Fig. 3) can be approximated first as a linear increase with decrease of temperature, espe-
/
ooo2o~
00015
"
W" ~.
i
~,~ 0.0010
"
,,a
H(2)" 0 0005 00000 - - ' ~ ' ~
/," ' -' ' ' ') " 10
m,,\ ................... 100
1000
H fOe] Fig. 2. The imaginary (absorption) part of ac susceptibility as a function of applied magnetic field. The characteristic fields are defined (see text).
40 T [ K ] 6 0
80
100
Fig. 3. The proposed phase diagram of intergrain junctions. Thc closed squares indicate reversibility field obtained by the microwave absorption. The open triangles show the critical field and the closed circles mean the irreversibility field of intergrain junctions obtained by ac susceptibility. cially close to Tc. The temperature dependences of H(2)(T) obey a power law as a function of temperature. However, exact analysis shows that all dependences can be described in the best way by the A - T line. H(1) and H(2) have been connected to the intergranular junctions. The H(1) field can be associated with critical field He1J and the H(2) field can be related to irreversibility field Hirrj of intergranular junctions [6]. As a conclusion, a H - T phase diagram of the sol-geL HTS samples is proposed (Fig. 3). The critical fields of intergrain junctions obtained by microwave absorption and ac susceptibility are comparable at temperatures close to Tc. At lower temperature the critical fields He1J obtained by ac susceptibility are higher than Hrev obtained by microwave absorption. The difference could be connected with a large difference of the frequency window of the two experimental techniques. The parameters C n and C 2 show dependence on the sintering temperature, and thus also on the granularity and porosity of these samples. We connect this fact with: (1) the change of total volume of intergranular weak links, and (2) the change of oxidation parameter with the changing of sintering temperature. Acknowledgments: This work is financed by the Polish Committee of Scientific Research through the project 209269101. References [1] G. Deutscher, Springer Series in Solid-State Sciences, vol. 90, eds. J.G. Bednorz and K.A. Miiller (Springer, Berlin, 1990), p. 174. [2] See, for instance, M. Tinkham and CJ. Lobb, Physical Properties of the New Superconductors, Solid State Physics, vol. 42, eds. H. Ehrenreich and D. Turnbull (Academic, San Diego 1989) p. 91. [3] T. Brylewski and K. Przybylski, Appl. Supercond. 1 (1993) 737. [4] W.M. Woch, A. Kolodziejczyk, Z. Trybuta and J. Stankowski, Proc. of Ampere Workshop, Poznafi 1994, to be published. [5] J.R.L. de Almeida and D.J. Thouless, J. Phys. A 11 (1978) 983. [6] Y. Yamaguchi, M. Tokumoto and K. Mitsugi, Physica C 185-189 (1991) 1861.
Journalof magnetism ~ H and magnetic J H materials
ELSEVIER
Mixed state of sol-gel Y-Ba-Cu-O type high-temperature superconductors W.M. Woch a,*, T. Sci~2or a, A. Kotodziejczyk a, j. Chmist a, Z. Trybuta b, J. Stankowski b, T. Brylewski c, K. Przybylski c Solid State Physics Dept., Uniuersity of Mining and Metallurgy, 30-059 Krak6w, Poland h Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznah, Poland c Physical Chemistry of Solids Dept., Unil~'ersityof Mining and Metallurgy, 30-059 Krak6w, Poland a
Abstract The microwave absorption and ac susceptibility vs. magnetic field and temperature were measured for Y - B a - C u - O type high-Tc superconductors which were prepared by the sol-gel method. Some characteristic fields were found and their temperature dependences were measured. The grain size and porosity effects on the penetration of a magnetic field into the sample were also studied.
The granular structure of high-temperature superconductors (HTS) plays an important role in determining their transport and magnetic properties [1]. The ceramic samples are basically modeled as consisting of homogeneous superconducting grains connected by both Josephson junctions and weak intergrain links [2]. The superconductivity is achieved in two steps, that is grains become superconducting first and then the whole sample, through intergrain junctions. Therefore, most of the magnetic properties show intergranular and intragranular characteristics at very low fields and higher fields, respectively. Thus, the intergranular critical state exists at low fields beyond which the grains are decoupled and the intragranular critical state appears at higher fields. The microwave absorption and ac susceptibility measurements of HTS are widely used to characterize their critical state. The bulk ceramic samples of YBazCu30 x were prepared by the sol-gel method [3]. These samples were pressurized under a pressure from 5 to 800 MPa and sintered in oxygen in the temperature interval from 905°C to 985°C. As a result, a series of samples with grain diameters of 3.0, 4.7, 5.6, 6.5, 7.1, 9.0 and 14.2 p~m and critical temperatures ranging from 90.3 to 91.4 K was obtained. The field modulation microwave absorption (FMMA) was measured using a standard X-band spectrometer in the
* Corresponding author. [email protected].
Fax:
+48-12-341247;
email:
following set up: amplitude of field modulation, microwave power, and sweep rate of applied magnetic field were less than 0 . 2 0 e , 2 mW and 10 O e / m i n , respectively. After each measurement step a sample was heated up above Tc and cooled down using the zero field cooling (ZFC) procedure. Measurements of the imaginary (absorption) X" part of the ac susceptibility were made using a standard mutual inductance bridge operating at an ac field of about 100 mOe and a frequency of 133 Hz. The measurements were carried out both as a function of the temperature and as a function of the applied dc magnetic field. The characteristic low field maximum (LFM) on FMMA curves was measured as a function of temperature in the magnetic field from 0.2 to about 200 Oe. The field in which the LFM has occurred is always a threshold field below which the remanency and hysteresis of the microwave absorption signal are not observed and the signal is fully reversible [4]. Therefore, this field was called reversibility field Hrev. The reversibility field as a function of temperature for the sample of 6.5 ~ m grain size is shown in Fig. 1. One can notice that the temperature dependence of the reversibility field could be divided into two regions: the first, low-temperature region from 5 to about 50 K and the second, high-temperature region from 50 K to the critical temperature. In the first region, Hrev(T) is well approximated by the glassy-like de Almeida-Thouless line ( A - T line) [5] in the form:
mrev(T ) = C1(1 - r / T c ) 3/2.
(1)
In the second region, however, experimental data of //rev
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01432-9
1312
W.M. Woch et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1311-I312
80
250-
60 • ' ~ )
-"
6.56m
200 '~
ft
A
•
6.5 ~tn
Hc/j
150.
O 40
100-
-r- 20
50 .
•
,
n
AA u : n ma " n ~
° o
muZ~l~u.
~
0
OI
( ' ' 2'0'
40 6'0 T [K]
20
'8'0~" 100
Fig. 1. The reversibility field as a function of temperature. Curve (1) was obtained using Eq. (1). The constant Cj is 80 Oe. Curve (2) was obtained using Eq. (2). The constant C 2 is 37 Oe. deviate from the A - T line. Therefore, in this region the classical formula for the critical field, Hc(T ) = 62(1 - ( T / T c ) 2 ) ,
(2)
was used for the fit of Hr~v(T). The agreement is quite good. This behavior is observed for the first time only for the sol-gel samples, most likely due to a narrow and more homogenous distribution of the intergrain junctions which are fairly decoupled close to T~. The reversibility field Hrev(T) of ceramic samples obtained by the standard solid state reaction procedure, as well as sol-gel samples but with large grains and wide grain size distribution, is well described by the A - T line within the whole temperature range below T~. Therefore, within the temperature range close to T~, reversibility fields correspond to the critical field of the intergrain junctions. At lower temperature Hr~ can be connected to the flux penetration field. The field H~ v decreases as the average grain size increases and it is equal to 15, 12 and 9 0 e at liquid nitrogen temperature for 4.7, 6.5 and 7.1 Fm samples, respectively. Some characteristic fields can be defined on the imaginary part of ac susceptibility ( X " ) vs applied magnetic field plots (Fig. 2). In this paper only two of them (H(1) and H(2)) have been discussed. H(1) is the field for which X" starts to increase from the initial near-zero value, and H(2) is the field of the first maximum. The temperature dependence of H(1)(T) (Fig. 3) can be approximated first as a linear increase with decrease of temperature, espe-
/
ooo2o~
00015
"
W" ~.
i
~,~ 0.0010
"
,,a
H(2)" 0 0005 00000 - - ' ~ ' ~
/," ' -' ' ' ') " 10
m,,\ ................... 100
1000
H fOe] Fig. 2. The imaginary (absorption) part of ac susceptibility as a function of applied magnetic field. The characteristic fields are defined (see text).
40 T [ K ] 6 0
80
100
Fig. 3. The proposed phase diagram of intergrain junctions. Thc closed squares indicate reversibility field obtained by the microwave absorption. The open triangles show the critical field and the closed circles mean the irreversibility field of intergrain junctions obtained by ac susceptibility. cially close to Tc. The temperature dependences of H(2)(T) obey a power law as a function of temperature. However, exact analysis shows that all dependences can be described in the best way by the A - T line. H(1) and H(2) have been connected to the intergranular junctions. The H(1) field can be associated with critical field He1J and the H(2) field can be related to irreversibility field Hirrj of intergranular junctions [6]. As a conclusion, a H - T phase diagram of the sol-geL HTS samples is proposed (Fig. 3). The critical fields of intergrain junctions obtained by microwave absorption and ac susceptibility are comparable at temperatures close to Tc. At lower temperature the critical fields He1J obtained by ac susceptibility are higher than Hrev obtained by microwave absorption. The difference could be connected with a large difference of the frequency window of the two experimental techniques. The parameters C n and C 2 show dependence on the sintering temperature, and thus also on the granularity and porosity of these samples. We connect this fact with: (1) the change of total volume of intergranular weak links, and (2) the change of oxidation parameter with the changing of sintering temperature. Acknowledgments: This work is financed by the Polish Committee of Scientific Research through the project 209269101. References [1] G. Deutscher, Springer Series in Solid-State Sciences, vol. 90, eds. J.G. Bednorz and K.A. Miiller (Springer, Berlin, 1990), p. 174. [2] See, for instance, M. Tinkham and CJ. Lobb, Physical Properties of the New Superconductors, Solid State Physics, vol. 42, eds. H. Ehrenreich and D. Turnbull (Academic, San Diego 1989) p. 91. [3] T. Brylewski and K. Przybylski, Appl. Supercond. 1 (1993) 737. [4] W.M. Woch, A. Kolodziejczyk, Z. Trybuta and J. Stankowski, Proc. of Ampere Workshop, Poznafi 1994, to be published. [5] J.R.L. de Almeida and D.J. Thouless, J. Phys. A 11 (1978) 983. [6] Y. Yamaguchi, M. Tokumoto and K. Mitsugi, Physica C 185-189 (1991) 1861.