Exploration of the superconducting properties of Y3Ba5Cu8O18 with and without Ca doping by magnetic measurements

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Exploration of the superconducting properties of Y

3

Ba

5

Cu

8

O

18

with and without

Ca doping by magnetic measurements

Ahmet Ekicibil

a,*

, Selda K

ılıç Cetin

a

, Ali Osman Ayas¸

b

, Atilla Cos¸kun

c

, Tezer F

ırat

d

, Kerim K

ıymac

a aDepartment of Physics, Faculty of Sciences and Letters, Cukurova University, 01330 Adana, Turkey

bDepartment of Physics, Faculty of Sciences and Letters, Adıyaman University, 02040 Adıyaman, Turkey cDepartment of Physics, Faculty of Sciences and Letters, Mugla University, 48000 Mugla, Turkey dSNTG Laboratory, Physics Engineering Department, Hacettepe University, 06800 Ankara, Turkey

a r t i c l e i n f o

Article history: Received 9 May 2011 Received in revised form 8 July 2011

Accepted 22 August 2011 Available online 3 September 2011 Keywords:

YBCO superconductors Critical current density Magnetic hysteresis AC susceptibility

a b s t r a c t

The superconducting Y3Ba5Cu8O18 (Y-358) and Y3Ba5Ca2Cu8O18 (YCa-358) compounds have been synthesized by using the solegel method. Hence, the influence of doping of Ca into the compound Y-358 has been studied by comparing the resistivities, DC magnetizations both M(H) and M(T),flux pinning properties, AC susceptibilities and critical current densities of the undoped and doped compounds, at low temperatures. The AC susceptibility and resistivity measurements showed that the superconducting transition temperature, Tc, is suppressed (about 6 K) with the addition of Ca into the main compound. The hysteresis loops of YCa-358 show peak like projecting parts at temperatures below 45 K and around zero applied field that may be due to the local modulation of the composition in YCa-358. Such a behavior has been observed for thefirst time. The critical current densities, Jc, determined from the hysteresis measurements decrease with the addition of Ca into Y-358. At 15 K, the maximum values of Jc, for the compounds Y-358 and YCa-358 are found to be 8 104A/cm2and 4.5 104A/cm2, respectively. Theflux pinning force, Fp,calculated from thefield dependence of the Jcvalues shows that the irre-versibility line shifts to lower magnetic fields with the doping of Ca into Y-358. Furthermore, the measurements of the inphase and out off components of the ACsusceptibilities clearly demonstrate that the superconducting volume fraction of Y-358 decreases with the addition of Ca.

Ó 2011 Elsevier Masson SAS. All rights reserved.

1. Introduction

Since the discovery of Y-based high temperature superconduc-tors (HTSs) in 1987[1], many attempts have been made to improve their superconducting properties, by using various methods. Y-based superconductors attract great technological interest, since their superconducting transition temperatures, Tc, exceed the

liquid-nitrogen temperature. For instance, the YBa2Cu3O7d(Y-123)

superconductor has Tc¼81 K[2]. However, the main disadvantage

of the Y-123 compound is the instability of its oxygen content[3]. Therefore, after the synthesis of Y-123, by using the conventional solid state reaction method, too many attempts were made tofind the new forms of the material offering higher Tcand better

super-conducting properties. Furthermore, many researches have been carried out to obtain new compositions of the YBCO supercon-ductors, like, YBa2Cu4O8(Y-124)[4], and Y2Ba4Cu7O15(Y-247)[5],

which are different from the point of the numbers of their CuO2

planes and CuO chains or double chains and their relative positions. The CuO2planes and the CuO chains are believed to be the carrier

reservoirs of the electron pairs. Doping of the carriers occurs by

pumping oxygen into the chains [6]. Some research groups

synthesized the bulk Y-124 by using a high-pressure oxygen tech-nique[7,8]. Murakami et al.[9] reported the preparation of the Y-124 phase by applying the so called solegel method. Further-more, it has been reported that Ca addition enhances the Tcof the

Y-124 phase up to 90 K[10].

The study of magnetic properties on the high-Tc

superconduc-tors is of great interest from the viewpoints of both understanding the basic phenomena in them and practical applications related to them. The HTSs YBCO family has a large potential application due to its higher critical current density, Jc, compared to other HTSs

families and hence, has become a majorfield of research area. The Jc

values are primarily limited by the insufficient flux pinning prop-erties[11]. In order to overcome this problem, rare earth elements can be doped into the compounds. The partial doping of rare earth elements, with different ionic radii, into YBCO compounds can lead

* Corresponding author. Tel.: þ90 322 338 60 84/2480; fax: þ90 322 338 60 60. E-mail address:ahmetcan@cu.edu.tr(A. Ekicibil).

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to the formation of a stressfield which could cause local flux pinning centers at the unit cell level[12]. Therefore, over the years, many researchers have employed new or additional doping elements with the intention of increasing their critical temperature (Tc) and critical

current density (Jc) values[13,14]. The main factors reducing the Jcin

HTSs polycrystalline samples are their grain boundaries and poor flux pinning properties[15,16]. The partial substitution of the rare earth elements into YBCO decreases the grain boundary problem and hence, causes an increase in the Meissner effect. Furthermore, it was also reported that after a further annealing treatment, the doped samples exhibit better critical current densities than the undoped compounds, e.g. Y-123[13,17].

The high Tc Y3Ba5Cu8O18(Y-358) superconducting compound,

having a crystal structure similar to that of the Y-123 with the exception of the number of CuO chains and CuO2planes, is a new

Y-based compound[6,18]. The compound Y-358 wasfirst prepared by the well known solid state reaction technique and was found to have a transition temperature of 102 K[6]. In our previous work

[18], we reported the preparation of the compounds Y-358 and

Y3Ca2Ba5Cu8O18(YCa-358) by using the so called solegel method.

There the structural, superconducting and transport properties were explored, by employing XRD, SEM, AFM, EDX, DTA, TGA, electrical resistivity, Hall coefficient (RH), Hall mobility (

m

H) and

magnetoresistance measurements. The transition temperatures, Tc.on, of Y-358 and YCa-358 were found to be 92.7 and 86.6 K,

respectively. For both compounds the signs of RHand

m

Hwere found

to be positive, indicating that the conductions are p-type. However, apart from the magnetoresistance, the magnetic properties of the compound Y-358 and its Ca doped form, YCa-358, have not been studied and reported yet, especially in its polycrystalline state. Therefore, in the present study, our aims are to report, DC magne-tization, both againstfield and temperature, and AC susceptibility measurements on the Y-358 and Ca doped Y-358 (YCa-358) superconducting compounds, and hence, especially to explore the effect of Ca doping on the critical current density of Y-358.

2. Experimental

The Ca free Y3Ba5Cu8O18 and Ca doped Y3Ba5Ca2Cu8O18

compounds have been prepared by the solegel method. Here after those compounds are labeled as Y-358 and YCa-358, respectively. In order to obtain the desired stoichiometries, appropriate amounts of Y2O3, Ba(NO3)2, CuO and CaCO3werefirst dissolved in dilute HNO3

solution at 150C and then citric acid and ethylene glycol were added to the mixture. A viscous residual was formed by slowly boiling this solution at 200C. Afterward, the obtained residual was dried slowly at 300C until a dry-gel was formed. Finally, in order to remove organic materials produced during chemical reactions, the residual precursor was burned in air at 600C. The materials obtained as above werefirst ground by using an agate mortar to have afine powdered form and then the pellets with 13 mm radii and 2 mm thicknesses were produced from each powdered composition by pressing them under a pressure of 3 tons. The pellets of the Y-358 and YCa-358 were then separately sintered at 890C and 900C for 72 h in air, respectively, and then cooled down to the room temperature in the furnace. The resistivity measurements of the compounds were carried out by using the standard four-probe method with silver paint contacts. The DC magnetic properties of the compounds were measured by using a Quantum Design PPMS with a closed cycle helium cryostat, from 10 K to 340 K in magneticfields up to 5 T. We have also explored the AC-magnetic properties of the compounds by measuring the in-phase and out off-in-phase components of their ACsusceptibilities. The AC-susceptibility apparatus was also the Quantum Design

PPMS, and the measurements were carried out in afield of 10 Oe at the frequencies of 10 and 1000 Hz.

3. Results and discussions

The resistivities,

r

(T), below 300 K, versus temperature,

obtained by employing the well known four probe technique on the samples of Y-358 and YCa-358, are shown inFig. 1. As it is clear from that figure the resistivities of the samples decrease slowly with decreasing temperature until their respective superconducting transition temperatures, Tc.on, and then drop to almost zero values.

The linear reduction in

r

with decreasing temperature from 300 K down to their Tc.onvalues indicates that at those temperatures our

samples have a dominantly metallic character. We should further point out that the magnitude of

r

increases by Ca doping into the

compound Y-358, indicating that the conductivity above Tc.on

decreases with the addition of Ca. However, inFig. 1, the slope of the line above Tc.onrelated to YCa-358 is more than that of the Y-358;

that means the conductivity of Ca doped Y-358 increase much faster than that of the Ca free Y-358 with decreasing temperature toward Tc. The onset of superconductivities of the compounds

Y-358 and YCa-358 as manifested by sharp drops in their resistiv-ities begin at the observed superconducting onset temperatures Tc.onofw92.7 and w86.6 K, respectively. On the other hand, their

zero resistivities, Tc.off, are achieved at w87.6 and w79.4 K,

respectively. Therefore, the transition widths to perfect supercon-ductivity are around 5 and 7.2 K for Y-358 and YCa-358, respec-tively, i.e., transitions are quite sharp and well defined. It should be mentioned that the measurements of the electrical resistivities indicate that the materials are stable, thermally recyclable and reproducible. As a result, according to the resistivity measurements the superconducting state is pressed by the doping of Ca atoms into the compound Y-358, and as pointed out above, the compounds show a metallic conducting behavior from 300 K down to their respective superconducting transition temperatures.

The hysteresis loops obtained for the compounds Y-358 and YCa-358, at three different temperatures (15, 45 and 75 K) in the 5 kOe range, are shown inFig. 2a and b, where the units of M(H) curves are give in terms of emu/cm3, instead of emu/gr, just for the sake of easy calculation of the superconducting critical current densities of the compounds that will be introduced below. It is noticeable from the comparison of thefigures that the hysteresis loops become smaller in size, at the comparable temperatures, with

Fig. 1. The resistivities,r(T) against temperature for the compounds Y3Ba5Cu8O18and

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the doping of Ca into the compound Y-358, indicating that the addition of Ca weakens the superconductivity of Y-358, in agree-ment with the resistivity results. Furthermore, it is quite obvious that for both compounds the absolute values of the hysteresis loops of the magnetization decrease (i.e., the hysteresis loops shrink and

become closer to the field axis) with increasing temperature,

indicating that the pinning forces become weaker toward Tc.on. This

is because the hysteresises are related to the presence of flux pinning sites in the material, and the fast decrease of hysteresis loops with increasing temperature and the symmetrical behavior of them imply the existence offlux pinning centers. Thus, we might suggest that the magnetization behaviors at lowfields are domi-nated by the bulk pinning centers rather than the surface and geometrical barriers. As can be seen fromFig. 2a and b, due to the pinning effects and large volumes of the superconducting regions, thefield penetration becomes difficult below 15 K for both samples. On the other hand, as expected, the appliedfields begin to signif-icantly penetrate into the samples at higher temperatures, espe-cially above 75 K, due to the decrease of superconducting regions with increasing temperature. This is probably the reason for the large decreases (shrinks) appearing in the width of the magnetic hysteresis loops above 75 K. This means also that the intrinsicflux pinning in the grains of the samples decreases rapidly at elevated temperatures. The pinning by ion tracks also becomes less efficient

due to the corresponding decreases of the coherence lengths of the samples with increasing temperature[19].

Both of the samples show strong maxima and minima at their Hc1values in their MeH curves, especially at lower temperatures.

This has been attributed to a strong increase of the intergranular currents at lower temperatures leading to a truly granular behavior

[20]. FromFig. 2a and b, it is clearly seen that the minima values of the magnetizations of the compounds at 15 K are located at Hc1h1.726 kOe and 1.573 kOe, respectively. In order to support our

conclusions mentioned above, that theflux pinning centers and

large superconducting regions are effective at lower temperatures and at lowerfields, the Hc1values of the compounds Y-358 and

YCa-358 are plotted as a function of temperature inFig. 3. As expected, the Hc1values decrease with increasing temperature and obviously

drop to zero at T¼ Tc.on, for both samples.

The dependence of hysteresis loop widths on magneticfield,

D

M(H), at temperatures 15, 45 and 75 K are also shown for the samples of Y-358 and YCa-358 in the inset ofFig. 2a and b. The characteristic features of those curves are the presence of maxima near 1400 and 1600 Oe for Y-358 and YCa-358 at 15 K, respectively, which are believed to be related to the influence of the grain surface barrier. However, if

D

M was defined by the surface barrier only, then it would fall down to zero when thefield drops to zero[21]. As it can be seen from the insets ofFig. 2a and b it is not the case due to probably the intragrain (i.e., bulk) pinning. The observed increase in the zerofield value of the widths of hysteresis loop with decreasing temperature is therefore a sign of bulk pinning strength. It is also obvious that the greater hysteresis broadening, demonstrated by peaks on the figures, takes place at fields around Hc1. Another

important observation that can be made is the formation of valleys

atfields around 0,5 kOe for T  45 K for the compound YCa-358

which probably arise due to the additional local composition modulations as explained later.

The critical current properties of the compounds can be ob-tained from the magnetic hysteresis curves shown inFig. 2a and b. It is well known that a larger hysteresis loop area means a larger superconducting critical current density, Jc. In this work, the

esti-mation of Jc from the magnetization hysteresis loops were

per-formed by using the Bean’s Critical State Model[22]that relates the critical current density to the magnetization Mþand Mobtained from the intersections of MeH loops with the fixed H lines. The magnetization irreversibility,

D

M ¼ jMþjþjMj, is related to the critical current density Jcas,

Fig. 2. Hysteresis curves for the compounds (a) Y3Ba5Cu8O18and (b) Y3Ba5Ca2Cu8O18

at 15, 45 and 75 K.

Fig. 3. Critical magnetic fields, Hc1, against temperature for the compounds

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Jc ¼ 20h

D

M

a1 a 3b

i

where a and b are the dimensions of the cross section of the sample perpendicular to the applied magneticfield.

We have estimated the critical current densities of the samples of Y-358 and YCa-358 at three different temperatures, 15, 45, and 75 K, and plotted them inFig. 4a and b against applied magnetic field for fields in the range 0  H  5000 Oe. The sample of Y-358 has larger Jcvalues compared with that of the YCa-358.

Further-more, it is very interesting that our results point out larger critical current densities compared with that of the other known high-Tc

superconductors; therefore, we may say that the structures of our

compounds have enhanced the flux pinning properties in the

samples.

Here we must insist on an important observation related to the behaviors of the hysteresis loops of the YCa-358 at lower temper-atures. To our knowledge the strange behavior (the little peaks) around zerofield has not been reported before. That is, the pro-jecting parts appearing especially at 15 K and 45 K for the sample of YCa-358, around zero appliedfield, inFig. 2b. Similar unexpected situations are observed in the inset ofFig. 2b and inFig. 4b. Valleys are formed around and below 0,5 kOe for temperature of 15 K and

45 K, respectively. These valleys in the inset ofFig. 2b and inFig. 4b are related through

D

M values used for the calculation of Jcvalues,

since Jcis proportional to

D

M; as in the above relation. In fact, the

little peaks inFig. 2b and the valleys inFig. 4b, although take place at differentfields, are also related. Since, if one pays attention to the hysteresis lop data inFig. 2b at 15 K, the relationship between the local peak at zero magneticfield inFig. 2b and the valley around 0,5 kOe inFig. 4b at 15 K can be explained. The peak forms when the field is descending and then reversed. After reversing the field almost around 0.5 kOe the valleys appears also inFig. 2b. In our calculations to obtain the critical current values, the data corre-sponding to that valley causes the valley mentioned in Fig. 4b. Because of that, for YCa-358 the Jc values below 1000 Oe first

unexpectedly decrease reaching the mentioned valley and then increase toward 4.5.104and 1.7.104A/cm2(at H¼ 0) at 15 K and 45 K, respectively. Therefore, both Figs.2b and4b indicate that the doping of Ca into the compound Y-358 causes a pronounced change in the character of Y-358. The reason for the mentioned peak like projecting parts inFig. 2b or the valleys inFig. 4b may be due to the defects or granular nature of the bulks that may cause variations in oxygen content of theflux pinning centers[23e25]. In other words, they may be due to the local composition modulation in the superconducting matrix of Y-358[26]. For the time being, this is a debatable point. The maximum value of Jcis about 8.104A/cm2at

Fig. 4. Critical current densities, Jc, against applied magneticfields for the compounds

(a) Y3Ba5Cu8O18and (b) Y3Ba5Ca2Cu8O18at 15, 45 and 75 K.

Fig. 5. Variation of pinning forces against applied magneticfields for the compounds (a) Y3Ba5Cu8O18and (b) Y3Ba5Ca2Cu8O18at 15, 45 and 75 K.

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15 K and at 1.6 kOe for the compound Y-358, whereas, the maximum Jc of YCa-358 is approximately 4.5.104 A/cm2 at the

corresponding temperature andfield values. Furthermore, it can be seen that, as expected, Jc is only weakly dependent on thefield

strength above 2 kOe for the compounds studied at temperatures above 45 K. Since Jcmust go to zero with increasingfield, and/or at

higher temperatures approaching Tc.on. Thus, the general features

and magnitudes inFig. 4a and b show that the Ca doping really ruins theflux pinning properties of the compound Y-358.

The Jcdata have been used to obtain the volume pinning force Fp

(force originating from the interactions between the vortex lattices and pinning defects per unit volume) in the compounds, studied at temperatures 15, 45 and 75 K, using the formula:

Fp ¼ Jc B

The plots of Fpversus HB/

m

0are shown inFig. 5a and b for Y-358

and YCa-358, respectively. The maximum values of Fpat 15 K for

Y-358 and YCa-Y-358 appeared at about 3.7 and 3.5 kOe, respectively, indicating that the irreversibility line gets shifted toward lower magneticfields with Ca doping. This supports our conclusion that theflux pinning strength of Y-358 significantly decreases with the doping of Ca.

The temperature dependence of the magnetizations (both FC and ZFC) of the compounds has been also measured under different

external magnetic fields of 50 and 100 Oe and at temperatures

below 100 K.Fig. 6a and b show the results of our experiments. It is

obviously seen that, as expected, applying the fields to the

compounds above their Tc.onvalues and then cooling them down

(FC) suppresses the superconductivity in the compounds. On the other hand, according to the ZFC data, again as expected, the compounds Y-358 and YCa-358 become diamagnetic below their onset temperatures ofw92 K and 85 K, respectively and diamag-netic saturations (field exclusions) are almost reached at the lowest temperatures. These behaviors are characteristics of all the bulk superconductors, mainly caused by their granular nature together with possible secondary phases, and thus the grain boundaries could then be considered as weak Josephson Junctions[27].

The AC susceptibility technique is a very important tool for studying the superconductivity in granular samples. The real part of the susceptibility,

c

0, gives the bulk susceptibility due to the superconductivity within the grains or intergranular regions. The suppression of superconductivity within the grains decreases the magnitude of

c

0. On the other hand, the imaginary part,

c

00, of the AC susceptibility is due to the AC energy losses corresponding to theflux penetration into the grain boundaries. This part of the AC susceptibility provides information about theflux pinning and the nature of the weak links between the grains[28].Fig. 7and b show the variations of the AC susceptibilities with temperature,

c

0(T, Hac)

and

c

00(T, Hac), measured on the samples of Y-358 and YCa-358 at

Fig. 6. Magnetizations against temperature for the compounds (a) Y3Ba5Cu8O18and

(b) Y3Ba5Ca2Cu8O18at an appliedfield of 50 Oe (inset figures are at 100 Oe).

Fig. 7. Plot of AC magnetic susceptibilities versus temperature for the compounds (a) Y3Ba5Cu8O18and (b) Y3Ba5Ca2Cu8O18at f¼ 1000 and 10 Hz in a field of H ¼ 10 Oe.

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the frequencies of 1000 Hz and 10 Hz in H¼ 10 Oe. By comparing the AC magnetic susceptibility data of the compounds Y-358 and YCa-358 it can be concluded that the critical temperatures and the magnitude of diamagnetic signals decrease with the doping of Ca into the compound Y-358, indicating that the doping of Ca decreases the superconducting volume fraction of the sample of Y-358. This is compatible with our previous conclusions. The diamagnetic transition temperatures (i.e. Tc.onvalues) are found to

be independent of frequency values, and are found to be 92 and 81 K for the samples of Y-358 and YCa-358, respectively, in agree-ment with the Tc.on(0) values determined from the resistivity

measurements. The doping of Ca into the compound Y-358 possibly introduces a change in the electronic state of the CuO2planes and

thus lowers the transition temperature, Tc. We should also point out

that as it can be seen from theFig. 7b the loss peak in

c

00has shifted to a lower temperature as a result of Ca doping. The observed shift of the peak in the

c

00 curves with Ca doping is thus due to the reduction in volume fraction of the superconducting phase. All of the above conclusions, obtained from the comparisons of the AC-susceptibility data of Y-358 and YCa-358 are compatible with our previous conclusions from the resistivity measurements and DC magnetization results.

4. Conclusion

The compounds with the nominal compositions Y3Ba5Cu8O18

and Y3Ba5Ca2Cu8O18were prepared by the solegel technique. The

effect of Ca doping on the electrical and magnetic properties of Y-358 have been investigated by measuring and comparing the electrical resistivities and DC magnetizations against temperature, magnetic hysteresis loops, and AC susceptibilities of Y-358 with and without Ca. From the hysteresis loops, the critical current densities andflux pinning properties have been derived, and they are found to be much better for the Ca free compound. In other words, the compound Y-358, compared to that of the YCa-358, exhibit higher intergranular coupling and hence higher critical current densities. From the resistivity and AC susceptibility measurements the critical temperatures Tc.onof the samples of Y-358 and YCa-358 have been

found; while the resistivity measurements indicate them to be 92.7 and 86.6 K, the susceptibility measurements show them to be 92 and 81 K, respectively. Hence, as expected, the diamagnetic onset temperatures obtained from the susceptibility measurements are nearly the same as the Tc.onvalues obtained from the resistivity

results. Therefore, from the comparison of the AC-susceptibilities of the compounds, it is concluded that the exclusion of magneticflux from the interior of the samples of Y-358 and YCa-358 start at 92 and 81 K and almost become completed (perfect diamagnetic state)

below 72 and 68 K, respectively. As a result, it is also concluded that the doping of Ca into Y-358 does not improve the interconnectivity of the grains, on the contrary ruins it.

Acknowledgments

This work was supported by Cukurova University under

FEF2009BAP10, AMYO2009BAP1, and FEF2009YL30 project

numbers. We wish to thank to Mr. Aydin Eraydin for graphical design.

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Figure

Fig. 1. The resistivities, r (T) against temperature for the compounds Y 3 Ba 5 Cu 8 O 18 and
Fig. 1. The resistivities, r (T) against temperature for the compounds Y 3 Ba 5 Cu 8 O 18 and p.2
Fig. 2. Hysteresis curves for the compounds (a) Y 3 Ba 5 Cu 8 O 18 and (b) Y 3 Ba 5 Ca 2 Cu 8 O 18
Fig. 2. Hysteresis curves for the compounds (a) Y 3 Ba 5 Cu 8 O 18 and (b) Y 3 Ba 5 Ca 2 Cu 8 O 18 p.3
Fig. 3. Critical magnetic fields, H c1 , against temperature for the compounds
Fig. 3. Critical magnetic fields, H c1 , against temperature for the compounds p.3
Fig. 5. Variation of pinning forces against applied magnetic fields for the compounds (a) Y 3 Ba 5 Cu 8 O 18 and (b) Y 3 Ba 5 Ca 2 Cu 8 O 18 at 15, 45 and 75 K.
Fig. 5. Variation of pinning forces against applied magnetic fields for the compounds (a) Y 3 Ba 5 Cu 8 O 18 and (b) Y 3 Ba 5 Ca 2 Cu 8 O 18 at 15, 45 and 75 K. p.4
Fig. 4. Critical current densities, J c , against applied magnetic fields for the compounds
Fig. 4. Critical current densities, J c , against applied magnetic fields for the compounds p.4
Fig. 6. Magnetizations against temperature for the compounds (a) Y 3 Ba 5 Cu 8 O 18 and
Fig. 6. Magnetizations against temperature for the compounds (a) Y 3 Ba 5 Cu 8 O 18 and p.5
Fig. 7. Plot of AC magnetic susceptibilities versus temperature for the compounds (a) Y 3 Ba 5 Cu 8 O 18 and (b) Y 3 Ba 5 Ca 2 Cu 8 O 18 at f ¼ 1000 and 10 Hz in a field of H ¼ 10 Oe.
Fig. 7. Plot of AC magnetic susceptibilities versus temperature for the compounds (a) Y 3 Ba 5 Cu 8 O 18 and (b) Y 3 Ba 5 Ca 2 Cu 8 O 18 at f ¼ 1000 and 10 Hz in a field of H ¼ 10 Oe. p.5

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