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Fishtail effect, magnetic properties and critical current density of Gd-added PMP YBCO
Physica C 297 Ž1998. 75–84
Fishtail effect, magnetic properties and critical current density of Gd-added PMP YBCO Yong Feng
a,b,c,)
, Lian Zhou a , J.G. Wen b, N. Koshizuka b, A. Sulpice c , J.L. Tholence c , J.C. Vallier c , P. Monceau c
a
b
Northwest Institute for Nonferrous Metal Research, P.O. Box 51, Xi’an, Shaanxi 710016, China SuperconductiÕity Research Laboratory, ISTEC, 1-10-13 Shinonome, 1-chome, Koto-ku, Tokyo 135, Japan c CRTBT and LCMI, CNRS, BP166, 38042 Grenoble Cedex 9, France Received 1 September 1997; revised 24 September 1997; accepted 13 November 1997
Abstract The magnetization curves of YBa 2 Cu 3 O y and Y0.4 Gd 0.6 Ba 2 Cu 3 O y samples prepared by a powder melting process technique were measured with a superconducting quantum interference device magnetometer at different temperatures. Fishtail effects are observed below 70 K with the H H c configuration in the Gd-added sample, while no peak effect is found in YBCO. The origin of the fishtail is discussed. It is found that Jc and flux pinning can be increased by the Gd addition. The magnitude of the improvement of Jc increases with the magnetic field. The reduction of the size of Y2 BaCuO5 particles, stress-field pinning and magnetic pinning induced by the substitution of Gd for Y may explain the enhancement of Jc and flux pinning. q 1998 Elsevier Science B.V. PACS: 74.72B; 74.60G; 74.60J Keywords: YBCO; Fishtail effect; Flux pinning; Gd addition
1. Introduction For the practical application of high-temperature superconductors, it is necessary to obtain high critical current densities Ž Jc .. Unfortunately, Jc is disappointingly low in sintered YBa 2 Cu 3 O y ŽYBCO. samples because of the serious weak links and the granularity in these materials. In zero field and at 77 K, only Jc values up to several 1000 Arcm2 are obtained and Jc dramatically drops even in a very )
Corresponding author. Tel.: q86 29 6224487; Fax: q86 29 623 1103; E-mail: [email protected].
small applied field. In recent years, much effort has been spent in enhancing the critical current density of YBCO and great progress has been made. To date, several methods such as melt-textured growth w1x, liquid phase process w2x, quench melt growth w3x, and powder melting process w4x have been developed to fabricate high-Jc YBCO superconductors. Recently, Egi et al. w5x have prepared high Jc NdBa 2 Cu 3 O y ŽNd123. single crystals by a travelling solvent floating zone ŽTSFZ. approach. Moreover, Yao et al. w6,7x have systematically investigated the growth dynamic of Nd123 and have prepared large Nd123 single crystals. Despite the remarkable en-
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 4 7 - 9
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hancement of the low-field Jc , its performance at a high magnetic field is disappointing. Therefore, it is necessary to further improve Jc in a high magnetic field in order to bring about their application to electrical engineering. One of the most effective ways is to introduce strong flux pinning centers in the YBCO system. The flux pinning process in YBCO is a very complex phenomenon, which is not yet clearly understood and should be further investigated. Recent studies show that oxygen defects, stacking faults, dislocations, twin boundaries, and columnar defects introduced by ion irradiations are found to be effective pinning centers in YBCO w8–12x. Jin et al. w13x suggested that fine-scale defects created during the decomposition of YBa 2 Cu 4 O 8 may act as pinning centers in YBCO. Furthermore, although the exact flux pinning mechanism of Y2 BaCuO5 ŽY211. is still unknown, it is widely accepted that both mechanical properties and flux pinning of YBCO are improved by the fine Y211 embedding w14x. Some scientists also found that the thickness of the interfaces between Y123 and Y211 is comparable to the coherence length in the ab-plane of YBCO superconductors. To date, the effectiveness of possible pinning centers has not been established. However, there is much evidence that the flux pinning of YBCO can be further enhanced through chemical doping of Pt, Rh, CeO 2 , etc. w15,16x. Some authors found that Jc values and flux pinning can be improved by adding elements to YBCO samples. Hf and Hf–Ca doping at the Y site could increase the intragrain Jc in the YBCO system. It is thought that the preferential substitution of Hf or Hf–Ca for Y can act as a pinning center w17,18x. We found that the substitutions of Ho for Y and Sn for Cu lead to an improvement of Jc and microstructure w19,20x. Because chemical doping can be easily controlled and is non-destructive and very effective in improving Jc , the further investigation of chemical doping is of great importance both for physical understanding and practical applications. In the present paper, the YBa 2 Cu 3 O y and Y0.4 Gd 0.6 Ba 2 Cu 3 O y superconductors were prepared by the powder melting process method and were investigated through differential thermal analysis, AC susceptibility, and superconducting quantum interference device ŽSQUID. magnetometer. The effects of the Gd substitution for
Y on superconducting properties and flux pinning are described.
2. Experimental Samples with nominal compositions of YBa 2 Cu 3 O y ŽYBCO. and Y0.4 Gd 0.6 Ba 2 Cu 3 O y ŽGd06. were fabricated by the powder melting process method. The detailed description of the preparation process has been reported previously w21x. In short, Y211 and BaCuO 2 powders were synthesized through a solid state reaction technique using Y2 O 3 , Gd 2 O 3 , BaCO 3 and CuO. Then, these powders were well ground in an appropriate ratio and were cold pressed into a rectangular shape. The bars were put into a tube furnace with the highest temperatures between 1030 and 10808C. The moving rate was around 2 mmrh. Finally, the samples were annealed at 5508C for 40 h in flowing oxygen to insure complete oxygenation of the samples. Also, annealing was continued to 4008C at a slow cooling rate Ž38Crh. for possible additional oxygen loading and then to room temperature at 58Crmin. The sample shape is rectangular and the dimensions are 0.8 = 0.3 = 0.06 cm3 and 0.9 = 0.35 = 0.07 cm3, respectively for the YBCO and Gd06 samples. In the experimental process, the magnetic field is parallel to the longer dimension of the samples. It means that the field is perpendicular to the c-axis of the sample. In order to confirm this, we first measured the magnetic hysteresis loop at 60 K in field perpendicular to the longer dimension of the specimen. Then, we slowly rotated the sample and tested again. It is found that the direction of the applied field in which the magnetic hysteresis loop is largest is just perpendicular to the longer dimension of the sample. This process insured that the longer dimension of the sample is perpendicular to the crystalline c-axis. Critical temperature was measured by a superconducting quantum interference device ŽSQUID. magnetometer. Magnetization measurements were carried out using a SQUID magnetometer with the magnetic field perpendicular to the c-axis of the sample at different temperatures. All the magnetization measurements were performed by first cooling the sample in zero field and then applying a field to begin
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Table 1 Critical temperatures for the two samples
YBCO Gd06
T ŽK.
DT ŽK.
92 92.8
1.0 1.1
the measurement. After each measurement finished at a given temperature, the sample was warmed above Tc to drive out any trapped flux and then cooled to the measuring temperature. We measured several YBCO and Gd06 samples. The results reported below are representative of these samples. Other results are very similar to those reported here.
3. Results and discussion Table 1 shows critical temperatures of YBCO and Gd06 samples. It is found that Tc values for the two samples are around 92 K. This result is in agreement with the previous work, in which the magnetic moment of rare-earth elements has no detrimental effect on Tc . The transition width is about 1 K, indicating that there is a good homogeneity in these samples. The typical temperature dependence of DC magnetization for the Gd06 specimen is shown in Fig. 1. The applied field was perpendicular to the c-axis. The DTA result reveals that the melting temperature ŽTm . is changed by the Gd addition. Tm is increased from 9758C ŽYBCO. to 10328C ŽGd06. in air.
Fig. 1. Temperature dependence of DC magnetization for the Gd06 sample with H H c.
Fig. 2. X-ray diffraction pattern of the Gd06 specimen.
X-ray diffraction pattern was performed on a Philips 1700 diffractometer with Cu K a radiation. The results show a strong enhancement in the strength of the Ž001. peaks in the YBCO and Gd06 samples, which indicates a perfect c-axis orientation in these specimens. The typical X-ray diffraction spectrum of the Gd06 sample is given in Fig. 2. Also, it can be observed in Fig. 2 that there is a small peak of the 211 phase in this spectrum. The weak peak reveals the presence of the 211 particles in the sample. Fig. 3 shows the SEM photographs of the Gd06 sample. These observations indicate that the plateshaped 123 crystals are well oriented with their ab-planes parallel to the longer dimension of the sample. The sample is very dense, showing hardly any voids or microcracks. In addition, grain boundaries are discontinuous and disappear in many regions. Thus, 123 crystals can grow with each other. The intergrowth between 123 grains can result in the elimination of weak links. Furthermore, it can be found that many dispersively distributed 211 particles exist in the 123 matrix. Figs. 4 and 5 give the field dependence of magnetization curves for the YBCO and Gd06 samples measured at different temperatures with the magnetic field perpendicular to the c-axis. It can be observed that the hysteresis of the magnetization increases when the temperature drops, which means that the flux pinning force of the specimens is gradually improved. This is because the hysteresis is attributed to the presence of flux pinning sites in materials. According to the results of Campbell et al. w22x, these magnetization curves belong to the typically
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Fig. 5. Magnetic hysteresis loops for the Gd06 sample at various temperatures with H H c.
magnetic hysteresis loops with strong flux pinning. It can be seen from Figs. 4 and 5 that the maximum diamagnetic field, Hm changes with temperature. Previously, Malozemoff w23x found that Hm could be described by Hm s AJc Ž 1 y D . ,
Fig. 3. Fracture photographs of the Gd06 sample. Ža. Low magnification. Žb. High magnification.
Fig. 4. The magnetization curves for the YBCO specimen at different temperatures with field perpendicular to the c-axis.
Ž 1.
where A is a constant related to the dimension of the samples and D is the diamagnetization factor. This equation suggests that Hm should be relevant to the dimension of the specimens, which has been confirmed by some authors. They observed that Hm drops with decreasing the dimension w23x. Figs. 6 and 7 illustrate Hm as a function of temperature for all the samples. Hm decreases as the temperature increases, a behavior which is similar to that of the
Fig. 6. The variation of maximum diamagnetic field Ž Hm . with temperature in the YBCO sample.
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Fig. 7. The maximum diamagnetic field Ž Hm . as a function of temperature for the Gd06 sample.
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Fig. 9. Critical current density Jc vs. magnetic field at different temperatures for YBCO Ž H H c ..
critical current density. For the Gd06 sample, Hm is about 0.3 T at 50 K, while it drops to 0.05 T when T s 80 K. On the other hand, it can be observed that the shape of the curves for the Gd06 sample strongly suggests the superposition of a reversible component and a hysteresis component at high temperatures. The reversible component can be related to paramagnetic Gd ions with negligible interaction between the local magnetic moments and the superconducting electrons. The temperature dependence of magnetization at H s 3 T for the Gd06 specimen is given in Fig. 8. It is found that the magnetization above Tc can be well described by the Curie–Weiss law 1rm A Ž T y u . , Ž 2. where u is the paramagnetic temperature. By computer fitting, u is around y4.15 for the Gd-added
sample. These observations clearly show that there is paramagnetism in the Gd06 specimen although Y is only partially substituted by Gd. The critical current density was calculated by using the Bean model w24x. In the case of field perpendicular to the c-axis of a sample, Jc is given by the following equation
Fig. 8. Temperature dependence of magnetization for the Gd06 sample at 3 T Ž H H c ..
Fig. 10. Magnetic field dependence of Jc at various temperatures for the Gd06 sample Ž H H c ..
Jc s 20 Ž Mqy My . rd. q
Ž 3.
y
Here, M and M are magnetization moments at increasing and decreasing magnetic fields, respectively, and d is the sample thickness along the direction of the field penetration. Figs. 9 and 10 illustrate the Jc –H properties of the YBCO and Gd06 samples at different temperatures with the H H c configuration. As for the YBCO sample, Jc decreases quickly with the magnetic field below 1 T, whereas Jc falls off much more slowly above 1 T.
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The result indicates that the weak link is significantly overcome and the flux pinning is relatively strong. In addition, no fishtail effect can be observed in YBCO, while this anomalous phenomenon is found in the Gd06 sample. Furthermore, the similar anomalies can be also seen in other high-Tc superconductors including melt-processed YBCO, NdBCO, Bi– Sr–Ca–Cu–O, etc. w25–27x. To date, two kinds of fishtail effects have been found in these materials. In YBCO, broad and temperature-dependent fishtails are observed. As for Bi–Sr–Ca–Cu–O superconductors, a sharp and temperature-independent peak is found. Although the origin of this phenomenon is not fully understood, several mechanisms have been developed to interpret it. Daeumling et al. w28x attributed the fishtail effect to the flux pinning created by the oxygen-deficient regions. However, studies on melt-textured YBCO subjected to prolonged oxygenation show that the peak effects continue to exist w29x. The location and shape of the fishtail effect in the specimen subjected to 40 h oxygenation are the same as those in the sample subjected to 90 h oxygenation, so it appears that the peak effect is not due to the flux pinning induced by the oxygen-deficient region. Moreover, the synchronization effects of the increased disorder in the vortex lattice and the matching effect between the vortex lattice and the twin structure are proposed to explain the fishtail effect w30x. Recently, some authors suggested that the peak effect in melt-processed YBCO can be well described by the collective pinning theory at high temperatures w31x. Also, a crossover from single to collective flux creep is believed to be the origin of the fishtail effect w32x. Here, we do not think that the flux pinning induced by the oxygen-deficient region is responsible for the peak effect in the Gd06 sample. It is interesting to note that the fishtail is observed for the H H c configuration in the Gd06 sample, which has not been seen in the oxygen-deficient YBCO. In addition, the peak effect is absent in Fig. 10 when T s 80 K. Therefore, we believe that the origin of fishtails in the Gd06 specimen are not oxygen defects. A similar result is found in NdBCO and SmBCO, in which the peak effects are created by Nd or Sm substitutions for the Ba sites w33x. It is important to note that the peak field Ž Hp . at which Jc reaches its maximum value is strongly tempera-
ture dependent and Hp shifts to a higher field with decreasing temperature as shown in Fig. 11. So, the fishtail effect is not attributed to the matching effect. On the other hand, it can be observed from Fig. 11 that Hp decreases linearly with temperature and s s d HprdT s 0.1 TrK. This is much smaller than the previous reports of s s 0.7–10 TrK w34x. Klein et al. w35x found a power law behavior Hp s 4.2 = 10 5 Ž1 y t . 3r2 in an untwinned YBCO single crystal. They proposed that the peak effect may be due to a percolating network of reversible zones since the temperature dependence of Hp is similar to that of the irreversibility field. However, the linear temperature dependence of Hp is found in the Gd06 sample. This kind of relation was also obtained in REBCO single crystals with RE s Y, Yb and Dy w36x. Thus, the percolating network of reversible zones is also not the reason of the fishtail in our sample. It is considered that the fishtail effect in the Gd06 specimen may be due to the cation defects orrand the paramagnetism in the sample induced by the local substitutions of Gd for Y. The exact mechanism of the peak effect in our sample is not clear and should be further investigated. Fig. 12 gives a plot of the reduced critical current density JcrJcp Ž Jcp corresponding to the highest Jc value. vs. the reduced field HrHp at different temperatures. The curves at 50 K and 60 K can be scaled in a single master curve, which means that the magnetization behavior is dominated by a single type of pinning center at 50 K and 60 K. Unfortunately,
Fig. 11. The peak field Hp as a function of temperature for the Gd06 specimen.
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Fig. 12. The reduced critical current density Jc r Jcp vs. the reduced field Hr Hp in the Gd06 sample.
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Fig. 14. Flux pinning force density Fp vs. magnetic field in the Gd06 specimen.
the plot at 70 K cannot be scaled in the same curve as those at 50 K and 60 K. This indicates that the flux pinning behavior at 70 K is different from those at 50 K and 60 K. In order to investigate the flux pinning characteristic, the flux pinning force density is calculated. Figs. 13 and 14 give the field dependence of Fp for the samples at various temperatures. Fp increases with increasing the field within the observed field at 50–70 K. When temperature is raised to 80 K, Fp initially increases with the field and reaches a maximum value. Then, Fp drops as further increasing the field. There is a single peak in each sample at 80 K. The dependence of Jc on temperature at different fields was studied and Figs. 15 and 16 show Jc values as a function of temperature for all the samples. It can be observed that the decrease of Jc on
temperature at low fields is much lower than that at high fields, which implies that the flux pinning mechanism is different in different fields. Martinez et al. w37x systematically investigated the Jc behavior of melt-textured YBCO in wide temperatures and magnetic fields. They found that the interfaces between Y123 and Y211 particles are the dominant pinning centers in the low field region, while other crystal defects become more active in high fields. Our recent work shows that the stacking faults can act as very effective pinning centers in the low fields and high fields, but the role of stacking faults as pinning centers is different under different fields w38x. In addition, the behavior of other defects such as point defects and dislocations changes with the magnetic field. From Figs. 9 and 10, it can be seen that Jc is increased by the Gd addition ranging from 1.05 to
Fig. 13. The flux pinning force density Fp as a function of field and temperature for YBCO.
Fig. 15. The dependence of Jc on temperature at different fields for the YBCO sample.
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Fig. 16. Jc as a function of temperature for the Gd06 sample.
3.5 times in different fields. Jc is about 8400 ŽYBCO. and 24 500 Arcm2 ŽGd06. at 60 K in 3 T. Fig. 17 illustrates the ratio of Jc in YBCO over Jc in the Gd06 sample. The magnitude of the enhancement of Jc in high fields is higher than that in low fields. It increases with the field from 1.1 in 0.1 T to 3.5 in 4 T at 50 K. This result clearly indicates that the Gd addition has different contributions to the increase in Jc at high fields and low fields. The improvement of Jc over pure YBCO is very interesting and it is expected that Jc will be enhanced high enough for electrical engineering applications through further refinement of the composition and optimization for processing conditions. Based on the above discussion, we can conclude that the substitution of Gd for Y can help improve Jc and flux pinning in YBCO superconductors. Also, some researchers have found that Jc can be in-
Fig. 17. Field dependence of the ratio of Jc in YBCO over Jc in Gd06 Ž Jc,Gd r Jc,Y ..
creased by other rare-earth element additions. The 20% substitutions of Sm and Eu for Y in sintered YBCO lead to an enhancement of the intragrain Jc from 11 000 Arcm2 to 25 000 and 27 000 Arcm2 at 77 K in 0.9 T w39x. Recent work demonstrates that the flux pinning is improved by the partial substitutions of Pr for Y. It is thought that the increase in flux pinning by Pr ions is mainly induced by the suppression of superconducting order parameters in the vicinity of Pr ions via a magnetic pair breaking w40x. In addition, Nd 2 O 3rLa 2 O 3 additions have been found to result in enhancement of the flux pinning in melt-textured YBCO. It is considered that the increased Jc may be due to pinning effects created by NdrLa ions being present on Y andror Ba sites in the YBCO lattice w41x. However, although the effects of the rare-earth element additions in YBCO samples on flux pinning have been investigated, the mechanism of the increase in Jc and flux pinning is not clear. On the basis of our results, we think that the enhancement of Jc may be related to the following reasons. First, the size of Y211 particles is significantly reduced from 3.2 mm ŽYBCO. to 0.96 mm ŽGd06.. The size of 211 particles was determined by a chemical method. The size distributions of 211 for YBCO and Gd06 are shown in Figs. 18 and 19. The reduction of the 211 size will be helpful to diminish microcracks and will lead to more interfaces between Y123 and Y211 phases. The interface of Y123 and Y211 is found to be effective pinning centers in YBCO. As a result, Jc and flux pinning are improved. This is confirmed by
Fig. 18. The distribution of the size of 211 particles in YBCO.
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can be observed in YBCO. The peak field Hp decreases linearly with the field. The fishtail may be attributed to the cation defects orrand the paramagnetism in the Gd-doped sample created by the Gd addition. It is found that Jc can be improved by the substitution of Gd for Y, which may be due to the reduction of the size of Y211 particles, magnetic pinning and stress-field pinning. This should be further investigated to separate the relative contributions of these aspects.
Fig. 19. The size distribution of 211 particles in the Gd06 specimen.
other reports, in which the refinement of the Y211 size caused by the additions of PtO 2 or CeO 2 can enhance Jc and obtain better microstructure w42x. Secondly, it is well known that the flux pinning will be introduced by the elastic interaction between the vortex lattice and the stress field in conventional hard superconductors w43x. It is expected that the local lattice mismatch can be created by the partial substitutions of rare-earth elements with different ionic radii for Y in YBCO, which will lead to the formation of the stress field. Consequently, the flux pinning can be introduced. There is a stress field caused by the Gd addition because the radius of Gd 3q is larger than that of Y 3q. Thus, an additional strong flux pinning will be formed in Gd-doped YBCO sample. So, Jc is increased. A recent calculation has given an evidence for it w44x. Furthermore, it can be observed in Fig. 8 that there is paramagnetism in the Gd06 specimen, which will induce the magnetic pinning. This may also be responsible for the improvement of Jc .
4. Conclusion We have carried out magnetic measurements of YBa 2 Cu 3 O y and Y0.4 Gd 0.6 Ba 2 Cu 3 O y prepared by the powder melting process method through a SQUID magnetometer. Anomalous peaks are found in the Gd-added sample below 70 K with the magnetic field perpendicular to the c-axis, whereas no fishtail
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Fishtail effect, magnetic properties and critical current density of Gd-added PMP YBCO Yong Feng
a,b,c,)
, Lian Zhou a , J.G. Wen b, N. Koshizuka b, A. Sulpice c , J.L. Tholence c , J.C. Vallier c , P. Monceau c
a
b
Northwest Institute for Nonferrous Metal Research, P.O. Box 51, Xi’an, Shaanxi 710016, China SuperconductiÕity Research Laboratory, ISTEC, 1-10-13 Shinonome, 1-chome, Koto-ku, Tokyo 135, Japan c CRTBT and LCMI, CNRS, BP166, 38042 Grenoble Cedex 9, France Received 1 September 1997; revised 24 September 1997; accepted 13 November 1997
Abstract The magnetization curves of YBa 2 Cu 3 O y and Y0.4 Gd 0.6 Ba 2 Cu 3 O y samples prepared by a powder melting process technique were measured with a superconducting quantum interference device magnetometer at different temperatures. Fishtail effects are observed below 70 K with the H H c configuration in the Gd-added sample, while no peak effect is found in YBCO. The origin of the fishtail is discussed. It is found that Jc and flux pinning can be increased by the Gd addition. The magnitude of the improvement of Jc increases with the magnetic field. The reduction of the size of Y2 BaCuO5 particles, stress-field pinning and magnetic pinning induced by the substitution of Gd for Y may explain the enhancement of Jc and flux pinning. q 1998 Elsevier Science B.V. PACS: 74.72B; 74.60G; 74.60J Keywords: YBCO; Fishtail effect; Flux pinning; Gd addition
1. Introduction For the practical application of high-temperature superconductors, it is necessary to obtain high critical current densities Ž Jc .. Unfortunately, Jc is disappointingly low in sintered YBa 2 Cu 3 O y ŽYBCO. samples because of the serious weak links and the granularity in these materials. In zero field and at 77 K, only Jc values up to several 1000 Arcm2 are obtained and Jc dramatically drops even in a very )
Corresponding author. Tel.: q86 29 6224487; Fax: q86 29 623 1103; E-mail: [email protected].
small applied field. In recent years, much effort has been spent in enhancing the critical current density of YBCO and great progress has been made. To date, several methods such as melt-textured growth w1x, liquid phase process w2x, quench melt growth w3x, and powder melting process w4x have been developed to fabricate high-Jc YBCO superconductors. Recently, Egi et al. w5x have prepared high Jc NdBa 2 Cu 3 O y ŽNd123. single crystals by a travelling solvent floating zone ŽTSFZ. approach. Moreover, Yao et al. w6,7x have systematically investigated the growth dynamic of Nd123 and have prepared large Nd123 single crystals. Despite the remarkable en-
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hancement of the low-field Jc , its performance at a high magnetic field is disappointing. Therefore, it is necessary to further improve Jc in a high magnetic field in order to bring about their application to electrical engineering. One of the most effective ways is to introduce strong flux pinning centers in the YBCO system. The flux pinning process in YBCO is a very complex phenomenon, which is not yet clearly understood and should be further investigated. Recent studies show that oxygen defects, stacking faults, dislocations, twin boundaries, and columnar defects introduced by ion irradiations are found to be effective pinning centers in YBCO w8–12x. Jin et al. w13x suggested that fine-scale defects created during the decomposition of YBa 2 Cu 4 O 8 may act as pinning centers in YBCO. Furthermore, although the exact flux pinning mechanism of Y2 BaCuO5 ŽY211. is still unknown, it is widely accepted that both mechanical properties and flux pinning of YBCO are improved by the fine Y211 embedding w14x. Some scientists also found that the thickness of the interfaces between Y123 and Y211 is comparable to the coherence length in the ab-plane of YBCO superconductors. To date, the effectiveness of possible pinning centers has not been established. However, there is much evidence that the flux pinning of YBCO can be further enhanced through chemical doping of Pt, Rh, CeO 2 , etc. w15,16x. Some authors found that Jc values and flux pinning can be improved by adding elements to YBCO samples. Hf and Hf–Ca doping at the Y site could increase the intragrain Jc in the YBCO system. It is thought that the preferential substitution of Hf or Hf–Ca for Y can act as a pinning center w17,18x. We found that the substitutions of Ho for Y and Sn for Cu lead to an improvement of Jc and microstructure w19,20x. Because chemical doping can be easily controlled and is non-destructive and very effective in improving Jc , the further investigation of chemical doping is of great importance both for physical understanding and practical applications. In the present paper, the YBa 2 Cu 3 O y and Y0.4 Gd 0.6 Ba 2 Cu 3 O y superconductors were prepared by the powder melting process method and were investigated through differential thermal analysis, AC susceptibility, and superconducting quantum interference device ŽSQUID. magnetometer. The effects of the Gd substitution for
Y on superconducting properties and flux pinning are described.
2. Experimental Samples with nominal compositions of YBa 2 Cu 3 O y ŽYBCO. and Y0.4 Gd 0.6 Ba 2 Cu 3 O y ŽGd06. were fabricated by the powder melting process method. The detailed description of the preparation process has been reported previously w21x. In short, Y211 and BaCuO 2 powders were synthesized through a solid state reaction technique using Y2 O 3 , Gd 2 O 3 , BaCO 3 and CuO. Then, these powders were well ground in an appropriate ratio and were cold pressed into a rectangular shape. The bars were put into a tube furnace with the highest temperatures between 1030 and 10808C. The moving rate was around 2 mmrh. Finally, the samples were annealed at 5508C for 40 h in flowing oxygen to insure complete oxygenation of the samples. Also, annealing was continued to 4008C at a slow cooling rate Ž38Crh. for possible additional oxygen loading and then to room temperature at 58Crmin. The sample shape is rectangular and the dimensions are 0.8 = 0.3 = 0.06 cm3 and 0.9 = 0.35 = 0.07 cm3, respectively for the YBCO and Gd06 samples. In the experimental process, the magnetic field is parallel to the longer dimension of the samples. It means that the field is perpendicular to the c-axis of the sample. In order to confirm this, we first measured the magnetic hysteresis loop at 60 K in field perpendicular to the longer dimension of the specimen. Then, we slowly rotated the sample and tested again. It is found that the direction of the applied field in which the magnetic hysteresis loop is largest is just perpendicular to the longer dimension of the sample. This process insured that the longer dimension of the sample is perpendicular to the crystalline c-axis. Critical temperature was measured by a superconducting quantum interference device ŽSQUID. magnetometer. Magnetization measurements were carried out using a SQUID magnetometer with the magnetic field perpendicular to the c-axis of the sample at different temperatures. All the magnetization measurements were performed by first cooling the sample in zero field and then applying a field to begin
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Table 1 Critical temperatures for the two samples
YBCO Gd06
T ŽK.
DT ŽK.
92 92.8
1.0 1.1
the measurement. After each measurement finished at a given temperature, the sample was warmed above Tc to drive out any trapped flux and then cooled to the measuring temperature. We measured several YBCO and Gd06 samples. The results reported below are representative of these samples. Other results are very similar to those reported here.
3. Results and discussion Table 1 shows critical temperatures of YBCO and Gd06 samples. It is found that Tc values for the two samples are around 92 K. This result is in agreement with the previous work, in which the magnetic moment of rare-earth elements has no detrimental effect on Tc . The transition width is about 1 K, indicating that there is a good homogeneity in these samples. The typical temperature dependence of DC magnetization for the Gd06 specimen is shown in Fig. 1. The applied field was perpendicular to the c-axis. The DTA result reveals that the melting temperature ŽTm . is changed by the Gd addition. Tm is increased from 9758C ŽYBCO. to 10328C ŽGd06. in air.
Fig. 1. Temperature dependence of DC magnetization for the Gd06 sample with H H c.
Fig. 2. X-ray diffraction pattern of the Gd06 specimen.
X-ray diffraction pattern was performed on a Philips 1700 diffractometer with Cu K a radiation. The results show a strong enhancement in the strength of the Ž001. peaks in the YBCO and Gd06 samples, which indicates a perfect c-axis orientation in these specimens. The typical X-ray diffraction spectrum of the Gd06 sample is given in Fig. 2. Also, it can be observed in Fig. 2 that there is a small peak of the 211 phase in this spectrum. The weak peak reveals the presence of the 211 particles in the sample. Fig. 3 shows the SEM photographs of the Gd06 sample. These observations indicate that the plateshaped 123 crystals are well oriented with their ab-planes parallel to the longer dimension of the sample. The sample is very dense, showing hardly any voids or microcracks. In addition, grain boundaries are discontinuous and disappear in many regions. Thus, 123 crystals can grow with each other. The intergrowth between 123 grains can result in the elimination of weak links. Furthermore, it can be found that many dispersively distributed 211 particles exist in the 123 matrix. Figs. 4 and 5 give the field dependence of magnetization curves for the YBCO and Gd06 samples measured at different temperatures with the magnetic field perpendicular to the c-axis. It can be observed that the hysteresis of the magnetization increases when the temperature drops, which means that the flux pinning force of the specimens is gradually improved. This is because the hysteresis is attributed to the presence of flux pinning sites in materials. According to the results of Campbell et al. w22x, these magnetization curves belong to the typically
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Fig. 5. Magnetic hysteresis loops for the Gd06 sample at various temperatures with H H c.
magnetic hysteresis loops with strong flux pinning. It can be seen from Figs. 4 and 5 that the maximum diamagnetic field, Hm changes with temperature. Previously, Malozemoff w23x found that Hm could be described by Hm s AJc Ž 1 y D . ,
Fig. 3. Fracture photographs of the Gd06 sample. Ža. Low magnification. Žb. High magnification.
Fig. 4. The magnetization curves for the YBCO specimen at different temperatures with field perpendicular to the c-axis.
Ž 1.
where A is a constant related to the dimension of the samples and D is the diamagnetization factor. This equation suggests that Hm should be relevant to the dimension of the specimens, which has been confirmed by some authors. They observed that Hm drops with decreasing the dimension w23x. Figs. 6 and 7 illustrate Hm as a function of temperature for all the samples. Hm decreases as the temperature increases, a behavior which is similar to that of the
Fig. 6. The variation of maximum diamagnetic field Ž Hm . with temperature in the YBCO sample.
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Fig. 7. The maximum diamagnetic field Ž Hm . as a function of temperature for the Gd06 sample.
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Fig. 9. Critical current density Jc vs. magnetic field at different temperatures for YBCO Ž H H c ..
critical current density. For the Gd06 sample, Hm is about 0.3 T at 50 K, while it drops to 0.05 T when T s 80 K. On the other hand, it can be observed that the shape of the curves for the Gd06 sample strongly suggests the superposition of a reversible component and a hysteresis component at high temperatures. The reversible component can be related to paramagnetic Gd ions with negligible interaction between the local magnetic moments and the superconducting electrons. The temperature dependence of magnetization at H s 3 T for the Gd06 specimen is given in Fig. 8. It is found that the magnetization above Tc can be well described by the Curie–Weiss law 1rm A Ž T y u . , Ž 2. where u is the paramagnetic temperature. By computer fitting, u is around y4.15 for the Gd-added
sample. These observations clearly show that there is paramagnetism in the Gd06 specimen although Y is only partially substituted by Gd. The critical current density was calculated by using the Bean model w24x. In the case of field perpendicular to the c-axis of a sample, Jc is given by the following equation
Fig. 8. Temperature dependence of magnetization for the Gd06 sample at 3 T Ž H H c ..
Fig. 10. Magnetic field dependence of Jc at various temperatures for the Gd06 sample Ž H H c ..
Jc s 20 Ž Mqy My . rd. q
Ž 3.
y
Here, M and M are magnetization moments at increasing and decreasing magnetic fields, respectively, and d is the sample thickness along the direction of the field penetration. Figs. 9 and 10 illustrate the Jc –H properties of the YBCO and Gd06 samples at different temperatures with the H H c configuration. As for the YBCO sample, Jc decreases quickly with the magnetic field below 1 T, whereas Jc falls off much more slowly above 1 T.
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The result indicates that the weak link is significantly overcome and the flux pinning is relatively strong. In addition, no fishtail effect can be observed in YBCO, while this anomalous phenomenon is found in the Gd06 sample. Furthermore, the similar anomalies can be also seen in other high-Tc superconductors including melt-processed YBCO, NdBCO, Bi– Sr–Ca–Cu–O, etc. w25–27x. To date, two kinds of fishtail effects have been found in these materials. In YBCO, broad and temperature-dependent fishtails are observed. As for Bi–Sr–Ca–Cu–O superconductors, a sharp and temperature-independent peak is found. Although the origin of this phenomenon is not fully understood, several mechanisms have been developed to interpret it. Daeumling et al. w28x attributed the fishtail effect to the flux pinning created by the oxygen-deficient regions. However, studies on melt-textured YBCO subjected to prolonged oxygenation show that the peak effects continue to exist w29x. The location and shape of the fishtail effect in the specimen subjected to 40 h oxygenation are the same as those in the sample subjected to 90 h oxygenation, so it appears that the peak effect is not due to the flux pinning induced by the oxygen-deficient region. Moreover, the synchronization effects of the increased disorder in the vortex lattice and the matching effect between the vortex lattice and the twin structure are proposed to explain the fishtail effect w30x. Recently, some authors suggested that the peak effect in melt-processed YBCO can be well described by the collective pinning theory at high temperatures w31x. Also, a crossover from single to collective flux creep is believed to be the origin of the fishtail effect w32x. Here, we do not think that the flux pinning induced by the oxygen-deficient region is responsible for the peak effect in the Gd06 sample. It is interesting to note that the fishtail is observed for the H H c configuration in the Gd06 sample, which has not been seen in the oxygen-deficient YBCO. In addition, the peak effect is absent in Fig. 10 when T s 80 K. Therefore, we believe that the origin of fishtails in the Gd06 specimen are not oxygen defects. A similar result is found in NdBCO and SmBCO, in which the peak effects are created by Nd or Sm substitutions for the Ba sites w33x. It is important to note that the peak field Ž Hp . at which Jc reaches its maximum value is strongly tempera-
ture dependent and Hp shifts to a higher field with decreasing temperature as shown in Fig. 11. So, the fishtail effect is not attributed to the matching effect. On the other hand, it can be observed from Fig. 11 that Hp decreases linearly with temperature and s s d HprdT s 0.1 TrK. This is much smaller than the previous reports of s s 0.7–10 TrK w34x. Klein et al. w35x found a power law behavior Hp s 4.2 = 10 5 Ž1 y t . 3r2 in an untwinned YBCO single crystal. They proposed that the peak effect may be due to a percolating network of reversible zones since the temperature dependence of Hp is similar to that of the irreversibility field. However, the linear temperature dependence of Hp is found in the Gd06 sample. This kind of relation was also obtained in REBCO single crystals with RE s Y, Yb and Dy w36x. Thus, the percolating network of reversible zones is also not the reason of the fishtail in our sample. It is considered that the fishtail effect in the Gd06 specimen may be due to the cation defects orrand the paramagnetism in the sample induced by the local substitutions of Gd for Y. The exact mechanism of the peak effect in our sample is not clear and should be further investigated. Fig. 12 gives a plot of the reduced critical current density JcrJcp Ž Jcp corresponding to the highest Jc value. vs. the reduced field HrHp at different temperatures. The curves at 50 K and 60 K can be scaled in a single master curve, which means that the magnetization behavior is dominated by a single type of pinning center at 50 K and 60 K. Unfortunately,
Fig. 11. The peak field Hp as a function of temperature for the Gd06 specimen.
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Fig. 12. The reduced critical current density Jc r Jcp vs. the reduced field Hr Hp in the Gd06 sample.
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Fig. 14. Flux pinning force density Fp vs. magnetic field in the Gd06 specimen.
the plot at 70 K cannot be scaled in the same curve as those at 50 K and 60 K. This indicates that the flux pinning behavior at 70 K is different from those at 50 K and 60 K. In order to investigate the flux pinning characteristic, the flux pinning force density is calculated. Figs. 13 and 14 give the field dependence of Fp for the samples at various temperatures. Fp increases with increasing the field within the observed field at 50–70 K. When temperature is raised to 80 K, Fp initially increases with the field and reaches a maximum value. Then, Fp drops as further increasing the field. There is a single peak in each sample at 80 K. The dependence of Jc on temperature at different fields was studied and Figs. 15 and 16 show Jc values as a function of temperature for all the samples. It can be observed that the decrease of Jc on
temperature at low fields is much lower than that at high fields, which implies that the flux pinning mechanism is different in different fields. Martinez et al. w37x systematically investigated the Jc behavior of melt-textured YBCO in wide temperatures and magnetic fields. They found that the interfaces between Y123 and Y211 particles are the dominant pinning centers in the low field region, while other crystal defects become more active in high fields. Our recent work shows that the stacking faults can act as very effective pinning centers in the low fields and high fields, but the role of stacking faults as pinning centers is different under different fields w38x. In addition, the behavior of other defects such as point defects and dislocations changes with the magnetic field. From Figs. 9 and 10, it can be seen that Jc is increased by the Gd addition ranging from 1.05 to
Fig. 13. The flux pinning force density Fp as a function of field and temperature for YBCO.
Fig. 15. The dependence of Jc on temperature at different fields for the YBCO sample.
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Fig. 16. Jc as a function of temperature for the Gd06 sample.
3.5 times in different fields. Jc is about 8400 ŽYBCO. and 24 500 Arcm2 ŽGd06. at 60 K in 3 T. Fig. 17 illustrates the ratio of Jc in YBCO over Jc in the Gd06 sample. The magnitude of the enhancement of Jc in high fields is higher than that in low fields. It increases with the field from 1.1 in 0.1 T to 3.5 in 4 T at 50 K. This result clearly indicates that the Gd addition has different contributions to the increase in Jc at high fields and low fields. The improvement of Jc over pure YBCO is very interesting and it is expected that Jc will be enhanced high enough for electrical engineering applications through further refinement of the composition and optimization for processing conditions. Based on the above discussion, we can conclude that the substitution of Gd for Y can help improve Jc and flux pinning in YBCO superconductors. Also, some researchers have found that Jc can be in-
Fig. 17. Field dependence of the ratio of Jc in YBCO over Jc in Gd06 Ž Jc,Gd r Jc,Y ..
creased by other rare-earth element additions. The 20% substitutions of Sm and Eu for Y in sintered YBCO lead to an enhancement of the intragrain Jc from 11 000 Arcm2 to 25 000 and 27 000 Arcm2 at 77 K in 0.9 T w39x. Recent work demonstrates that the flux pinning is improved by the partial substitutions of Pr for Y. It is thought that the increase in flux pinning by Pr ions is mainly induced by the suppression of superconducting order parameters in the vicinity of Pr ions via a magnetic pair breaking w40x. In addition, Nd 2 O 3rLa 2 O 3 additions have been found to result in enhancement of the flux pinning in melt-textured YBCO. It is considered that the increased Jc may be due to pinning effects created by NdrLa ions being present on Y andror Ba sites in the YBCO lattice w41x. However, although the effects of the rare-earth element additions in YBCO samples on flux pinning have been investigated, the mechanism of the increase in Jc and flux pinning is not clear. On the basis of our results, we think that the enhancement of Jc may be related to the following reasons. First, the size of Y211 particles is significantly reduced from 3.2 mm ŽYBCO. to 0.96 mm ŽGd06.. The size of 211 particles was determined by a chemical method. The size distributions of 211 for YBCO and Gd06 are shown in Figs. 18 and 19. The reduction of the 211 size will be helpful to diminish microcracks and will lead to more interfaces between Y123 and Y211 phases. The interface of Y123 and Y211 is found to be effective pinning centers in YBCO. As a result, Jc and flux pinning are improved. This is confirmed by
Fig. 18. The distribution of the size of 211 particles in YBCO.
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can be observed in YBCO. The peak field Hp decreases linearly with the field. The fishtail may be attributed to the cation defects orrand the paramagnetism in the Gd-doped sample created by the Gd addition. It is found that Jc can be improved by the substitution of Gd for Y, which may be due to the reduction of the size of Y211 particles, magnetic pinning and stress-field pinning. This should be further investigated to separate the relative contributions of these aspects.
Fig. 19. The size distribution of 211 particles in the Gd06 specimen.
other reports, in which the refinement of the Y211 size caused by the additions of PtO 2 or CeO 2 can enhance Jc and obtain better microstructure w42x. Secondly, it is well known that the flux pinning will be introduced by the elastic interaction between the vortex lattice and the stress field in conventional hard superconductors w43x. It is expected that the local lattice mismatch can be created by the partial substitutions of rare-earth elements with different ionic radii for Y in YBCO, which will lead to the formation of the stress field. Consequently, the flux pinning can be introduced. There is a stress field caused by the Gd addition because the radius of Gd 3q is larger than that of Y 3q. Thus, an additional strong flux pinning will be formed in Gd-doped YBCO sample. So, Jc is increased. A recent calculation has given an evidence for it w44x. Furthermore, it can be observed in Fig. 8 that there is paramagnetism in the Gd06 specimen, which will induce the magnetic pinning. This may also be responsible for the improvement of Jc .
4. Conclusion We have carried out magnetic measurements of YBa 2 Cu 3 O y and Y0.4 Gd 0.6 Ba 2 Cu 3 O y prepared by the powder melting process method through a SQUID magnetometer. Anomalous peaks are found in the Gd-added sample below 70 K with the magnetic field perpendicular to the c-axis, whereas no fishtail
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