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Enhanced flux pinning by Fe point defects in Bi2Sr2Ca(Cu1−xFex)2O8+δ single crystals
Physica C 337 Ž2000. 221–224 www.elsevier.nlrlocaterphysc
Enhanced flux pinning by Fe point defects in Bi 2 Sr2 Ca žCu 1yx Fe x / 2 O 8qd single crystals X.L. Wang a,) , J. Horvat a , G.D. Gu b, K.K. Uprety a , H.K. Liu a , S.X. Dou a a
Institute for Superconducting and Electronic Materials, UniÕersity of Wollongong, Wollongong, NSW 2522, Australia b Physics School, UniÕersity of New South Wales, Sydney 2050, Australia
Abstract We study the flux pinning of Bi 2 Sr2 CaCu 2 O 8q d ŽBi2212. single crystals doped with up to 2.2% of Fe on the Cu positions using measurements of magnetization hysteresis loop. We found that the critical current density increased with the doping for doping less than 0.5 at.%. This proves that Fe point defects contribute to the flux pinning. However, high doping levels deteriorate the Jc . The effect of Fe doping on the peak effect was also studied. The peak position shifts to high temperature, but the peak field decreased with increasing doping levels. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Flux pinning; Fe; Single crystals
Introducing various artificial defects into the high temperature superconductors ŽHTS. has been stimulating the studies on vortex nature in the HTS. Aiming at finding effective pinning centres into Bi– Sr–Ca–Cu–O ŽBSCCO., a very weak intrinsic pinning material, many doping investigations have been done on this compound for a decade. Extended pinning centres, such as columnar defects, dislocation networks, grain boundaries, and nanometer particle inclusions have been regarded as strong pinning centres in the BSCCO w1x. Since the vortex interaction in HTS has to be considered in a collective pinning model which was based on the long-range order of the vortex w2x, random point defects would destroy the large range vortex lattice. It is interesting that it might be that for high quality HTS single
)
Corresponding author. E-mail address: [email protected] ŽX.L. Wang..
crystals random point defects provide the main source of pinning w2x. It has been regarded that the elemental pinning force at a single point defect is very weak. However, on the other hand, the point defects located in a unit cell might cause the reduction of anisotropy and therefore enhance the Josephson coupling. Reduction of anisotropy in Bi2212 is caused by oxygen defects or Pb point defects in the BiO 2 layer w3–6x. It is interesting to note that all the point defects in CuO layer kill the superconductivity because the point defects on Cu sites destroy the long-range order of Cu interaction. However, as the vortex can also be interacted by a magnetic coupling w7x, point defects on a Cu site created by ferromagnetic ions, such as Fe or Ni w8,9x, may enhance the magnetic coupling for vortex interactions and therefore the pinning would be increased. In this work we aim to further investigate the Fe doping effect on flux pinning in the high quality
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 1 0 5 - 2
X.L. Wang et al.r Physica C 337 (2000) 221–224
222
X
Fig. 1. x vs. temperature for different doping levels.
Bi2212 single crystal. Results show that the flux pining in the Bi2212 can be enhanced under a very low Fe doping level. The Fe-doped Bi2212 crystals were grown by the floating zone method ŽFZM.. Details of the crystal growth were given in Ref. w10x. Four sets of crystals with different nominal y values of 0, 0.005, 0.013, 0.022 in the Bi 2.1 Sr1.9 Ca 1.0 ŽCu 1yy Fe y . 2 O 8q d were used for the flux pinning studies. Magnetization hysteresis loops were characterised by Physical Property Measurement System ŽPPMS., Quantum Design. The transition temperature Tc was determined by ac susceptibility measurement. As shown in Fig. 1, the Tc decreases with the increase of y. However, the transition widths are
Fig. 2. Field dependence of Jc for different doping levels at 20 and 30 K. Real line: y s 0; dot dash lines: y s 0.005; open circles: y s 0.013; closed circles: y s 0.022.
Fig. 3. Jc vs. T r Tc for different y.
very small for y s 0 and 0.005 samples and moderate for higher y value samples, suggesting a uniform distribution of oxygen or Fe substitution for Cu in the 2212 matrix. The field dependence of the critical current Jc is shown in Fig. 2. The Jc was obtained from M-H loops by using the formula w11x: Jc Ž T . s
20D M a a 1y 3b
ž
, a-b
/
where, D M is the magnetization difference between the lower and the upper part of the loop at a particular field, and a and b are the sample dimensions. It can be seen that the Jc seems to decrease with increasing Fe doping levels at different real tempera-
Fig. 4. Hpeak as a function of relative temperature T r Tc for different doping levels.
X.L. Wang et al.r Physica C 337 (2000) 221–224
tures. However, if Jc was plotted as a function of reduced temperature TrTc at different fields, we found that the Jc for y s 0.005 is actually larger than for the undoped sample. For y ) 0.005, the Jc deteriorated as shown in Fig. 3. It seems that the optimum doping level is only limited to very small value of y. The enhancement of the Jc is in well agreement with that observed in Fe, or Ni doped Bi2212 single crystals with very small doping contents w8,9x. Our results further confirm that the Fe point defects indeed contribute to flux pinning w9x. Further evidence of the pinning enhancement from the Fe doping can be seen from the feature of peak effect in the Fe-doped Bi2212 crystals. We found that the peak effect is always present for all the doping levels and can persist at high temperature ranges of TrTc . Fig. 4 shows field values at the secondary peak position as a function of TrTc . It clearly shows that for y s 0.005 sample, the peak effect is present over a wider temperature range with larger values of fields than that without doping. When y ) 0.005, the peak effect can persist at high temperatures but within a small range of field compared with low Fe doping samples. The enhancement of Jc and flux pinning in the peak effect may be due to the unchanged pinning potential for a optimum Fe doing levels which might be similar to Ni doped Bi2212 crystals w8x. With large doping, it is possible that the mean size of vortex bundles decreases strongly beyond the optimum concentration of point defect, thus reducing the effective activation energy and Jc . A detailed investigation on vortex nature in the Fe doped Bi2212 crystal needs to be done. Introducing extra magnetic interactions would enhance the vortex coupling w12x. Since the Fe is a magnetic ion, the vortex would be coupled strongly through magnetic interaction. This would enhance the flux pinning. In principle, more Fe ions mean stronger flux pining. However, due to the strong ferromagnetism of the Fe, more Fe ion incorporated into Cu sites would cancel the supercurrents which is diamagnetic and destroy the long-range coupling of Cu ion in the CuO layer which would deteriorate the Tc . Fig. 5 shows Tc or Jc changes with different Fe content. We can see that the Tc is significantly Žlinearly. depressed with increasing doping content. But the Jc at a fix TrTc in a 0.35 T takes maximum at y s 0.005 and then decreases with further doping.
223
Fig. 5. Tc or Jc for different Fe doping levels.
This indicates that Fe doping causes two effects. One is the linear decrease in Tc and another is enhancement of flux pinning. If doping beyond 0.005, although the pinning is still effective at relative high temperature, the total number of vortices contributing to the Jc would be small, therefore, Jc becomes low as shown in Figs. 2 and 3. Jc is low for all the Fe doping samples as a function of real temperature. However, the Jc is actually enhanced for the y s 0.005 sample measured at different fields. This is strong evidence that the Fe point defects are really effective pinning centres in the Bi2212. For large doping level, as we expected, the decrease of Jc is due to the cancellation of supercurrents by the strong ferromagnetic ion of iron.
References w1x S.X. Dou, P.N. Mikheenko, X.L. Wang, H.K. Liu, High Temperature Superconductors, Annual Reports Vol. 93 The Royal Society of Chemistry, 1998, Section C. w2x G. Blatter, M.V. Feigelman, V.B. Geshkenbein, A.I. Larkin, V.M. Vinokur, Rev. Mod. Phys. 64 Ž1996. 1125. w3x S. Ooi, T. Shibauchi, T. Tamegai, Physica C 302 Ž1998. 339. w4x Y. Yamaguchi, K. Oka, Z. Zou, S. Nakajima, M. Murakami ŽEds.., Advances in Superconductivity IX, Proceedings of the 19th International Symposium on Superconductivity, October 21–24, 1996, Sapporop, Springer-Verlag, Tokyo, 1997, p. 231. w5x I. Chong, Z. Hiroi, M. Izumi, J. Shimoyama, Y. Nakayama, K. Kishio, T. Terashima, Y. Bando, M. Takano, Science 276 Ž1997. 770. w6x K. Kishio, J. Shimoyama, T. Motohashi, H. Kobayashi, Y.
224
X.L. Wang et al.r Physica C 337 (2000) 221–224
Nakayama, K. Kitazawa, Proceedings of the 1990 International Workshop On Superconductivity, July 12–15, Okinawa, Japan, 1998, p. 99. w7x A.E. Koshelev, V.M. Vinokur, Phys. Rev. B 57 Ž1998. 8026. w8x R. Noetzel, K. Westerholt, Phys. Rev. B 58 Ž1998. 15108. w9x B. Noetzel, K. vom Hedt, Physica C 260 Ž1996. 290.
w10x G.D. Gu, K. Takamuku, N. Koshizuka, S. Tanaka, J. Cryst. Growth 137 Ž1994. 472. w11x E.M. Giorgy, R.B. van Dover, K.A. Jackson, L.F. Schneemeyer, J.V. Waszcak, Appl. Phys. Lett. 55 Ž1989. 283. w12x K.K. Uprety, D. Domingucz, Phys. Rev. B 51 Ž1995. 5955.
Enhanced flux pinning by Fe point defects in Bi 2 Sr2 Ca žCu 1yx Fe x / 2 O 8qd single crystals X.L. Wang a,) , J. Horvat a , G.D. Gu b, K.K. Uprety a , H.K. Liu a , S.X. Dou a a
Institute for Superconducting and Electronic Materials, UniÕersity of Wollongong, Wollongong, NSW 2522, Australia b Physics School, UniÕersity of New South Wales, Sydney 2050, Australia
Abstract We study the flux pinning of Bi 2 Sr2 CaCu 2 O 8q d ŽBi2212. single crystals doped with up to 2.2% of Fe on the Cu positions using measurements of magnetization hysteresis loop. We found that the critical current density increased with the doping for doping less than 0.5 at.%. This proves that Fe point defects contribute to the flux pinning. However, high doping levels deteriorate the Jc . The effect of Fe doping on the peak effect was also studied. The peak position shifts to high temperature, but the peak field decreased with increasing doping levels. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Flux pinning; Fe; Single crystals
Introducing various artificial defects into the high temperature superconductors ŽHTS. has been stimulating the studies on vortex nature in the HTS. Aiming at finding effective pinning centres into Bi– Sr–Ca–Cu–O ŽBSCCO., a very weak intrinsic pinning material, many doping investigations have been done on this compound for a decade. Extended pinning centres, such as columnar defects, dislocation networks, grain boundaries, and nanometer particle inclusions have been regarded as strong pinning centres in the BSCCO w1x. Since the vortex interaction in HTS has to be considered in a collective pinning model which was based on the long-range order of the vortex w2x, random point defects would destroy the large range vortex lattice. It is interesting that it might be that for high quality HTS single
)
Corresponding author. E-mail address: [email protected] ŽX.L. Wang..
crystals random point defects provide the main source of pinning w2x. It has been regarded that the elemental pinning force at a single point defect is very weak. However, on the other hand, the point defects located in a unit cell might cause the reduction of anisotropy and therefore enhance the Josephson coupling. Reduction of anisotropy in Bi2212 is caused by oxygen defects or Pb point defects in the BiO 2 layer w3–6x. It is interesting to note that all the point defects in CuO layer kill the superconductivity because the point defects on Cu sites destroy the long-range order of Cu interaction. However, as the vortex can also be interacted by a magnetic coupling w7x, point defects on a Cu site created by ferromagnetic ions, such as Fe or Ni w8,9x, may enhance the magnetic coupling for vortex interactions and therefore the pinning would be increased. In this work we aim to further investigate the Fe doping effect on flux pinning in the high quality
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 1 0 5 - 2
X.L. Wang et al.r Physica C 337 (2000) 221–224
222
X
Fig. 1. x vs. temperature for different doping levels.
Bi2212 single crystal. Results show that the flux pining in the Bi2212 can be enhanced under a very low Fe doping level. The Fe-doped Bi2212 crystals were grown by the floating zone method ŽFZM.. Details of the crystal growth were given in Ref. w10x. Four sets of crystals with different nominal y values of 0, 0.005, 0.013, 0.022 in the Bi 2.1 Sr1.9 Ca 1.0 ŽCu 1yy Fe y . 2 O 8q d were used for the flux pinning studies. Magnetization hysteresis loops were characterised by Physical Property Measurement System ŽPPMS., Quantum Design. The transition temperature Tc was determined by ac susceptibility measurement. As shown in Fig. 1, the Tc decreases with the increase of y. However, the transition widths are
Fig. 2. Field dependence of Jc for different doping levels at 20 and 30 K. Real line: y s 0; dot dash lines: y s 0.005; open circles: y s 0.013; closed circles: y s 0.022.
Fig. 3. Jc vs. T r Tc for different y.
very small for y s 0 and 0.005 samples and moderate for higher y value samples, suggesting a uniform distribution of oxygen or Fe substitution for Cu in the 2212 matrix. The field dependence of the critical current Jc is shown in Fig. 2. The Jc was obtained from M-H loops by using the formula w11x: Jc Ž T . s
20D M a a 1y 3b
ž
, a-b
/
where, D M is the magnetization difference between the lower and the upper part of the loop at a particular field, and a and b are the sample dimensions. It can be seen that the Jc seems to decrease with increasing Fe doping levels at different real tempera-
Fig. 4. Hpeak as a function of relative temperature T r Tc for different doping levels.
X.L. Wang et al.r Physica C 337 (2000) 221–224
tures. However, if Jc was plotted as a function of reduced temperature TrTc at different fields, we found that the Jc for y s 0.005 is actually larger than for the undoped sample. For y ) 0.005, the Jc deteriorated as shown in Fig. 3. It seems that the optimum doping level is only limited to very small value of y. The enhancement of the Jc is in well agreement with that observed in Fe, or Ni doped Bi2212 single crystals with very small doping contents w8,9x. Our results further confirm that the Fe point defects indeed contribute to flux pinning w9x. Further evidence of the pinning enhancement from the Fe doping can be seen from the feature of peak effect in the Fe-doped Bi2212 crystals. We found that the peak effect is always present for all the doping levels and can persist at high temperature ranges of TrTc . Fig. 4 shows field values at the secondary peak position as a function of TrTc . It clearly shows that for y s 0.005 sample, the peak effect is present over a wider temperature range with larger values of fields than that without doping. When y ) 0.005, the peak effect can persist at high temperatures but within a small range of field compared with low Fe doping samples. The enhancement of Jc and flux pinning in the peak effect may be due to the unchanged pinning potential for a optimum Fe doing levels which might be similar to Ni doped Bi2212 crystals w8x. With large doping, it is possible that the mean size of vortex bundles decreases strongly beyond the optimum concentration of point defect, thus reducing the effective activation energy and Jc . A detailed investigation on vortex nature in the Fe doped Bi2212 crystal needs to be done. Introducing extra magnetic interactions would enhance the vortex coupling w12x. Since the Fe is a magnetic ion, the vortex would be coupled strongly through magnetic interaction. This would enhance the flux pinning. In principle, more Fe ions mean stronger flux pining. However, due to the strong ferromagnetism of the Fe, more Fe ion incorporated into Cu sites would cancel the supercurrents which is diamagnetic and destroy the long-range coupling of Cu ion in the CuO layer which would deteriorate the Tc . Fig. 5 shows Tc or Jc changes with different Fe content. We can see that the Tc is significantly Žlinearly. depressed with increasing doping content. But the Jc at a fix TrTc in a 0.35 T takes maximum at y s 0.005 and then decreases with further doping.
223
Fig. 5. Tc or Jc for different Fe doping levels.
This indicates that Fe doping causes two effects. One is the linear decrease in Tc and another is enhancement of flux pinning. If doping beyond 0.005, although the pinning is still effective at relative high temperature, the total number of vortices contributing to the Jc would be small, therefore, Jc becomes low as shown in Figs. 2 and 3. Jc is low for all the Fe doping samples as a function of real temperature. However, the Jc is actually enhanced for the y s 0.005 sample measured at different fields. This is strong evidence that the Fe point defects are really effective pinning centres in the Bi2212. For large doping level, as we expected, the decrease of Jc is due to the cancellation of supercurrents by the strong ferromagnetic ion of iron.
References w1x S.X. Dou, P.N. Mikheenko, X.L. Wang, H.K. Liu, High Temperature Superconductors, Annual Reports Vol. 93 The Royal Society of Chemistry, 1998, Section C. w2x G. Blatter, M.V. Feigelman, V.B. Geshkenbein, A.I. Larkin, V.M. Vinokur, Rev. Mod. Phys. 64 Ž1996. 1125. w3x S. Ooi, T. Shibauchi, T. Tamegai, Physica C 302 Ž1998. 339. w4x Y. Yamaguchi, K. Oka, Z. Zou, S. Nakajima, M. Murakami ŽEds.., Advances in Superconductivity IX, Proceedings of the 19th International Symposium on Superconductivity, October 21–24, 1996, Sapporop, Springer-Verlag, Tokyo, 1997, p. 231. w5x I. Chong, Z. Hiroi, M. Izumi, J. Shimoyama, Y. Nakayama, K. Kishio, T. Terashima, Y. Bando, M. Takano, Science 276 Ž1997. 770. w6x K. Kishio, J. Shimoyama, T. Motohashi, H. Kobayashi, Y.
224
X.L. Wang et al.r Physica C 337 (2000) 221–224
Nakayama, K. Kitazawa, Proceedings of the 1990 International Workshop On Superconductivity, July 12–15, Okinawa, Japan, 1998, p. 99. w7x A.E. Koshelev, V.M. Vinokur, Phys. Rev. B 57 Ž1998. 8026. w8x R. Noetzel, K. Westerholt, Phys. Rev. B 58 Ž1998. 15108. w9x B. Noetzel, K. vom Hedt, Physica C 260 Ž1996. 290.
w10x G.D. Gu, K. Takamuku, N. Koshizuka, S. Tanaka, J. Cryst. Growth 137 Ž1994. 472. w11x E.M. Giorgy, R.B. van Dover, K.A. Jackson, L.F. Schneemeyer, J.V. Waszcak, Appl. Phys. Lett. 55 Ž1989. 283. w12x K.K. Uprety, D. Domingucz, Phys. Rev. B 51 Ž1995. 5955.