- Email: [email protected]
Magnetic properties and critical currents of bulk MgB2 polycrystalline superconductor
Physica C 372–376 (2002) 1262–1265 www.elsevier.com/locate/physc
Magnetic properties and critical currents of bulk MgB2 polycrystalline superconductor D.A. Cardwell *, N. Hari Babu, M. Kambara, A.M. Campbell IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK
Abstract Bulk, polycrystalline MgB2 has been fabricated in the undoped state and with Cu, Ag and Zn doped on the Mg site. We report the results of magnetic measurements on these samples over a wide range of temperatures and applied magnetic fields and use these to discuss the effect of grain boundaries on the current carrying properties of this system in applied field. Analysis of the normalised pinning force as a function of reduced magnetic field in Zn doped and undoped samples reveals increased pinning strength in the former at higher magnetic fields. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.60.Jg; 74.60.Ge; 74.60.Ec Keywords: MgB2 ; Critical currents; Pinning force; Critical fields
1. Introduction Superconductivity below 39 K has recently been discovered in MgB2 [1]. Significantly, the grain boundaries in this material are strongly coupled and the current flow is unhindered by the grain boundaries (unlike in high Tc superconductors) [2,3], underlining the potential of this material for high transport current applications. Metal clad MgB2 wires have already been made successfully [4,5] with iron-clad wire able to carry critical current densities ðJc ’sÞ > 105 A/cm2 at 1 T [5]. Unfortunately, however, Jc in both wire and bulk specimens falls-off rapidly with applied field due to
*
Corresponding author. Tel.: +44-1223-337050; fax: +441223-337074. E-mail address: [email protected] (D.A. Cardwell).
lack of pinning in this material. Although the Tc of MgB2 is higher than conventional low Tc Nb3 Sn, for example, its Jc at high fields and irreversibility field are typically much lower. In this paper, we investigate flux pinning in MgB2 by chemical doping with Ag, Cu and Zn. From the magnetic properties of these samples, we conclude that the addition of Zn to MgB2 produces a new type of pinning centre, which is effective at high fields.
2. Experimental Cylindrical pellets composed of MgB2 , Mg0:8 Ag0:2 B2 , Mg0:8 Cu0:2 B2 and Mg0:8 Zn0:2 B2 were prepared from a mixture of high purity Mg, B, Ag, Cu and Zn powders using an evacuated press die-set to minimize the air trapped in the sample. Each pellet was sandwiched between Mo/Al2 O3 plates
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 8 8 - 7
D.A. Cardwell et al. / Physica C 372–376 (2002) 1262–1265
and placed on an alumina crucible with pure Mg powder placed close to the sample to suppress oxidation and evaporation of Mg from the sample during processing. The arrangement was heat treated in a Mg-coated tube furnace under an applied load of 10 N at 850 °C under flowing 4%H2 þ Ar gas for 10 h. Detailed processing conditions are given in [2]. Small pieces were cut from each sintered pellet and their magnetic properties measured using a SQUID magnetometer. Zero field cooled (ZFC) curves were recorded by cooling each specimen in zero field. Magnetic field was then applied and the magnetic moment measured as the sample was warmed. Field cooled (FC) curves were measured in a similar way but after each sample was cooled under applied magnetic field.
3. Results and discussion Fig. 1 shows the temperature dependence of the normalised ZFC magnetic moment at 1 mT applied field for MgB2 , Mg0:8 Cu0:2 B2 , Mg0:8 Ag0:2 B2 and Mg0:8 Zn0:2 B2 polycrystalline superconductors. The onset Tc values for these samples are 38.5, 36, 33.8 and 38.5 K respectively. The reduction in Tc for the Cu and Ag doped samples indicates that Cu and Ag substitute in the MgB2 lattice and alter the
Fig. 1. Temperature variation of normalised magnetic moment for various MgB2 doped samples.
1263
Fig. 2. Hirr , Hc2 and Hc1 as a function of temperature for polycrystalline bulk MgB2 .
carrier density. Addition of Zn to MgB2 does not suppress Tc , however, suggesting that Zn remains in the form of precipitates within the MgB2 grains or at grain boundaries. Fig. 2 shows Hc1 , Hirr and Hc2 for the undoped MgB2 polycrystalline sample. Hc1 is determined from the virgin MðH Þ curve as the point at which M deviates from l0 H . Hc2 and Hirr are determined from the FC and ZFC curves. The inset to Fig. 2 shows typical ZFC and FC curves at 4 T applied field for undoped MgB2 . The arrows indicate the transition and irreversibility temperatures at this field. It is apparent from Fig. 2 that the irreversibility line is much lower than the Hc2 line, as is observed for high Tc superconductors. The wide field domain between Hirr and Hc2 suggests a lack of pinning disorder in this material, unlike low Tc Nb3 Sn superconductor where Hirr is nearly double that observed for MgB2 . The Tc of Nb3 Sn, on the other hand, is nearly half that for MgB2 . Increasing Hirr towards Hc2 is only possible by increasing pinning in the material. The bulk (inter-grain) Jc was determined from the measured magnetic hysteresis loops using the Bean critical state model. The measured Jc ðBÞ curves at different temperatures are shown in Fig. 3. The polycrystalline sample exhibits a very high bulk Jc at low fields, suggesting that randomly oriented grain boundaries can support significant
1264
D.A. Cardwell et al. / Physica C 372–376 (2002) 1262–1265
negligible above Hirr , as is also the case for high Tc superconductors. Based on direct summation of elementary pinning forces, Dew-Hughes [6] proposed the following general expression for the normalised pinning force density; Fp =Fp;max / hp ð1 hÞq
ð1Þ
where the dimensionless parameters p and q depend on the specific characteristics of flux pinning.
Fig. 3. Jc ðHÞ for a bulk polycrystalline and powder (grain) MgB2 specimens.
transport currents. The intra-grain Jc for a powder specimen measured at 10 K but assuming a much smaller current length scale (10 lm) was determined using a similar technique to be 5 106 A/ cm2 at 0 T, although the error in this value could be as high as 20% due to difficulties in estimating the grain size from optical micrographs. At low fields the inter-grain Jc of the polycrystalline sample is lower than the intra-grain Jc of the powder sample. At high fields, however, the bulk (inter-grain) Jc was observed to approximate well to the intra-grain Jc . In both cases Jc is strongly dependent on applied field and decreases rapidly at high fields. The bulk Jc ’s in MgB2 doped with Cu, Ag and Zn are also the order of 104 –105 A/cm2 at 10 K, indicating that the reduced weak link nature of the MgB2 grain boundaries are unaffected by doping in this system. The flux pinning force has been analysed in powder, sintered MgB2 , sintered Mg0:8 Cu0:2 B2 , Mg0:8 Ag0:2 B2 and Mg0:8 Zn0:2 B2 specimens in order to study the characteristics of the intrinsic pinning mechanisms in this system. In general this is performed by scaling the normalised pinning force density, Fp =Fp;max , against reduced applied magnetic field (h ¼ H =Hc2 ), where Fp;max is the maximum pinning force density and Hc2 is the upper critical field. Here, it is necessary to replace Hc2 with irreversibility filed Hirr since Jc is almost
Fig. 4. Fp =Fp;max as a function of reduced field for (a) MgB2 powder (b) sintered MgB2 and (c) sintered Mg0:8 Zn0:2 B2 .
D.A. Cardwell et al. / Physica C 372–376 (2002) 1262–1265
As a result, different types of pinning centres are described by different values of p and q. These, in turn, yield different values of hmax . Fig. 4(a) shows the pinning force density curve for the undoped MgB2 powder specimen, which exhibits good scaling behaviour. The maximum peak in pinning force density occurs at a reduced field hmax ¼ 0:2, which suggests that normal and surface pinning dominate within the MgB2 grains. This is evident from the solid line fit with p ¼ 1=2 and q ¼ 2 which approximates well to the experimental data. hmax is unchanged for the sintered specimen, where the grains are in physical contact, as shown in Fig. 4(b). This suggests that the type of pinning across grain boundaries is the same as that in the grains. The origin of the surface pinning in both the grains and the bulk is as yet unknown. Doping with Cu and Ag does not change the peak position and shape of the normalized pinning curve. For the Zn doped specimen, however, hmax appears at h ¼ 0:32 and the shape of the scaling curve is much broader in the medium field range, as shown in Fig. 4(c). This is quite different to the data for the undoped sample. According to the Dew-Hughes model, the peak can appear at 0.33 if normal and point pinning is the dominant mechanism. Point pinning in MgB2 containing Zn might originate from Zn precipitates in the MgB2 matrix. The peak and high pinning force at higher reduced field suggests that superconducting Mg0:8 Zn0:2 B2 could carry high currents at high fields. Further work is underway to optimise the Zn content of MgB2 to yield a large pinning force at high fields in this material.
1265
4. Conclusions We have observed strong evidence for high inter-granular critical current densities and a large bulk magnetic flux pinning in superconducting polycrystalline MgB2 . Doping of this system with Cu, Ag and Zn does not affect the reduced weaklink nature of the grain boundaries of MgB2 . Cu and Ag doping is observed to reduce Tc whereas Zn doping has little effect on Tc . Analysis of the normalised pinning force for various doped materials reveals that a surface pinning mechanism is dominant in MgB2 grains. Bulk polycrystalline MgB2 is also shown to exhibit surface pinning. The addition of Zn to MgB2 , on the other hand, appears to introduce point pinning centres which yields a high pinning force at high fields.
Acknowledgements N. Hari Babu is a Leverhulme Fellow funded by the Leverhulme Trust.
References [1] [2] [3] [4]
J. Nagamatsu et al., Nature 410 (2001) 3. M. Kambara et al., Supercond. Sci. Technol. 14 (2001) L5. D.C. Larbalestier et al., Nature 410 (2001) 186. B. Glowacki et al., Supercond. Sci. Technol. 14 (2001) 193. [5] S. Jin et al., Nature 411 (2001) 563. [6] D. Dew-Hughes, Phil. Mag. 30 (1974) 293.
Magnetic properties and critical currents of bulk MgB2 polycrystalline superconductor D.A. Cardwell *, N. Hari Babu, M. Kambara, A.M. Campbell IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK
Abstract Bulk, polycrystalline MgB2 has been fabricated in the undoped state and with Cu, Ag and Zn doped on the Mg site. We report the results of magnetic measurements on these samples over a wide range of temperatures and applied magnetic fields and use these to discuss the effect of grain boundaries on the current carrying properties of this system in applied field. Analysis of the normalised pinning force as a function of reduced magnetic field in Zn doped and undoped samples reveals increased pinning strength in the former at higher magnetic fields. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.60.Jg; 74.60.Ge; 74.60.Ec Keywords: MgB2 ; Critical currents; Pinning force; Critical fields
1. Introduction Superconductivity below 39 K has recently been discovered in MgB2 [1]. Significantly, the grain boundaries in this material are strongly coupled and the current flow is unhindered by the grain boundaries (unlike in high Tc superconductors) [2,3], underlining the potential of this material for high transport current applications. Metal clad MgB2 wires have already been made successfully [4,5] with iron-clad wire able to carry critical current densities ðJc ’sÞ > 105 A/cm2 at 1 T [5]. Unfortunately, however, Jc in both wire and bulk specimens falls-off rapidly with applied field due to
*
Corresponding author. Tel.: +44-1223-337050; fax: +441223-337074. E-mail address: [email protected] (D.A. Cardwell).
lack of pinning in this material. Although the Tc of MgB2 is higher than conventional low Tc Nb3 Sn, for example, its Jc at high fields and irreversibility field are typically much lower. In this paper, we investigate flux pinning in MgB2 by chemical doping with Ag, Cu and Zn. From the magnetic properties of these samples, we conclude that the addition of Zn to MgB2 produces a new type of pinning centre, which is effective at high fields.
2. Experimental Cylindrical pellets composed of MgB2 , Mg0:8 Ag0:2 B2 , Mg0:8 Cu0:2 B2 and Mg0:8 Zn0:2 B2 were prepared from a mixture of high purity Mg, B, Ag, Cu and Zn powders using an evacuated press die-set to minimize the air trapped in the sample. Each pellet was sandwiched between Mo/Al2 O3 plates
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 8 8 - 7
D.A. Cardwell et al. / Physica C 372–376 (2002) 1262–1265
and placed on an alumina crucible with pure Mg powder placed close to the sample to suppress oxidation and evaporation of Mg from the sample during processing. The arrangement was heat treated in a Mg-coated tube furnace under an applied load of 10 N at 850 °C under flowing 4%H2 þ Ar gas for 10 h. Detailed processing conditions are given in [2]. Small pieces were cut from each sintered pellet and their magnetic properties measured using a SQUID magnetometer. Zero field cooled (ZFC) curves were recorded by cooling each specimen in zero field. Magnetic field was then applied and the magnetic moment measured as the sample was warmed. Field cooled (FC) curves were measured in a similar way but after each sample was cooled under applied magnetic field.
3. Results and discussion Fig. 1 shows the temperature dependence of the normalised ZFC magnetic moment at 1 mT applied field for MgB2 , Mg0:8 Cu0:2 B2 , Mg0:8 Ag0:2 B2 and Mg0:8 Zn0:2 B2 polycrystalline superconductors. The onset Tc values for these samples are 38.5, 36, 33.8 and 38.5 K respectively. The reduction in Tc for the Cu and Ag doped samples indicates that Cu and Ag substitute in the MgB2 lattice and alter the
Fig. 1. Temperature variation of normalised magnetic moment for various MgB2 doped samples.
1263
Fig. 2. Hirr , Hc2 and Hc1 as a function of temperature for polycrystalline bulk MgB2 .
carrier density. Addition of Zn to MgB2 does not suppress Tc , however, suggesting that Zn remains in the form of precipitates within the MgB2 grains or at grain boundaries. Fig. 2 shows Hc1 , Hirr and Hc2 for the undoped MgB2 polycrystalline sample. Hc1 is determined from the virgin MðH Þ curve as the point at which M deviates from l0 H . Hc2 and Hirr are determined from the FC and ZFC curves. The inset to Fig. 2 shows typical ZFC and FC curves at 4 T applied field for undoped MgB2 . The arrows indicate the transition and irreversibility temperatures at this field. It is apparent from Fig. 2 that the irreversibility line is much lower than the Hc2 line, as is observed for high Tc superconductors. The wide field domain between Hirr and Hc2 suggests a lack of pinning disorder in this material, unlike low Tc Nb3 Sn superconductor where Hirr is nearly double that observed for MgB2 . The Tc of Nb3 Sn, on the other hand, is nearly half that for MgB2 . Increasing Hirr towards Hc2 is only possible by increasing pinning in the material. The bulk (inter-grain) Jc was determined from the measured magnetic hysteresis loops using the Bean critical state model. The measured Jc ðBÞ curves at different temperatures are shown in Fig. 3. The polycrystalline sample exhibits a very high bulk Jc at low fields, suggesting that randomly oriented grain boundaries can support significant
1264
D.A. Cardwell et al. / Physica C 372–376 (2002) 1262–1265
negligible above Hirr , as is also the case for high Tc superconductors. Based on direct summation of elementary pinning forces, Dew-Hughes [6] proposed the following general expression for the normalised pinning force density; Fp =Fp;max / hp ð1 hÞq
ð1Þ
where the dimensionless parameters p and q depend on the specific characteristics of flux pinning.
Fig. 3. Jc ðHÞ for a bulk polycrystalline and powder (grain) MgB2 specimens.
transport currents. The intra-grain Jc for a powder specimen measured at 10 K but assuming a much smaller current length scale (10 lm) was determined using a similar technique to be 5 106 A/ cm2 at 0 T, although the error in this value could be as high as 20% due to difficulties in estimating the grain size from optical micrographs. At low fields the inter-grain Jc of the polycrystalline sample is lower than the intra-grain Jc of the powder sample. At high fields, however, the bulk (inter-grain) Jc was observed to approximate well to the intra-grain Jc . In both cases Jc is strongly dependent on applied field and decreases rapidly at high fields. The bulk Jc ’s in MgB2 doped with Cu, Ag and Zn are also the order of 104 –105 A/cm2 at 10 K, indicating that the reduced weak link nature of the MgB2 grain boundaries are unaffected by doping in this system. The flux pinning force has been analysed in powder, sintered MgB2 , sintered Mg0:8 Cu0:2 B2 , Mg0:8 Ag0:2 B2 and Mg0:8 Zn0:2 B2 specimens in order to study the characteristics of the intrinsic pinning mechanisms in this system. In general this is performed by scaling the normalised pinning force density, Fp =Fp;max , against reduced applied magnetic field (h ¼ H =Hc2 ), where Fp;max is the maximum pinning force density and Hc2 is the upper critical field. Here, it is necessary to replace Hc2 with irreversibility filed Hirr since Jc is almost
Fig. 4. Fp =Fp;max as a function of reduced field for (a) MgB2 powder (b) sintered MgB2 and (c) sintered Mg0:8 Zn0:2 B2 .
D.A. Cardwell et al. / Physica C 372–376 (2002) 1262–1265
As a result, different types of pinning centres are described by different values of p and q. These, in turn, yield different values of hmax . Fig. 4(a) shows the pinning force density curve for the undoped MgB2 powder specimen, which exhibits good scaling behaviour. The maximum peak in pinning force density occurs at a reduced field hmax ¼ 0:2, which suggests that normal and surface pinning dominate within the MgB2 grains. This is evident from the solid line fit with p ¼ 1=2 and q ¼ 2 which approximates well to the experimental data. hmax is unchanged for the sintered specimen, where the grains are in physical contact, as shown in Fig. 4(b). This suggests that the type of pinning across grain boundaries is the same as that in the grains. The origin of the surface pinning in both the grains and the bulk is as yet unknown. Doping with Cu and Ag does not change the peak position and shape of the normalized pinning curve. For the Zn doped specimen, however, hmax appears at h ¼ 0:32 and the shape of the scaling curve is much broader in the medium field range, as shown in Fig. 4(c). This is quite different to the data for the undoped sample. According to the Dew-Hughes model, the peak can appear at 0.33 if normal and point pinning is the dominant mechanism. Point pinning in MgB2 containing Zn might originate from Zn precipitates in the MgB2 matrix. The peak and high pinning force at higher reduced field suggests that superconducting Mg0:8 Zn0:2 B2 could carry high currents at high fields. Further work is underway to optimise the Zn content of MgB2 to yield a large pinning force at high fields in this material.
1265
4. Conclusions We have observed strong evidence for high inter-granular critical current densities and a large bulk magnetic flux pinning in superconducting polycrystalline MgB2 . Doping of this system with Cu, Ag and Zn does not affect the reduced weaklink nature of the grain boundaries of MgB2 . Cu and Ag doping is observed to reduce Tc whereas Zn doping has little effect on Tc . Analysis of the normalised pinning force for various doped materials reveals that a surface pinning mechanism is dominant in MgB2 grains. Bulk polycrystalline MgB2 is also shown to exhibit surface pinning. The addition of Zn to MgB2 , on the other hand, appears to introduce point pinning centres which yields a high pinning force at high fields.
Acknowledgements N. Hari Babu is a Leverhulme Fellow funded by the Leverhulme Trust.
References [1] [2] [3] [4]
J. Nagamatsu et al., Nature 410 (2001) 3. M. Kambara et al., Supercond. Sci. Technol. 14 (2001) L5. D.C. Larbalestier et al., Nature 410 (2001) 186. B. Glowacki et al., Supercond. Sci. Technol. 14 (2001) 193. [5] S. Jin et al., Nature 411 (2001) 563. [6] D. Dew-Hughes, Phil. Mag. 30 (1974) 293.