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Appl. Phys. Lett., Vol. 83, No. 2, 14 July 2003
Li et al.
pinning of grain boundaries.3 In the sample doped with het-
eronanoprecipitates (Y2O3), the maximum pinning force
density shifts toward higher field, and reveals that vortex
pinning is not only augmented by grain boundaries, but also
by inclusions. However, the field dependence of the pinning
force density ͓solid line in Fig. 4͑b͔͒, measured from the
sample with nanodefect traps, is much greater than that ob-
served in those samples in which pinning is enhanced only
by grain boundaries and inclusions. The maximum pinning
force density is increased to 1.34 from 0.76 T for pinning by
grain boundaries. The volume pinning force density of
5.5 GN/m3 at 20 K is comparable to that of NbTi at 4.2 K.
This indicates that the density of the trapped defects in the
semicrystalline nanodefect wells is an order of magnitude
greater than that found in other vortex pinning sources. How-
ever, similar Jc(0) in all samples ͓Fig. 4͑a͔͒ suggests that the
highly aligned nanostructure of the present sample does not
greatly contribute to the critical current density in zero field,
supporting the notion that the MgB2 is not compromised by
a weak-link problem.2,3
In summary, the significant enhancement of flux pinning
in the high-field performance of MgB2 reveals the formation
of a vortex pinning source: semicrystalline nanodefect traps
in self-aligned nanostructured MgB2 . It is observed that the
͓211͔ zone is the favored orientation for crystal growth and it
suggests that the ͑211͒ plane may have lower surface energy
of the system. Dislocation incorporation from small-angle
domain boundaries results in large-scale alignment of the
nanodomains in three dimensions. When orthogonal disloca-
tions of different signs meet at a point in the ͑211͒ plane,
nanodefect wells are formed to release the strain caused by
the rotations of nanodomains. Such wells trap numerous
crystal defects from the boundaries to act as the vortex pin-
ning centers with intense pinning force, thus enhancing the
high-field performance of MgB2 significantly.
FIG. 4. ͑a͒ The magnetic Jc(H) for MgB2 with nanodefect traps at 20 K
compared with the data reported for other vortex pinning sources. ͑b͒ The
field dependence of flux pinning force density shows that the pinning force
density of the defect traps is much greater than other vortex pinning sources,
such as grain boundaries and heteronanoparticles.
nanostructured MgB2 . However, defect traps can only re-
lease local strain. On a large scale, many similar semicrys-
talline defect traps are formed and dispersed throughout the
material, as shown in Fig. 1͑a͒. It is believed that the addi-
tion of SiC in MgB2 facilitates the formation of self-aligned
nanostructured MgB2 through promotion of incipient melt-
ing, but the nature of facilitation is still under investigation.
Nonetheless, whatever the mechanism of formation, it is
clear that nanodefect traps substantially enhance the high
field performance of MgB2 . Figure 4͑a͒ compares the mag-
netic Jc(H) for material having nanodefect traps at 20 K
with literature data. It can be seen that the sample with nano-
defect traps has high Jc as well as excellent field perfor-
mance, particularly in high field. These superior properties
indicate that the pinning force of the semicrystalline defect
trap is much greater than that of the other vortex pinning
sources. The flux pinning enhancement of the semicrystalline
nanodefect trap is also demonstrated by the field dependence
of flux pinning force density as shown in Fig. 4͑b͒, which
compares the pinning force density of the nanodefect traps
with the reported data of the other pinning sources in bulk
materials. The dotted line presents the field dependence at 20
K of the relative pinning force density measured from a
sample in which the supercurrent density is mostly deter-
mined by the flux pinning of grain boundaries. It shows that
the flux-pinning force density curve, as the function of the
1 J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, and J. Akimitsu,
Nature ͑London͒ 410, 63 ͑2001͒.
2 D. Larbalestier, A. Gurevich, D. M. Feldmann, and A. Polyanskii, Nature
͑London͒ 414, 368 ͑2001͒.
3 D. C. Larbalestier, L. D. Cooley, M. O. Rikel, A. A. Polyanskil, J. Jiang,
S. Patnalk, X. Y. Cai, D. M. Feldmann, A. Gurevich, A. A. Squitieri, M. T.
Naus, C. B. Eom, E. E. Hellstrom, R. J. Cava, K. A. Regan, N. Rogado,
M. A. Hayward, T. He, J. S. Slusky, K. Khalifah, K. Inumaru, and M.
Haas, Nature ͑London͒ 410, 186 ͑2001͒.
4 Y. Bugoslavsky, G. K. Perkins, X. Qi, L. F. Cohen, and A. D. Caplin,
Nature ͑London͒ 410, 563 ͑2001͒.
5 Y. Bugoslavsky, L. F. Cohen, G. K. Perkins, M. Pollchetti, T. J. Tate, R.
Gwilliam, and A. D. Caplin, Nature ͑London͒ 411, 561 ͑2001͒.
6 C. B. Eom, M. K. Lee, J. H. Chol, L. J. Belenky, X. Song, L. D. Cooley,
M. T. Naus, S. Patanik, J. Jiang, M. Rikei, A. Polyanski, A. Gurevich, X.
Y. Cai, S. D. Bu, S. E. Babcock, E. E. Hellstrom, D. C. Larbalestier, N.
Rogado, K. A. Regan, M. A. Hayward, T. He, J. S. Slusky, K. Inumaru, M.
K. Haas, and R. J. Cava, Nature ͑London͒ 411, 558 ͑2001͒.
7 X. Z. Liao, A. C. Erquis, Y. T. Zhu, J. Y. Huang, D. E. Peterson, F. M.
Mueller, and H. F. Xu, Appl. Phys. Lett. 80, 4398 ͑2002͒.
8 J. Wang, Y. Bugoslavsky, A. Berenov, L. Cowey, A. D. Caplin, L. F.
Cohen, L. D. Cooley, X. Song, and D. C. Larbalestier, Appl. Phys. Lett.
81, 2026 ͑2002͒.
9
¨
H. L. Suo, C. Beneduce, X. D. Su, and R. Flukiger, Supercond. Sci.
Technol. 15, 1058 ͑2002͒.
10 S. X. Dou, S. Soltanian, J. Horvat, X. L. Wang, S. H. Zhou, M. Ionescu,
H. K. Liu, P. Munroe, and M. Tomsic, Appl. Phys. Lett. 81, 3419 ͑2002͒.
11 P. C. Canfield, D. K. Finnemore, S. L. Bud’ko, J. E. Ostenson, G. Laper-
applied field, matches the curve contributed purely by the
tot, C. E. Cunningham, and C. Petrovic, Phys. Rev. Lett. 86, 2423 ͑2001͒.
128.143.1.222 On: Thu, 11 Dec 2014 21:07:18