Effect of High Magnetic Field during Melt-Solidification Process on Crystal Alignment of REBCO Bulk
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Effect of High Magnetic Field during Melt-Solidification Process on Crystal Alignment of REBCO Bulk R. Yamagiwa, S. Horii, H. Eto, K. Otzschi, J. Shimoyama, and K. Kishio Department of Superconductivity, University of Tokyo, Tokyo, 113-8656, Japan ABSTRACT Disk-shaped samples of REBa2Cu3Oy ( RE123, RE = Y, Ho ) with 40%-by-molar RE2BaCuO5 ( RE211, same RE of RE123 ) were melt-solidified without seeding under various magnetic fields ( µ0Ha = 0 ~ 8 T ) and at various cooling rates ( Cr = 5.0, 10.0°C/h ) in ambient atmosphere. Crystal alignment was evaluated by the X-ray diffraction, magnetization measurement using a SQUID magnetometer and observation of microstructure using a scanning electron microscope and a polarizing microscope. The melt grown RE123 crystals were c-axisaligned under magnetic fields parallel to the applied field, (Ha), while the effect of alignment were systematically changed by the conditions of cooling rate and RE elements and the magnitude of Ha. The best c-axis-aligned sample showed that the value of anisotropy ( where the ratio of MH//Ha / MH⊥Ha ) was approximately 20 at 85K and at µ0H = 0.3 T for the condition of RE = Y, Cr = 5.0°C/h , µ0Ha = 8 T.
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INTRODUCTION Recent and remarkable development of cryogenic and superconducting technologies have brought us downsized high magnetic fields environment. Now, a high magnetic field of 10 Tesla can be easily generated without liquid Helium, and therefore are expected as an effective process of development of functional materials. In the high temperature superconducting ( HTS ) materials research, especially in REBa2Cu3Oy( RE123 ) system, many studies under magnetic field have been eagerly carried out and several effects were reported. Farrel et al. [1] have successfully aligned Y123 powders at room temperature in an organic matrix under 9.4 Tesla and reported that this effect was due to that the anisotropy energy of RE123, ∆E, (∆E = µ0∆χVHa2/2, where V is the volume of each particle, Ha is a magnitude of heat-treatment field and ∆χ = χ// - χ⊥ is the anisotropy of the paramagnetic susceptibility), was larger than the thermal disordering energy ( ~ kBT ). Using a similar process, Ferreira et al. [2] reported that c-axis of RE123 powders was aligned along a parallel direction to applied magnetic field in heat-treatment process (Ha) for RE = Dy, Ho, Nd, Sm and Y , while along a perpendicular direction of Ha for RE = Er, Eu, Tm or Yb. They concluded that the anisotropy of magnetic susceptibility was originated from both the CuO2 plane and the sign of the second-order Stevens factor αJ of the CEF Hamiltonian [3] for RE 3+ ion site. On the other hand, the magnetic field study was carried out not only for powders but also for melt-solidified bulks. De Rango et al. [4] reported that the c-axis of the grown crystals was aligned parallel to the direction of Ha. Further, Awaji et al. [5] showed that the crystallinity of melt-textured YBCO bulks using the seed crystals was improved drastically with increase of Ha. However, one has not understood the alignme
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