Magnetic torque of oriented bulk Bi2212 by the shock-loading method
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M. Kikuchi, H. Hikosaka, and Y. Syono Institute ,for Materials Research, Tohoku University, Katahira, Sendai 980, Japan (Received 10 April 1995; accepted 12 June 1995)
The magnetic torque of oriented bulk specimen of Bi2Sr2CaCu20s+8by the shock-loading method was measured under magnetic fields up to 8 kOe in the temperature range from 4.2 to 100 K. The torque amplitude of the shocked specimen was much larger than that of the as-grown bulk specimen measured as a reference. The angular dependence of the torque was reproduced by assuming the Gauss-type distribution for the crystalline orientation. It became clear that the magnetic torque of the shocked specimen showed the high degree of crystalline orientation in which the c-axis is perpendicular to the pellet disk plane and that the flux pinning perpendicular to the pellet disk plane was enhanced by shock-loading.
1. INTRODUCTION
Bi2Sr2CaC~208+6 (Bi2212) has a layered structure and cleaves easily at the BiO plane like mica. Since each grain has a thin flat shape, the crystalline orientation is achieved by shock-loading compression in addition to increasing density.' The optimum parameter to prepare a crystallographically oriented specimen with high density, i.e., shock pressure, packing density of starting pellets, and post-annealing temperature, was investigated precisely.2 It was found that the most effective orientation was achieved under the condition that shock pressure is 8 GPa, packing density of the starting pellet is 67%, and post-annealing temperature is 850 "C.2 By the investigation of the transport and magnetic proprieties in the Bi2212 single crystal^,^-^ it is clear that Bi2212 shows highly anisotropic character originated from the layered structure. It is expected that the oriented bulk specimen show the magnetic anisotropy if the degree of the crystalline orientation is large enough. It is also expected that the shock-loading induces the crystal defects which act as pinning center^.^,^ It is of much interest to study how large enhancement both in anisotropy and pinning force is achieved in a shockoriented specimen. In this report, the enhancement of the magnetic anisotropy and pinning in the shock loading bulk Bi2212 were investigated by the magnetic torque measurements. II. EXPERIMENTAL
Samples in this measurements were prepared as follows. Polycrystalline Bi2212 pellet was synthesized 2444
J. Mater. Res., Vol. 10, No. 10, Oct 1995
http://journals.cambridge.org
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by solid state reaction in air using Bi2O3, SrC03, CaCu3, and CuO. The mixed reagents with stoichiometric metal ratio were pressed and heated at 800 "C for 12 h and then at 870 "C for 12 h. This specimen is called "as-grown bulk". In the next step, it was ground to the size from 44 to 74 p m . After that, the powder was pre-aligned by tapping and pelletized into a disk with the packing density of 67%. Bulk density of the specimen before being shock-loaded is important.2 This specimen is called "tapped". In the last step, shock-loading experiments were carried ou
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