Comparison of Flux Dynamics in Two Samples of YBa 2 Cu 3 O 7 with Different Pinning
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COMPARISON OF FLUX DYNAMICS IN TWO SAMPLES OF YBa 2 Cu 3 07 WITH DIFFERENT PINNING M. Turchinskaya, L.H. Bennett, L.J. Swartzendruber, M. Hill, and J.E. Blendell, and K. Sawano* NIST, Gaithersburg, MD, 20899, *Nippon Steel Corp., Kawasaki 211, Japan.
A.
Roitburd, C.K. Chiang,
ABSTRACT The kinetics of magnetization in two samples of YBa 2 Cu3 07, prepared by different methods, are studied. Magnetization vs. time data were obtained at various temperatures for a number of applied fields. The data display (after some short period of time) logarithmic behavior and (after a sufficiently long time) exponential behavior. The pinning parameter "activation volume" was determined from the experimental data for both samples.
INTRODUCTION We have investigated the pinning by measuring the kinetics of magnetization in two crystalline samples of YBa 2 Cu 3 07 prepared by different processes. One sample was produced by a quench and melt growth process (QMG) [1]. The other sample was sintered in the solid state (SS) [2]. Previous investigations [31 showed, that the width of hysteresis loops of samples prepared by the QMG process depended linearly on size. Hence, QMG-type samples can be considered as bulk superconductors without weak links [1]. SStype samples are assemblies of grains with large numbers of weak links at the grain boundaries [4]. SAMPLE CHARACTERISTICS The QMG sample was cut from a single grain (containing subgrains) with the c direction generally in the plane perpendicular to its long axis. Its dimensions are 0.9 x 0.9 x 5.1 mm. There are fine Y2 BaCuO5 particles within the YBa 2 Cu 3 0 7 matrix [1]. The SS sample has a uniform grain structure with a grain size less than 5 pm. Its dimensions are 2.3 x 1.7 x 5.8 mm. The ac susceptibility of the two samples is almost identical. Fig. 1 shows sharp transition for both samples at 93 K, the same transition widths (10%-90%) of about 2 K and nearly a 100% Meissner effect below Tc. However, the nonequilibrium magnetic properties are quite different, as seen both by the differing value of the peak in X" as well as by the shape of the 77 K hysteresis loops (Fig. 2). SCHEME OF EXPERIMENT The samples were cooled to the measuring temperature in zero magnetic field. The magnetic field was then applied and the magnetization, M(t), was measured as a function of time using either a vibrating sample magnetometer (VSM) or a SQUID system. To allow the field to stabilize, times less than 20 s for the VSM and less than 20 m for the SQUID were not measured. THEORETICAL CONSIDERATIONS After some time which is necessary for the critical state to be established, the creep of magnetization can be described as a thermoactivated motion of vortices to the center of the sample [5]. The kinetics of M(t) are
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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connected primarily with the velocity of a bundle of vortices, in the form: V =
which we take
(1)
Voexp(-U/kT).[2sinh(dB/dr.a/kT)]
where U is the activation energy of a vortex bundle, V0 is its initial velocity (
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