1PO2-60 Application of the General Flux Conservation Equations for Different Shape Superconductors in Electromagnetic Measurements

F. Wang 1, X.N. Xu 1, Y.L. Tang 1, L.J. Shen 2, X. Jin 1, and C.C. Lam 2. 1 Department of Physics and National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, China. 2 Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China.

Presenting Author: F. Wang

In the investigation of flux creep dynamics behaviors of high-T_c superconductors, the commonly used methods such as fixed field relaxation, external field sweep and ac response etc. are all based on the flux conservation equations of slab or cylinder samples, which hardly satisfy the accuracy of analysis in practical research. For the convenience of investigation we derive the general flux conservation equation for different shape samples by introducing the general parameter corresponding to different sample geometries. By using it in the three measurements mentioned above we make a detailed discussion on its application in measures and provide the measurement equations of effective pinning potential and electric current density, etc.. They are all intimately associated with a crucial parameter determined by sample shapes, which should be considered for the accuracy of analysis in researches. We listed its different forms corresponding to the sample shapes so that it can be used directly in practical measurements.

1PO2-61 Correpondence between Magnetic Time Relaxation and Field Sweeping Rate Dependence of Magnetic Hysteresis

S.A. Aruna 1, H.M. Shao 1, Y. Rui 1, H.L. Ji 1, X.N. Xu 1, X.X. Yao, X.S. Rong 2, B. Ying 2, and Z.X. Zhao 2. 1 Department of Physics and National Laboratory of Solid State Microstructures, Center for Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing, 210093, P.R. China. 2 National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China.

Presenting Author: S.A. Aruna

The dependence of the magnetization in the flux creep experiments M(t) and field sweep rate of magnetic hysteresis M(dH/dt) are derived and found to correspond with each other in both the first and second order Taylor expansion of the effective activation energy U_eff(J). The agreement of U_eff(J) extracted from the magnetic relaxation and magnetic hysteresis loops of a polycrystalline HgBa₂Ca₂Cu₃O_8+d sample is a strong support of this correspondence. At the end of this paper, the temperature dependence of the magnetic critical current density is also discussed.

1PO2-62 Magnetic relaxation measurements in a detwinned YBa₂Cu₃O_7-x single crystal

E. Moraitakis 1, M. Pissas 1, and A. Bondarenko 2. 1 Institute of Materials Science, NCSR Demokritos, 15310, Aghia Paraskevi, Athens, Greece. 2 Kharkov State University, 310077, Svoboda sq. 4, Kharkov, Ukraine.

Presenting Author: E. Moraitakis

We performed global magnetic relaxation measurements in a detwinned YBa₂Cu₃O_7-x single crystal, in the temperature range T: 10-80 K and for applied fields parallel to c-axis. The data gives an inverse relaxation rate 1/S=U_c/k_T + mln(t/t0) which increases abruptly with the magnetic field and shows a plateau near the observed kink in the global magnetization vs field curves. For higher fields near the "fishtail peak" the analysis of the relaxation data is in agreement with a crossover from elastic to plastic deformation of the flux lattice. The influence of surface barriers which are observed at higher temperatures are discriminated with relaxation measurements of flux entry and exit in the crystal.

1PO2-63 Irreversibility Limits of the Abrikosov and Josephson Flux Dynamics in High-T_c Superconductors.

Valdemar N. Vieira, Job P. da Silva Fo, and Jacob Schaf, Instituto de Física - Universidade Federal do Rio Grande do Sul, CEP 91501-970, Porto Alegre-RS, Brazil

Presenting Author: J. Schaf

The magnetic irrversibility of an high quality YBa₂Cu₃O₇ single crystal, a twinned YBa_1.5Sr_0.5Cu₃O₇ single cristal and an YBa_1.75Sr_0.25Cu₃O₇ polycristalline superconductor was determined in great detail up to 5T. The irreversibility limits Tirr(Hi) of all the samples can be well represented by the power law: H=Ho(1- t)a , where t =T_irr(H)/T_irr(0), H is the applied magnetic field, Ho and T_irr(0) » T_c are fitting parameters and a = 3/2 or 1/2.

While T_irr(H_i) of the pure single crystal is well represented by a unique power law in the whole field range for a =3/2, the T_irr(H_i) of the doped single crystal and the polycrystalline sample follow several well defined regimes. In the very lowest fields T_irr(H_i) obey the power law for a =3/2, a de Almeida-Thouless like line (AT1). For intermediate fields the T_irr(H_i) follow the power law with a =1/2, a Gabay-Toulouse like line. Finally in the major high field region the T_irr(H_i) fall back to the power law with a =3/2, a second de Almeida-Thouless like line (AT₂).

We attribute the a =3/2 power law behavior of the T_irr(H_i) in the pure single crystal and in the high field regions of the granular superconductors to the intragrain Abrikosov flux dynamics, while the low field AT-like and GT-like behaviors of the T_irr(H_i) of the granular superconductors are viewed as signatures of the Josephson flux dynamics.

1PO2-64 Magnetic Flux Creep in YBCO films in the Regime of Critical Fluctuations

Igor P. Krylov, Physics Dept., University of the Western Cape, PB X17, Bellville 7535, South Africa

Presenting Author: I.P. Krylov

Magnetization of YBCO films was investigated using a sensitive SQUID magnetometer in a perpendicular field B<50 G in the vicinity of the critical temperature T_c» 89 K. After zero-field cooling the trapped magnetic flux was generated in the specimen by ramping the applied field to a finite value and then back to zero. The remanent magnetic field of persistent currents circulating in the film was measured as a function of temperature and time. The isothermal time decay of the current density j, determined by the flux creep, followed a power-law relaxation jµ t^-p with the parameter p<<1. In agreement with theory, this power-law was interpreted as a result of logarithmic dependence of the creep activation energy U=U₀ln(j/j_c), with the critical current density, j_c and the value p=kT/U₀. In the range 10 K>T_c-T>1 K, the values U₀/k~10⁴ K demonstrated a weak temperature dependence, but as the temperature was raised still closer to T_c, the curve U₀(T) turned sharply down to zero at T=T_c. This result was interpreted as a crossover to the regime of critical fluctuations, in agreement with the traditional estimate of the critical region ½ T_c-T½ £ GixT_c» 1 K where the Ginsburg number, Gi» 1.5x10^-2 could be evaluated from the known YBCO parameters.

The temperature dependence of the critical current j_c(T) was measured very accurately during the fast warming runs. In the critical region T_c-T£ 1 K, the dependence j_cµ (T_c-T)² was observed in all samples. The sample dependent deviations from the square law were observed only very close to T_c, typically at T_c-T£ 0.1 K. The creep freezing effect permitted to check the accuracy of the sample temperature measurements ~2 mK.

In the critical region the logarithmic dependence U=U₀ln(j/j_c) could be explained by the current-assisted thermal unbinding of vortex-antivortex pairs generated by the critical fluctuations. On this basis the results on U₀(T) and j_c(T) were compared with the critical exponents of the 3D XY model.

1PO2-65 Low Field Flux Dynamics of Various Under-doped Bi-2212 Single Crystals

Y.Y. Hsu, Y.P. Sun, and H.C. Ku, Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan

Presenting Author: Y.Y. Hsu

Magnetic hysteresis M(H,T) and magnetic relaxation M(t) results on various under-doped single crystals Bi₂Sr₂Ca_1-xPr_xCu₂O_8+d are reported (x < 0.5). Superconducting H-T phase diagram with applied field parallel to single crystal c-axis indicates that all H_c1(T), H_1p(T), H_on(T), H_2p(T), and H_irr(T) lines shift to the lower reduced temperature (T/T_c) and lower field region with narrower H_on/H_2p window. For optimum-doped Bi₂Sr₂Ca_0.89Pr_0.11Cu₂O_8+d with T_c = 88 K, the mixed state the anomalous second peak field and its onset field H_2p/H_on varies from 900/800 G at 15 K to 210/85 G at 40 K with a plateau around 20-30 K. Low field magnetic relaxation M(t) data near H_2p/H_on indicates a distinct change of flux creep behavior from collective creep model for H < H_on to vortex glass model for H > H_on. Such a crossover behavior implies a change in vortex coupling mechanism from vortex quasi-lattice to entangled vortex glass. Details of systematic analysis on various under-doped Bi₂Sr₂Ca_1-xPr_xCu₂O_8+d single crystals will be discussed.

*Research supported by NSC of ROC contract NSC89-2112-M007-023 and -051.

1PO2-66 Critical current density and irreversibility behaviors in Re-doped HgBa₂Ca₂Cu_2.9Re_0.1O_y

X.J. Fan, X.F. Sun, X. Zhao, J. Zhang, and X.-G. Li, Structure Research Laboratory, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P.R. China

Presenting Author: X.J. Fan

The magnetic properties of polycrystalline HgBa₂Ca₂Cu_2.9Re_0.1O_y have been investigated as functions of temperature and magnetic field by using a vibrating sample magnetometer. It was found that the variations of the penetrating field H_p can be described using surface pinning effect at T ³ 40K. The critical current J_c follows a power law J_c µ 1/Hⁿ at T £ 20K which shows the dominant effect of bulk pinning, and an exponential law J_c µ exp(-a Hⁿ) at 40K £ T £ 90K which implies that the effect of the surface pinning becomes important. For the irreversibility line, the experimental data can be scaled as H_irr µ T^-1/n with n = 0.48 at T £ 90K and H_irrµ (1-T/T_c)^m with m = 2 at T ³ 100K. The experimental results can be explained in the framework of the vortex creep model in terms of different pinning potential, i.e., the classical Anderson-Kim relation U µ H^-n (1-J/J_c0) at T £ 20K and the collective creep relation U µ H^-nln(J_c0/J) at 40K £ T £ 90K.

1PO2-67 Current Ramping Rate Dependence of the Critical Current in 2H-NbSe₂ Crystals

Z.L. Xiao 1, E.Y. Andrei 1, P. Shuk 2 and M. Greenblatt 2. 1 Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08855. 2 Department of Chemistry, Rutgers University, Piscataway, NJ 08854.

Presenting Author: Z.L. Xiao

The critical current of a field-cooled vortex lattice in pure 2H-NbSe₂ superconducting crystals is found to depend on the ramping rate of the current. By measuring the critical current at current ramping rates of 10^-5 A/s to 5x10² A/s we find that at a fixed temperature and magnetic field the critical current increases with increasing current ramping rate. These experiments show that metastable states, which are missed in the usual slow transport measurements, do exist in the vortex lattice of pure 2H-NbSe₂ superconducting crystals. The initial field-cooled vortex lattice is more disordered than that probed by the usual slow transport measurement. A discussion on the reorganization of a driven vortex lattice including possible nucleation and propagation of ordered regions, formation and growth of moving channels will be presented.

1PO2-68 Irreversibility temperature from magnetic relaxation

Mahesh Chandran, Dept. of Cond. Matt. Phy. & Mat. Sci., Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005

Presenting Author: M. Chandran

1PO2-69 Investigation of the Low Field Irreversibility Line in Bi₂Sr₂CaCu₂O_8+x Thin Films

J.-F. de Marneffe, Y.-Z. Zhang, and R. Deltour, Brussels Free University, Solid State Physics Laboratory, CP 233, Bvd du Triomphe, B-1050 Brussels, Belgium

Presenting Author: J.-F. de Marneffe

Bi₂Sr₂CaCu₂O_8+x epitaxial superconducting (T_c~83 K) thin films have been deposited by Inverted Cylindrical Magnetron (ICM) sputtering on SrTiO₃ single crystal substrates. Their structural characteristics, investigated by X-ray diffraction, Scanning Electron Microscopy and Energy Dispersive X-ray Analysis, indicated that the films were pure, homogeneous, and oriented with their c-axis perpendicular to the substrate surface. Measurements of the Irreversibility Line (IL) under low applied magnetic field (1<H<10⁴ G), indicating the onset of dissipative flux motion, have been made using resistivity and AC susceptibility. The temperature, frequency, AC & DC magnetic field dependence of the peak of the absorptive component of the susceptibility signal could allow us to deduce the temperature, field and current functional dependences of the vortex pinning energy U₀(T,B,J). A comparison is made with measurements of the changes in a-b plane resistivity under magnetic field applied along the c-axis direction, which enable us to deduce the U₀(T,B,J) dependences by a direct method. A U(H)~H^-1/2 dependence, typical of a dislocation-mediated thermally activated flux motion, is found in a magnetic field range of 10³~H~10⁴ G; whereas a transition to higher pinning energy appears below 10³ G, which may be interpreted as a 2D-3D transition in the vortex system.