Critical current density and flux creep in epitaxial YBa2Cu3O7−x thin films

Critical current density and flux creep in epitaxial YBa2Cu3O7−x thin films

Critical current density and flux creep in epitaxial YBazCu307_x thin films G.C. Xiong*, F.R. Wang, S.Z. Wang, Q.D. Jiang, J.Y. Li, Z.J. Yin, C.Y. Li ...

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Critical current density and flux creep in epitaxial YBazCu307_x thin films G.C. Xiong*, F.R. Wang, S.Z. Wang, Q.D. Jiang, J.Y. Li, Z.J. Yin, C.Y. Li and D.L. Yin* Department of Physics, Peking University, Beijing 100871, China *CCAST (World Laboratory), PO Box 8730, Beijing, China The superconducting transition and critical current density were studied for epitaxial YBa2Cu307_x thin films as a function of temperature, magnetic field and electric voltage gradient. A linear section of the curve of the logarithm of apparent voltage versus transport current density, In(E)-J, was discovered in a region of low electric voltage gradient. According to the Anderson-Kim flux creep model a linear dependence is expected, as E = Eoexp[ - ( U o - JD)/kBT]. From the linear fits in the set of I n ( E ) - J curves, the value of D was obtained at different temperatures in variable magnetic fields. JD is a direct measurement of the dissipation energy caused by flux creep. The activation energy, Uo, was estimated from these measurements. The behaviour of the parameter D is discussed in terms of the flux creep model.

Keywords: critical current density; flux creep; Y - B e - C u - O compounds; epitaxial thin films

The current carrying capacity of the high Tc superconductor YBa2Cu307_x (YBCO) in a magnetic field is of both scientific and technological interest. The critical current density of a sample is determined by the balance of two opposing forces acting on the magnetic flux lines: the pinning force, Fp, and the Lorentz force, Fl. Two regions of flux line movement can be distinguished: flux creep, when the pinning force dominates; and flux flow, when the Lorentz force dominates ~-3. Thermally activated creep motion of magnetic flux lines inside high Tc oxide superconductors has been noted in many studies 4-10. The corresponding activation energies were found to be ~0.15 eV for Y - B a - C u - O , and 0.2 for Bi compounds. As a result of flux creep in these materials with relatively low activation energies, the critical density of a sample is limited and the resistive transition is broadened in an applied magnetic field. In this paper the transport properties of epitaxial YBCO thin films are analysed under various conditions and then discussed in terms of the flux creep model. The dissipation energy of the thermal flux creep and activation energy were estimated.

YBCO thin films have a single crystalline structure over all the film area. In these experiments epitaxial YBCO thin films perpendicular to the c-axis were used, with a typical Jc of the order of 106 A cm -2 at 77 K. Details of the preparation and characteristics of the epitaxial YBCO thin films have been described earlier tl. The superconducting properties of these thin films were measured using a standard d.c. four probe method. The geometries of the applied magnetic field both parallel and perpendicular to the basal ab planes were used in the experiments. Figure 1 shows the temperature dependence of the resistivity and the broadening of the resistive transition for a sample due to the applied magnetic field. The transitions and critical current densities of the epitaxial YBCO thin films are not nearly as sensitive to the applied magnetic field as are powder samples. This implies high quality films with few weak links.

Results and discussion In the Anderson-Kim flux creep model, ifFp > FI, the flux motion is assumed to creep over the barriers. In this case, the hopping rate is R = R0 exp [ - Uo/kBT ] sinh(F1X/kBT)

Experimental details To obtain high quality YBCO thin films a modified planar d.c. magnetron sputtering system was used. Deposited at high substrate temperatures, the YBCO thin films grow epitaxially on (100) SrTiO3 single crystal substrates. Reflection high energy electron diffraction patterns and electron channelling patterns reveal that the epitaxial 0011 - 2275/90/050448-

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where: R0 = appropriate frequency factor; U0 = activation energy; and X -- average hopping distance for bundles under an average Lorentz force, FI. Expressing the dissipation energy, Ud, as u~ = F , X = J H V X

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from straight lines in the high voltage region. This can be partly attributed to contributions from normal state electrons. From the linear fit in the set of l n ( E ) - J curves, the value of D/kBT was obtained from the slope of the fitted lines. JD is a direct measurement of the dissipation energy caused by thermal flux creep. Values of D for an epitaxial YBCO thin film with a perpendicular magnetic field of 0 and 5.91 T are shown in Figure 3, in which the values of D are plotted as a function of [1 - T/Tc(H)]. It is clear from these plots that the data form essentially straight lines with a slope of - 2 in the low temperature region. In the high temperature region, the value of D is not sensitive to the strength of the applied magnetic field. In the A n d e r s o n - K i m model, the flux lines were supposed to be bound together as flux bundles. When the

Figure 1 Temperature dependence of electrical resistivity of epitaxial YBCO thin film in various m a g n e t i c fields. (a) M a g n e t i c field parallel to basal ab planes: o , H = 0; O, H = 0 . 4 2 "12 V, H = 1.44 ~ V , H = 3 . 3 4 ~ .&, H = 5 . 0 6 ~ /k, H = 6.31 T. (b) Magnetic field perpendicular to basal ab planes: o , H = 0, O, H = 1 T', V , H = 2 T ; , A , H = 3 ~ A , H = 4"E V , H = 5 . 9 1 T

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Critical current density and flux creep: G.C. Xiong et al. flux lines are flexible, the volume of the flux bundle is expected to scale as C~a~b~c, where C is a constant factor and ~a, ~b and ~c are the coherence lengths. With the G i n z b u r g - Landau formula = 0.74~0(1 - t) -1/2 where t = T/Tc. The volume of the flux bundle is expected to be proportional to (1 - t) -2/3. Assuming that only a few of the flux lines can hop over a pinning centre in one period of thermal creep, the hopping distance of the flux bundle will be a multiple of the Ginzburg-Landau coherence length. In this scheme, D = HVX and is expected to be proportional to [ 1 - T/Tc(H)] -2. The experimental value H = 5.91 T yields a value of VX 1.7 x 10 -35 m 4 at low temperatures. To the knowledge of the authors, this is the first documented observation on the "behaviour of the activation volume of the flux bundle and the effective hopping distance of flux creep. Further details about the flux creep will be discussed elsewhere. At zero temperature the thermal flux creep should also be zero, provided Fp < Ft. Thus, we obtain U0(0)= J¢D(O). Using an intrinsic critical current density value of 1 × 10 7 A cm -2, a value of U0 = Ud = 0.37 eV is given for zero field. This is comparable with the values estimated by Palstra and Sun. It is also clear from Figure 3 that the low temperature dissipation energy and the activation energy are related to the applied magnetic field. In conclusion, the current densities of epitaxial YBCO thin films show a linear relationship with the logarithm of the apparent electric voltage. From the linear fit of the In(E) - J curves, the correlation parameter D = HVX was determined. The dissipation energy of the flux creep, the

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activation energy and the activation volume were estimated. As a function of temperature the parameter D exhibits universal behaviour in variable magnetic fields and is proportional to [ 1 - T/Tc(H)] -2 at low temperatures. This phenomenon can be understood by assuming that the flux lines are flexible and that only a few flux lines can hop over the barriers during each period of thermal flux creep at low temperatures.

Acknowledgements The authors would like to thank Professors Z.Z. Gan and Y.D. Dai for helpful discussions and S.G. Wang, X.H. Zeng and Y. Zhang for patterning the films. The work was supported by the National Center for Research and Development on Superconductivity, China.

References 1 2 3 4 5 6

Anderson, P.W. Phys Rev Lett (1962) 9 309 IGm, Y.B. Rev Mod Phys (1964) 36 39 Tinkham, M. Phys Rev Lett (1964) 13 804 Yeshurun, Y. and Malozemoff,A.P. Phys Rev Lett (1988)60 2202 Tinkham, M. Phys Rev Lett (1988) 61 1658 Palstra, T.T.M., Batlogg, B. Schneemeyer,L.F. and Waszczak, J.V. Phys Rev Lett (1988) 61 1662 7 Mannhart, J., Chandhari, P., Dimos, D., Tsuei, C.C. and McGuire, T.R. Phys Rev Lett (1988) 61 2476

8 Sun, J.Z., Char, K., Hahn, M.R., Geballe, T.H. and Kapitulnik, A. Appl Phys Lett (1989) 54 663 9 Palstra, T.T.M., Batlogg, B., van Dover, R.B., Schneemeyer, L.F. and Waszczak, J.V. Appl Phys Lett (1989) 54 763 10 Hettinger, J.D., Swanson, A.G., Skocpol, W.J. and Brooks, J.S. Phys Rev Lett (1989) 62 2044 11 Xiong, G.C. and Wang, S.Z. Appl Phys Lett (1989) 55 (8)