We have investigated magnetic flux relaxation at low temperatures in a 90 nm epitaxial coevaporated film on YBa2Cu3O7−δ superconductor. This film is characterized by a high critical current Jc of 3.5 × 1011Am−2 at 4.2 K, and 1.3 × 1010Am−2 at 77 K. Although Jc depends strongly on the applied magnetic field B the magnetic relaxation is only weakly dependent on B. Also the magnetic relaxation does not vanish at zero temperature, indicating that besides a thermally activated process there is another intrinsic process working. ... Read more

Large relaxation effects have been observed in many high Tc superconductors and the time dependence of the magnetization is usually found to follow a logarithmic time decay. This is in agreement with the results of simple Monte Carlo simulations which lead to M(t, T, B) = M(O, T, B) [1 − (kT/U)In(1 + t/τ)], where M(O, T, B) is the magnetization at time t = 0, temperature T and magnetic field B, and τ is a relaxation time related to the attempt frequency for hopping of flux lines from one pinning region to an adjacent one. The activation energy, U, depends on both T and B. The purpose of this paper is to discuss some of the difficulties encountered in measurements of magnetization relaxation effects in high Tc superconductors in general and thin films in particular. First, how information about the energy barriers for flux motion may be extracted from experimental data on M(t, T, B) is discussed and experimental evidence is given for the necessity to introduce a distribution of activation energies to reproduce the observed temperature dependence of the relaxation rate, S = −dlnM/dlnt. Second, by means of Monte Carlo simulations, the influence of an initial partially critical state (where the critical current, Jc, flows only in a thick surface layer of the sample) on the time dependence of the magnetization relaxation is studied. The results of these simulations are compared with the predictions of an approximate analytical treatment of the relaxation problem. Third, it is shown that the initial conditions have a crucial influence on the magnetization relaxation. As an example, the effect is considered of a small overshoot during the application of a magnetic field before measuring M(t, T, B), which leads to remarkable micro-creep behaviour. Finally, some data are presented which imply that magnetization relaxation is still large at low temperatures (down to 1.5 K) in good quality epitaxial thin films and the possibility of flux line tunnelling at low temperatures is discussed. ... Read more

In this paper we present magnetic flux relaxation mea critical current Jc measurements on epitaxial coevaporated films of YBa2Cu3O7-δ superconductor. We measured a critical current of 3.5 ∗ 1011 A/m2 at 4.2 K and 1.3 ∗ 1010 A/m2 at 77 K. Jc depends strongly on the magnetic field B for B applied perpendicularly to the surface. The magnetization relaxation M(t) is logarithmic in time. In contrast to single crystals and polycrystalline samples the normalized magnetization relaxation tends towards a constant value at low temperatures and is weakly field-dependent. ... Read more

Critical currents of YBa2Cu3O7 - δ epitaxial films, obtained by co-evaporation of yttrium, copper and BaF2 in an ultrahigh vacuum system, were measured using resistometric and magnetic torque methods. Typical values of jc $ ̌2 × 1011 A m-2 at 4.2 K and 4 × 1010 A m-2 at 77 K were found. In spite of their good quality these films exhibited large magnetic relaxation even at 4.2 K. Using the results of Monte-Carlo simulations of thermally activated flux motion for the analysis of the relaxation data typical values of 25 ± 3 meV were found for the activation energy U of flux motion at 4.2 K. It is shown that the occurrence of highjc values and small activation energies is not inconsistent in high Tc superconductors because of their short coherence lengths. ... Read more