Magnetization relaxation in YBCO films with improved supercurrent transport properties

Abstract

Abstract. The relaxation of the irreversible magnetization in optimally doped YBCO films with natural and artificial pinning centres was measured in zero-field cooling conditions using SQUID magnetometry. The external magnetic field H was oriented along the c axis. An appropriate method for the determination of the characteristic vortex pinning energy from the normalized vortex-creep activation energy is discussed. This is based on the existence of a crossover elastic (collective) vortex creep at low temperatures T − plastic vortex creep at high T, caused by the T dependent macroscopic currents induced in the sample during magnetization measurements. IntroductionYBa 2 Cu 3 O 7−δ (YBCO) still represents one of the best choices for large scale applications of hightemperature superconductors (HTS), such as the fabrication of superconducting coated conductors [1], and the increase of vortex pinning through artificial routes is a field of intense activity [2][3][4][5][6][7]. It was found that BaZrO 3 inclusions in YBCO thin films, for example, significantly enhance vortex pinning for the external magnetic field H parallel to the c axis [2]. The pinning behavior in HTS is widely investigated using standard magnetization relaxation measurements, with a fixed relaxation time window.As known [8], the vortex pinning energy barrier would easily be extracted from the relaxation of the irreversible magnetization M(t), where t is the relaxation time, if the current density J dependence of the vortex-creep activation energy U would be linear [9], U(J) = U 0 (1 − J/J c ), where U 0 is the

Vortex creep in TFA–YBCO nanocomposite films

“…This is clearly evidenced in figure 4(a), where we have represented the elastic-to-plastic transition line, μ H T ( ) 0 cr , for all the samples. This result is in agreement with other works [32,35,36], where T cr was found to increase for films with enhanced pinning properties. In our case, the crossover from the elastic to the plastic regime creep correlates with the increase in the amount of isotropic nanostrain in the nanocomposites, as demonstrated in figure 4(b).…”

On the determination of vortex creep parameters in superconductors using standard magnetization relaxation data

“…T cr decreases with increasing H (see the main panel of figure 2), and for not very thin films T cr ∝ H −1/2 [22]. At the same time, T cr increases for samples with improved pinning properties [23].…”

“…This is not surprising [27], since at T = T cr the creep exponent changes sign, i.e., in the vicinity of T cr it remains close to zero, which is equivalent with a logarithmic U (J ). Here we are interested in the actual U (J ) at T = 25 K, very close to T cr (40 kOe), where U (J ) is certainly logarithmic and U 0 should be [23] U * (T cr ) ≈ U * (25 K) = 310 K (taken from the main panel of figure 2). Thus, U (J, 25 K) from the main panel of figure 4 was multiplied by a factor U * (25 K)/U 0 = 310/293 = 1.06 to get the exact U (J ) at T = 25 K. With the m(t) data set registered at T = 25 K (H = 40 kOe) one obtains U (t).…”

Vortex dynamics driven by AC magnetic field in YBCO thin films with complex pinning structures

“…In order to confirm the value of p, we made m(t) measurements in DC magnetic field, more exactly, we measured the magnetization relaxation rate S=Δln(|m|)/Δln(t) versus temperature T as is shown figure 3. As might be seen for and a plastic creep process similar to nonuniform single vortex creep dominates de flux dynamics [17,18]. In this scenario, it is known that for a magnetic relaxation curve at given H dc and T, U * (J)=U c (J c0 /J) p [10], so the creep exponent p can be obtained from the linear fit of U * versus 1/J in log-log scale which in our case provides p~−1 (see the inset to figure 3).…”