Screening technique measurement of ac imaginary susceptibility and critical current density of YBa2Cu3O7−δ films

Abstract

The screening technique was used to study systematically the ac susceptibility of superconducting YBa2Cu3O7−δ thin films in the temperature range of 32–83 K. We used the Fourier transform of the pick-up signal, coming from the secondary coil, to estimate the critical current density. From the experimental data we observed regularities in the imaginary part of ac susceptibility, and confirmed that the flux creep mechanism is dominant.

Study of higher-order harmonics of complex ac susceptibility in YBa2Cu3O7−δ thin films by the mutual inductive method

“…For the detection of harmonic generation we have used the so-called mutual inductive method [5,7,8,9]. In this technique a multiturn excitation coil is pressed against the film and a pickup coil is coaxially placed on the rear of the film to sense the deformed wave.…”

“…On the other hand, we can introduce the temperature dependence of the magnetic susceptibility via B m . If the critical state is present, thus a linear relation between B m and J c , from equation (7), can be assumed [9]. The parameter r is close to 1 whenever T is more than 10 K below T c and ∼ 3.5 for temperatures very close to T c [9].…”

“…Since the pioneer works of Bean [1] the analysis of the harmonics of the complex ac susceptibility χ n = χ n − iχ n has become one of the most common methods used for the characterization of high-temperature superconductors [3,4]. Conventionally, the ac susceptibility technique [2] is used in most physics laboratories; however, the inductive method has shown to be an alternative and highly efficient method not only to study the electromagnetic properties and the vortex dynamics in superconducting thin films but also some important aspects of the models [5,6,7,8,9]. The physical information contained in the harmonics of susceptibility have been extensively studied [7,8,10,11].…”

“…Conventionally, the ac susceptibility technique [2] is used in most physics laboratories; however, the inductive method has shown to be an alternative and highly efficient method not only to study the electromagnetic properties and the vortex dynamics in superconducting thin films but also some important aspects of the models [5,6,7,8,9]. The physical information contained in the harmonics of susceptibility have been extensively studied [7,8,10,11]. The real part of the fundamental susceptibility χ is directly related to the shielding supercurrents (or diamagnetic response) flowing in the film; it gives information on how the magnetization in the sample is evolving; whereas the energy dissipation occurring in the sample is displayed by the imaginary component χ .…”

“…Great efforts have been made to understand the physics implied in these harmonics not only for superconductors in bulk [12,13,14,15,16,17,18] but also for superconducting thin films [9,19]. The third harmonic χ 3 has shown to be an indicative of flux penetration into the sample and has been embraced as a criterion for the determination of the critical current density J c [5,6,7,9]. In a similar fashion, the decrease in the value of higher harmonics has been related to a reduction of the oxygen contents in YB 2 C 3 O 7−δ crystals [16].…”