A new technique for measuring the microwave penetration depth in high-T-c superconducting thin films

The electron spin resonance (ESR) technique has been used to obtain the microwave (MW) penetration depth of YBa2Cu3O7 thin films. An ESR-signal-generating marker was placed in between two high-temperature superconducting (HTSC) films to probe the MW field penetrating through the films. Below the superconducting transition temperature (T-c) the HTSC film started to screen the marker inside the sandwich. A meaningfully diminishing ESR signal was reproducibly recorded for various samples with different markers such as paramagnetic diphenyl pierylhydrazy (DPPH), Mn++ impurity in MgO and a ferromagnetic (FM) permalloy film. The temperature dependence of the ESR signal intensity above and below T-c has been studied to deduce the penetration depth from the measured signal intensity. A very rapid change of A just below T-c, slowed down later at lower temperatures and became smoothly changing. The value lambda(ab) = 1020 Angstrom has been measured for c-axis films at 77 K.