Imaging of the grain structure of thin HTS film by a single-layer flat-coil-oscillator test-method (SFCO-technique)

An imaging technique has been created (using a focused $\mathrm{He-Ne}$ laser beam as a probing signal) capable of imaging the grain structure of HTS thin films with $2-3 \ \mu m$ spatial resolution. It is based on detection of an inductance change of a single-layer flat pick-up coil, placed at the face of the specimen. This leads to frequency changes of a stable tunnel diode oscillator. Test device enabled $2D$-mapping of the grain structure of the bridge-shaped $\mathrm{YBaCuO}$ film. Basically, the method is capable of imaging fine peculiarities of normal-metallic to superconductive phase transition and $2D$-current distribution, as well as may identify localized defects in thin HTS-materials with sub-$\mu$ spatial resolution, using non-bolometric response. However, the achieved $2-3 \ \mu m$ resolution of a bolometric nature (in a given device with $3 mm$-size coil) is limited and depends on how narrow is possible to focus the probing beam, while the own spatial resolution of the tested flat-coil technique is better than $0.1 ~\mu m$, and can be improved by $1-2$ orders of the value by reducing pick-up coil size.