Magnetization measurements in epitaxial films of the YBa2Cu3Oy compound

Systematic measurements of magnetization in epitaxial YBa2Cu3Oy (Y-Ba-Cu-O) films have been performed. Y-Ba-Cu-O films have been prepared by coevaporation in an ultrahigh-vacuum system for molecular-beam epitaxy. These epitaxial films are oriented with their c axis perpendicular to the MgO(100) substrates. The dc magnetization of these films was measured by means of a superconducting-quantum-interference-device magnetometer, and the sample size was precisely controlled by means of a micropatterning technique. At low temperatures and low magnetic fields, the zero-field-cooled magnetization showed almost perfect diamagnetism, and the demagnetization effect was observed as the sample size was changed. In the transition region of zero-field-cooled magnetization, the temperature and magnetic-field dependences of the magnetization also showed the sample-size dependence and were consistent with that of the Bean critical-state model, which indicated that there was no granularity in these epitaxial Y-Ba-Cu-O films. The magnetization data were consistent with the flux-density distribution, which was directly observed by means of the magneto-optical Faraday effect. In addition, the field-cooled magnetization was carefully measured, and the susceptibility of thin films in the field-cooled process was estimated. From these experimental results, a quantitative discussion of the magnetization and the observation of granularity characteristics in the oxide superconductor films have become possible under various temperature and magnetic-field conditions