Changes in the elemental composition and microstructure of an YBa$_2$Cu$_3$O$_{7-\delta}$ target during magnetron sputtering

YBa$_2$Cu$_3$O$_{7-\delta}$ high-temperature superconductor targets subjected to ion-plasma sputtering in a magnetron-sputtering system are studied by X-ray diffraction analysis and secondary ion-mass spectrometry. It is found that dc magnetron sputtering is accompanied by diffusive oxygen depletion and restructuring of the target surface caused by radiation and heat effects of bombarding ions. It is assumed that these changes in the target are responsible for a decrease in the growth rate of YBa$_2$Cu$_3$O$_{7-\delta}$ films.