Magneto-optical imaging of columnar YBCO films

The columnar growth of superconducting films is a new approach that provides high density of evenly distributed extended defects on the nanometer scale. This growth can be activated by tightly-packed nanoparticles deposited on substrate prior to the pulse deposition of superconducting film. The magnetization measurements demonstrate that critical current density (Jc) in columnar films is higher than in common epitaxial films. However, in magnetization measurements calculation of Jc is model sensitive. An assumption of ideal critical state frequently proves to be wrong giving incorrect Jc averaged over the entire sample. Missing details of current distribution can be obtained by magneto-optical imaging (MOI) that also allows obtaining local Jc in the samples. In this paper we report MOI study of previously not imaged relatively thick (about one micrometer) columnar YBa2Cu3Ox films in order to clarify if they fit a critical state model and to what extent their properties are modified by the columnar growth. MOI demonstrates a homogeneous penetration of magnetic flux that fits a critical state model. The long-length influence of defects, especially those residing on the edges is less pronounced in columnar than in common epitaxial films. The measurement of the depth of flux front at partial flux penetration allows estimating average low-field Jc, which is in good agreement with extrapolated magnetization measurements. The spatial details of the distribution of magnetic flux provide valuable information for tailoring deposition parameters in order to obtain highest possible value of Jc. MOI could be an ideal tool for measuring Jc when other methods are not available or not appropriate. Using MOI and magnetometry, an important effect of the increase in Jc in aged columnar films has been observed.