Domain structure of epitaxial Co films with perpendicular anisotropy

Epitaxial hcp cobalt films with pronounced c-axis texture have been prepared by pulsed lased deposition either directly onto Al₂O₃(0001) single-crystal substrates or with an intermediate ruthenium buffer layer. The crystal structure and epitaxial growth relation was studied by x-ray diffraction, pole figure measurements, and reciprocal-space mapping. Detailed vibrating-sample magnetometer analysis shows that the perpendicular anisotropy of these highly textured Co films reaches the magnetocrystalline anisotropy of hcp-Co single-crystal material. Films were prepared with thickness t of 20<t<100 nm to study the crossover from in-plane magnetization to out-of-plane magnetization in detail. The analysis of the periodic domain pattern observed by magnetic force microscopy allows us to determine the critical minimum thickness below which the domains adopt a pure in-plane orientation. Above the critical thickness the width of the stripe domains is evaluated as a function of the film thickness and compared with domain theory. Especially the discrepancies at smallest film thicknesses show that the system is in an intermediate state between in-plane and out-of-plane domains, which is not described by existing analytical domain models.