Morphological stripe-bubble transition in remanent magnetic domain patterns of Co/Pt multilayer films and its dependence on Co thickness

Abstract

We report a morphological transition in the magnetic domain pattern exhibited by perpendicular anisotropy ferromagnetic ${[\mathrm{Co}/\mathrm{Pt}]}_{50}$ multilayer films at room temperature and remanence. We found that the remanent magnetic domain morphology and the associated domain density, defined as the number of domains of a given magnetization direction per area, strongly depend on the magnetic history. When the magnitude of the previously applied external field approaches a specific value, typically 75--95% of the saturation field, the magnetic pattern, which generally forms a maze of interconnected stripe domains, decays into a shorter stripe pattern, and the domain density increases. We mapped out this morphological transition as a function of the previously applied field magnitude as well as the Co thickness. We found that a Co thickness close to 30 \AA{} yields the highest domain density with the formation of a pure bubble domain state. Three-dimensional micromagnetic simulations confirm the formation of a pure bubble state in that parameter region and allow an estimation of the perpendicular anisotropy (here $2\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.16em}{0ex}}\mathrm{J}/{\mathrm{m}}^{3}$ for an input magnetization of 1080 kA/m), as well as the interpretation of distinct features of the samples' hysteresis loop based on the corresponding domain pattern.