Magnetic domains in thin films with perpendicular anisotropy: An extensive study

Thin films of tetragonal ordered alloys grown by molecular beam epitaxy provide a unique template for the study of magnetic configurations in thin films with perpendicular anisotropy. These configurations have been investigated by magnetic force microscopy over a wide thickness range, for large and weak perpendicular anisotropies, in both equilibrium and out of equilibrium states. FePd samples ordered into the tetragonal L1(0) structure have been obtained by simultaneous evaporation of Fe and Pd on Pd(100). These samples exhibit large perpendicular anisotropy. Magnetic domains have been imaged as a function of increasing thickness from 1.4 to 100 nm. Both an exponential decrease of the domain size at low thickness and a slow increase at higher thickness have been observed, allowing quantitative confirmation of the theoretical evolution expected. Due to this exponential decrease of their equilibrium size, the domains appearing at the beginning of growth exhibit a large size, well above the equilibrium one when the layer thickness is thereafter increased. The growth of the layer then leads to highly out of equilibrium magnetic configurations, where the domain shape evolves through wall undulation at low thickness and thereafter through a fingering instability. This fingering instability preserves large parts of the initial domains, allowing a quantitative estimation of the strain to which they are submitted. Samples grown with a low chemical order exhibit weak perpendicular anisotropy. The magnetization then lies into the plane below a critical thickness. Above this thickness, the images confirm the square root dependence of the period of the stripe structure upon thickness. Stripes as small as 37 nm have been observed. (C) 1999 American Institute of Physics. [S0021-8979(99)28908-6].