Magnetic domain crossover in FePt thin films

We have investigated the crossover in the magnetic domain structure of FePt thin films as a function of film thickness. We have directly observed by magnetic force microscopy (MFM) that at a critical thickness d₂ₑ30 nm the orientation of the magnetization in the magnetic domains changes from in-plane alignment to a system of stripes in which a component perpendicular to the film plane points alternately in opposite directions. The same critical thickness was also estimated from in-plane magnetization vs field measurements. From the MFM images we have also found that the stripe period is an increasing function of the film thickness following a square root law. These data were interpreted with two different models that yield parameters (magnetization, anisotropy, and exchange stiffness) compatible with those determined from magnetization measurements. Films with thicknesses above d₂ₑ show a strong dependence of the domain configuration on the magnetic history. Rotatable anisotropy was found in these samples, with a rotational anisotropy field that became stronger with the increase in film thickness. Bubblelike domains could be also observed when the sample is saturated perpendicular to the film plane. All magnetic measurements as a function of film thickness can be interpreted using the same values of magnetization, anisotropy, and exchange stiffness.