Domain formation near the reorientation transition in perpendicularly magnetized, ultrathin Fe/Ni bilayer films (invited)

Ultrathin films with perpendicular magnetization convert from a single domain state into a multidomain structure as the reorientation phase transition to an in-plane magnetization is approached. Reorientation transitions in magnetic ultrathin films result from the interplay of interfacial magnetic anisotropy, the dipolar interaction, and two-dimensional thermodynamics. These transitions can be driven by changing either the film thickness or temperature. Experimental and theoretical studies of this effect are briefly discussed in the context of the thickness–temperature phase diagram of the reorientation transition. We then describe magnetic susceptibility experiments on ultrathin Fe/Ni(111) bilayers. Our experiments indicate an exponential increase in domain density of a multidomain structure with temperature and identify the region of the thickness–temperature reorientation transition phase diagram where this condensation is most pronounced. The temperature dependence of the domain density agrees quantitatively with theoretical predictions. Films that are slightly too thin to exhibit the reorientation transition with temperature are a special case. They undergo a ferromagnetic-to-paramagnetic transition from the perpendicularly magnetized state and exhibit domain-like behavior many tens of Kelvin above estimates of the Curie temperature. This surprising observation is interpreted using the two-dimensional dipolar Ising model.