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

Ultrathin films with perpendicular magnetization convert from a single doma in state into a multidomain structure as the reorientation phase transition to an in-plane magnetization is approached. Reorientation transitions in m agnetic ultrathin films result from the interplay of interfacial magnetic a nisotropy, the dipolar interaction, and two-dimensional thermodynamics. The se transitions can be driven by changing either the film thickness or tempe rature. Experimental and theoretical studies of this effect are briefly dis cussed in the context of the thickness-temperature phase diagram of the reo rientation 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 p hase diagram where this condensation is most pronounced. The temperature de pendence of the domain density agrees quantitatively with theoretical predi ctions. Films that are slightly too thin to exhibit the reorientation trans ition with temperature are a special case. They undergo a ferromagnetic-to- paramagnetic transition from the perpendicularly magnetized state and exhib it domain-like behavior many tens of Kelvin above estimates of the Curie te mperature. This surprising observation is interpreted using the two-dimensi onal dipolar Ising model. (C) 1999 American Institute of Physics. [S0021-89 79(99)78408-2].