Local and long-range realizations of a spin-reorientation surface phase transition

The spin-reorientation transition of an ultrathin film from perpendicular to in-plane magnetization is driven by a competition between dipole and anisotropy energies. In situ measurements of the magnetic susceptibility of Fe/2 monolayers (ML) Ni/W(110) films as a function of Fe coverage, made as the films are deposited at constant temperature, show two clear peaks that are described quantitatively as a long-range and a local realization of the transition. In the long-range realization, the susceptibility probes the striped domain pattern that is formed in response to the balance of energetics on a mesoscopic scale. Here the reorientation transition occurs at a noninteger layer thickness. In the local realization, the susceptibility probes the response of small islands with in-plane anisotropy in the third atomic Fe layer that are grown on the second atomic Fe layer, which has perpendicular anisotropy. It is a response to the local finite-size, metastable energetics due to discrete steps in thickness. An excellent quantitative description of the susceptibility data is obtained when both local and long-range aspects of the spin-reorientation transition are included.