Coupling-induced reorientation phase transitions in ultrathin Fe/Gd films

A phenomenological explanation for the reorientation phase transitions in an Fe/Gd ultrathin film system on the basis of Landau's theory of phase transitions is proposed. We model the film as a strongly coupled bilayer-like system consisting of the surface Fe overlayers and the interfacial Gd layer(s) below them. The total free energy of the system is accordingly obtained and the relevant phases and the order of the phase transitions involved are thus determined. Qualitative accordance between the theory and experiments is obtained. An alternative mechanism is proposed that attributes primarily the observed first-order phase transition in the system to the strong coupling between the Fe and the Gd film and its induced vertical magnetization component of the latter film. Competition between the antiferromagnetic coupling and the anisotropy energy of the Fe-rich ultrathin film is responsible for the other continuous reorientation. The effects of an applied external field including several field-induced first- and second-order phase transitions are predicted for experimental verification of the theory.