Inverse transition in the dipolar frustrated Ising ferromagnet: The role of domain walls

Abstract

We present a theoretical study aimed at elucidating the origin of the inverse symmetry-breaking transitionobserved in ultrathin magnetic films with perpendicular anisotropy. We study the behavior of the dipolar frustratedIsing model in a mean field approximation as well as two other models with simple domain walls. By a numericalanalysis we show that the internal degrees of freedom of the domain walls are decisive for the presence of theinverse symmetry-breaking transition. In particular, we show that in a sharp domain wall model the inversetransition is absent. At high temperatures the additional degrees of freedom of the extended domain wallsincrease the entropy of the system leading to a reduction of the free energy of the stripe phase. Upon lowering thetemperature the domain walls become narrow, and the reduction of the number of degrees of freedom associatedwith them manifests in a reduction of entropy which eventually induces an inverse transition to the competinghomogenous phase. We also show that, for a growing external field at constant temperature, the stripe widthgrows strongly when approaching the critical field line and diverges at the transition. These results indicate thatthe inverse transition is a continuous phase transition and that the domain wall profiles and the temperature havelittle effect on the critical behavior of the period of the domain as a function of the applied field.