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

Abstract

We present a theoretical study aimed to elucidate the origin of the inverse symmetry breaking transition observed in ultrathin magnetic films with perpendicular anisotropy. We study the behavior of the dipolar frustrated Ising model in a mean field approximation as well as two other models with simple domain walls. By a numerical analysis we show that the internal degrees of freedom of the domain walls are decisive for the presence of the inverse symmetry breaking transition. In particular, we show that in a sharp domain wall model the inverse transition is absent. At high temperatures the additional degrees of freedom of the extended domain walls increase the entropy of the system leading to a reduction of the free energy of the stripe phase. Upon lowering the temperature the domain walls become narrow and with the corresponding degrees of freedom effectively frozen, which eventually induces an inverse transition to the competing homogenous phase. We also show that, for growing external field at constant temperature, the stripe width grows strongly when approaching the critical field line and diverges at the transition. These results indicate that the inverse transition is a continuous phase transition and that the domain wall profiles as well as the temperature has little effect on the critical behavior of the period of the domain as function of the applied field.

Modulated systems in external fields: Conditions for the presence of reentrant phase diagrams

“…Both the asymmetry and modulation length grow with the field in a way similar to what was observed in the dipolar frustrated Ising model in Ref. [8], and seems to diverge at the critical field where the homogenous phase sets in, as it will be discussed later. Remarkably, both transition lines become almost independent of n max for relatively small values of it (n max = 5) even at very small temperatures.…”

“…Comparing expression (6) with a more microscopic one, e.g. the spectrum of the dipolar frustrated Ising ferromagnet considered in [8,18], it can be shown that a ∝ 1/δ 2 , where δ = J/g, J being the strength of the short range exchange interaction and g the intensity of the competing dipolar interaction. In the modulated sector of the dipolar frustrated Ising model, δ 1 and therefore a takes typically a small value.…”

“…As shown in the figure, in this case at low fixed temperature the model goes through two successive transitions as the external field is raised. At zero and low fields the stripe configurations are the equilibrium phase of the model [7,8,18], but at a critical field the magnetized background triggers an instability towards bubble solutions, which are the equilibrium ones at intermediate fields until a transition to a uniformly magnetized paramagnetic state takes place at a second critical field. As the field is raised the stripes develop a finite magnetization in the form of an asymmetry favoring the direction parallel to the field.…”

“…al. studied a mean field version of the dipolar frustrated Ising ferromagnet (DFIF), and showed that the stripe phase in a field presents reentrant behavior [8]. Furthermore, by comparing the DFIF with two simpler models, the authors concluded that the reentrant behavior in this kind of systems has its origin on the entropy gain from domain walls degrees of freedom of modulated structures.…”

“…Nevertheless, the question of the physical origin of reentrant behavior remains obscured by the inherent complexity of the thermodynamic behavior of disordered systems. Magnetically frustrated systems without quenched disorder, where low temperature phases and ground states display known symmetries, seem to be better candidates for getting a better understanding of ISB [5,7,8]. Besides ultrathin ferromagnetic systems with dipolar frustration, other frustrated systems without quenched disorder showing inverse transitions are, e.g.…”