Inverse transition in a two-dimensional dipolar frustrated ferromagnet

Abstract

We show that the mean-field phase diagram of the dipolar frustrated ferromagnet in an external field presents an inverse transition in the field-temperature plane. The presence of this type of transition has recently been observed experimentally in ultrathin films of Fe/Cu(001). We study a coarse-grained model Hamiltonian in two dimensions. The model supports stripe and bubble equilibrium phases, as well as the uniform phase. At variance with common expectations, already in a single-mode approximation, the model shows a sequence of uniform-bubbles-stripes-uniform phase transitions upon lowering the temperature at a fixed external field. Going beyond the single-mode approximation leads to the shrinking of the bubbles phase, which is restricted to a small region near the zero-field critical temperature. Monte Carlo simulations results with a Heisenberg model are consistent with the mean-field results.

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

“…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.…”

“…Note that, up to this point, the model defined is quite general. Previously considered coarse grained models have been mainly of the Ginzburg-Landau type, defined by an expansion in powers of the order parameter, typically up to φ 4 , which limits the validity of results to temperatures near the critical point [5,7]. Instead, in line with the mean field approximation, we consider an entropy density function to be of the form:…”

“…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.…”

“…Subsequent theoretical work analyzed the existence of ISB from a scaling hypothesis [5,6]. Reentrant behavior was shown on a coarse-grained model of the Landau-Ginzburg type [7], although no attempt was made to explain the nature of the reentrance, mainly due to limitations in the very definition of the model, which was not able to capture the low temperature sector of the phase diagram. Recently, * billoni@famaf.unc.edu.ar † cannas@famaf.unc.edu.ar ‡ stariolo@if.uff.br…”

Effects of rigid or adaptive confinement on colloidal self-assembly. Fixed vs. fluctuating number of confined particles

“…Based on the similarity between the two types of self-assembly, we can expect that our results may also concern amphiphilic systems such as surfactants or lipids in water and block copolymers, at least on a qualitative level. Similarly, magnetic systems with competing interactions [29][30][31][32] The H * j contains the pair interactions between the particles at the sites of the M-th box, as well as the interactions between the particles at the sites labeled 3M +1 and 3M +2 (if they exist for given L). …”