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Diaz-Mendez R.,Nanophysics Group | Diaz-Mendez R.,University of Habana | Mendoza-Coto A.,University of Habana | Mulet R.,University of Habana | And 2 more authors.
European Physical Journal B | Year: 2011

We study the Langevin dynamics of a ferromagnetic Ginzburg-Landau Hamiltonian with a competing long-range repulsive term in the presence of an external magnetic field. The model is analytically solved within the self consistent Hartree approximation for two different initial conditions: disordered or zero field cooled (ZFC), and fully magnetized or field cooled (FC). To test the predictions of the approximation we develop a suitable numerical scheme to ensure the isotropic nature of the interactions. Both the analytical approach and the numerical simulations of two-dimensional finite systems confirm a simple aging scenario at zero temperature and zero field. At zero temperature a critical field h c is found below which the initial conditions are relevant for the long time dynamics of the system. For h < h c a logarithmic growth of modulated domains is found in the numerical simulations but this behavior is not captured by the analytical approach which predicts a t 1/2 growth law at T = 0. © 2011 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

Diaz-Mendez R.,Nanophysics Group | Diaz-Mendez R.,University of Habana | Mulet R.,University of Habana
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We explore the equilibrium properties of a two-dimensional Ising spin model with short-range exchange and long-range dipolar interactions as a function of the applied magnetic field H. The model is studied through extensive Monte Carlo simulations that show the existence of many modulated phases with long-range orientational order for a wide range of fields. These phases are characterized by different wave vectors that change discontinuously with the magnetic field. In particular, the emergence of novel anharmonic phases that keep the orientational order but are characterized by several wave vectors is studied in detail. We provide numerical evidence supporting the existence of first-order transitions between modulated phases. At higher fields our results suggest a Kosterliz-Thouless scenario for the transition from a bubble to a ferromagnetic phase. © 2010 The American Physical Society.

Mendoza-Coto A.,Federal University of Rio Grande do Sul | Diaz-Mendez R.,Nanophysics Group | Diaz-Mendez R.,University of Habana
Journal of Magnetism and Magnetic Materials | Year: 2013

We solve the Langevin dynamics of a continuum model with a spherical constraint, considering a ferromagnetic exchange and a long-range antiferromagnetic interaction. Analytical results within the Hartree approximation show an equivalence in the form of spatial and auto-correlation functions in the long time regime between this model and the recently studied Ginzburg-Landau frustrated model. The low-temperature behavior is discussed in the context of glassy dynamics. The emergence of interesting features regarding the establishment of the saturated phase is also analyzed in the view of recent literature. © 2013 Elsevier B.V. All rights reserved.

Garcia-Zaldivar O.,University of Habana | Diaz-Mendez R.,Nanophysics Group | Diaz-Mendez R.,University of Habana | Calderon-Pinar F.,University of Habana | Pelaiz-Barranco A.,University of Habana
Physica Status Solidi (B) Basic Research | Year: 2012

The behavior of the dielectric response in relaxor ferroelectric materials is studied by means of Monte Carlo simulations of a simple two-dimensional (2D) generic model. It is showed that the shifting with the frequency of the peak of the temperature dependence of the dielectric permittivity ε can be obtained just by considering a randomly distributed pinning anisotropy for the out-plane projection of the polar nanoregions. It is also showed that the consideration of a weak long-range dipolar interaction between nanodomains continuously drives the system to a non-relaxor ferroelectric state in which a single peak is observed for the dielectric permittivity. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Diaz-Mendez R.,Nanophysics Group | Diaz-Mendez R.,University of Habana | Mulet R.,University of Habana
Journal of Magnetism and Magnetic Materials | Year: 2012

The dynamical properties of a 2D Heisenberg model with dipolar interactions and perpendicular anisotropy are studied using Monte Carlo simulations in two different ordered regions of the equilibrium phase diagram. We find a temperature defining a dynamical transition below which the relaxation suddenly slows down and the system apart from the typical Arrhenius relaxation to a VogelFulcherTamann law. This anomalous behavior is observed in the scaling of the magnetic relaxation and may eventually lead to a freezing of the system. Through the analysis of the domain structures we explain this behavior in terms of the domains dynamics. Moreover, we calculate the energy barriers distribution obtained from the data of the magnetic viscosity. Its shape supports our comprehension of both, the VogelFulcherTamann dynamical slowing down and the freezing mechanism. © 2011 Elsevier B.V. All rights reserved.

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