Interests


Complex systems networks and data


      Complex systems exhibit highly nonlinear macroscopic behavior resulting from the structure of interactions among the many microscopic entities that compose them. These systems often exhibit long-range temporal and/or spatial statistical correlations. In the statistical characterization of these systems it is essential to study the type and structure of the interactions. In particular, in many complex systems these structures do not have the symmetries of the regular networks that are prevalent in physics but present different types of disorder and/or topologies, reason for which they are called complex networks. Even though complex systems and networks represent relatively new objects of study, the standard techniques of statistical physics have been successfully adapted to study these new objects both in their structure and dynamics, i.e. the theoretical tool par excellence for their study is statistical physics; both from the conceptual and methodological point of view. Systems that are defined as complex are extremely diverse and come from different areas of science and in most cases require the collection and analysis of large volumes of data .


Publications:





Percolation process in a two-dimensional random network (Delaunay triangulation) due to a centrality-based attack by betweenness. [Almeira et al. Chaos, Solitons and Fractals 153 (2021) 111529].




Stochastic model for describing ball possession time in soccer matches. [Chacoma et al. Physical Review E, 102, 042120, (2020)].

Perovskitas y láminas magnéticas ultradelgadas


      Multiferroic magnetic perovskites are currently widely studied due to their important practical applications as transducers. An important phenomenon that has been observed in such systems, e.g. YFe(1-x)Cr(x)O3, is the reversal of magnetization when the system is cooled with an applied field, and usually a spin reorientation is also observed at even lower temperatures. These materials like many other multiferroics have a perovskite structure with two sublattices one consisting of transition metal cations and another lattice formed by rare earth cations. Below a critical temperature the transition metal ions arrange their magnetic moments in an antiferromagnetic order and at a lower temperature the rare earth cations are usually arranged as well.
      On the other hand, magnetic systems in the form of ultrathin films are important because they allow the study of important concepts in the area of nanomagnetism. These systems show a diversity of magnetic structures due to the competition between short-range interactions and long-range dipole interactions. The observed patterns are of great interest because their phenomenology is quite general as they are present in many other systems that also exhibit competitive interactions. The morphological characteristics of these magnetic structures depend on the relationship between the magnitudes describing these interactions and the temperature. In particular, it is of great interest to know how the transitions between phases occur, which have different patterns associated with them, depending on the variables that characterize the interactions and the temperature.


Publications:






Spin configuration in a low-temperature YFe(0,5)Cr(0,5)O3 perovskite having a weak ferromagnetic component.






Stripe width as a function of anisotropy near the spin reorientation transition. By introducing dislocations the system adjusts the width of the stripes so that it increases with anisotropy.



Magnetic nanoparticles


      Magnetic particles of nanometer size or nanoparticles currently have many fields of application that motivate their experimental and theoretical study. In addition, the study of magnetic nanoparticles is also a starting point to understand the phenomenology of more complex magnetic systems such as: magnetic relaxation, thermal stability of magnetic systems, magnetization processes, etc. In particular, the study of the inversion dynamics of a magnetic nanoparticle is of great importance since it is related to the recording processes in magnetic media and the stability of the stored information. In this sense, Monte Carlo type numerical simulations are of great help to deal with these problems in a good approximation to real systems.



Publications:







Diagram of states with different magnetic orientation in a nanoparticle. To pass from one state to the other the system must overcome an energy barrier.




Trajectory of the total magnetization m of a nanoparticle changing its magnetization state by an applied magnetic field H that is opposite to the initial orientation of the magnetization. This trajectory was obtained by means of a Monte Carlo type dynamics.


Exchange bias phenomenon



      The current interest in the study of the exchange bias phenomenon arises mainly from its importance in the design of spin valves. Although this phenomenon was reported more than 50 years ago by Meiklejohn and Bean, when they studied cobalt particles covered by an oxide layer, at present there are numerous experimental systems where the phenomenon is observed, leaving many aspects unresolved in its theoretical and experimental approach. The phenomenon manifests itself when an inhomogeneous system, such as one formed by a ferromagnetic and an antiferromagnetic phase, is cooled under an applied magnetic field to a temperature below the Neel temperature of the antiferromagnetic material. If a hysteresis loop is then performed, it is generally observed that the loop moves in the opposite direction to the cooling field. Systems in the form of thin films with a ferromagnetic and an antiferromagnetic layer are among the most studied since this is the form in which they find their most relevant technological applications. Some simple analytical models account for the main characteristics of the phenomenon, however, a better approach to the problem requires the use of numerical simulations. In particular, Monte Carlo methods are useful and are often used as a complement to micromagnetic simulations and analytical calculations.


Publicaciones:



Figure 1


Figure 2
Figure 1: Magnetic orientation of different atomic planes close to the interface in a two-film system composed of a ferromagnetic FM and an antiferromagnetic AFM film. The magnetization is rotated by the presence of the applied external field H. In a first approximation the spins of the FM film rotate coherently while a wall is formed in the AFM film.












Figure 2: Hysteresis loops obtained by Monte Carlo simulations of a system as shown in Fig. 1. Due to the coupling of the FM layer with the AFM the hysteresis loop is shifted to the left by a value identified as the bias field, HE.






Amorphous magnetic materials
      Ferromagnetic amorphous materials based on rare earth alloys and transition metals, e.g. TbFe or Dy(x)Gd(1-x)Ni can be described to a good approximation by the random anisotropy model (RAM). In this model the local anisotropy experienced by each rare earth ion varies from site to site due to the coupling of the (non-zero) orbital momentum of this ion with the surrounding crystal field, which varies from site to site because the crystal structure of the material is amorphous. The Heisenberg-type spin model with random site anisotropy was introduced by Harris, Plischke, and Zuckermann and is useful for describing a wide variety of systems. As ions interact by exchange, neighboring ions have similar orientations. However, random anistropy destroys the long-range ferromagnetic order. Due to the disordered anistropy these systems have similar characteristics to spin glasses, e.g. slow dynamics and presence of aging, etc. Thus, many questions about the relaxation dynamics of this model are not satisfactorily answered.


Publications:


Schematic drawing showing the orientation of atomic spins in an amorphous ferromagnetic material.