Mean field and Monte Carlo simulations techniques are applied to study the behavior of a mixed Ising spin model in a square lattice where spins 7/2 are located in alternating sites with spins 3/2. There is an antiferromagnetic interaction between nearest neighbors, ferromagnetic interactions between next-nearest neighbors, crystal fields, and a magnetic external field. The role of the different interactions in the magnetic behavior of the system is explored. It is found that depending on the combination of parameters in the Hamiltonian this system presents multiple hysteresis loops, exchange bias, compensation temperatures, and discontinuous magnetizations. The results are compared with previous studies that do not include the ferromagnetic next-nearest neighbor interactions and found that they can have a strong effect in the magnetic behavior of the system.

We perform Monte Carlo simulations in order to study the magnetic properties of the mixed spin-S = ± 3/2, ± 1/2 and spin-σ = ± 5/2, ± 3/2, ± 1/2 Ising model. The spins are alternated on a square lattice such that S and σ are nearest neighbors. We found that when the Hamiltonian includes antiferromagnetic interactions between the S and σ spins, ferromagnetic interactions between the spins S, and a crystal field, the system presents compensation temperatures in a certain range of the parameters. The compensation temperatures are temperatures below the critical point where the total magnetization is zero, and they have important technological applications. We calculate the finite-temperature phase diagrams of the system. We found that the existence of compensation temperatures depends on the strength of the ferromagnetic interaction between the S spins.

No abstract

No abstract

The magnetic properties of a ferrimagnetic mixed spin‐3/2 and spin‐5/2 Ising model with two crystal fields, in a longitudinal magnetic field, are studied by Monte Carlo simulations. The role of the different interactions in the Hamiltonian is explored. We investigate the thermal variations of the total magnetization and present phase diagrams. Depending on the region of the parameter space, we find some interesting phenomena such as compensation temperatures, and discontinuities in the magnetizations. We find that when an external field is present, the discontinuities are due to a simultaneous reversal of the spins of the sublattices. They signal the change between two different antiferromagnetic orderings, and are basically due to a competition between the different interactions in the Hamiltonian. We plot the dependence of the temperature at which the new phase occurs, for the different parameters in the Hamiltonian. Our results differ radically from earlier results obtained by other authors based on mean field theories. We find compensation temperatures at some particular combinations of parameters. Mean field theory predicts a wide range of parameters where the system presents even several compensation temperatures; in most of these regions we found none. In contrast with the mean field analysis that only looks at the total magnetizations, our study includes an analysis of the behavior of the sublattice magnetizations.

Se determinó la densidad de DL-alanina en soluciones acuosas del líquido iónico trifluorometanosulfonato de 1-Butil-3-metil imidazolio en el intervalo de temperaturas de 283.15 a 313.15 K utilizando un densímetro de tubo vibratorio Anton Paar DMA 5000. Se calcularon los volúmenes molares aparentes, volúmenes molares aparentes a dilución infinita, la segunda derivada del volumen molar parcial a dilución infinita con respecto a la temperatura, el volumen molar parcial de transferencia a dilución infinita y los números de hidratación. Se encontró que los valores de la segunda derivada del volumen molar aparente límite con temperatura son negativos a todas las concentraciones del solvente mixto. Esto indica que se favorecen las interacciones soluto-solvente y que la DL-alanina actúa como un desestabilizador de la estructura del solvente. Palabras clave: propiedades volumétricas, volúmenes molares aparentes, DL-alanina en soluciones acuosas del líquido iónico

In this article, we study the dynamics of a new proposed age-structured population mathematical model driven by a seasonal forcing function that takes into account the variability of the climate. We introduce a generalized force of infection function to study different potential disease outcomes. Using nonlinear analysis tools and differential inequalities theorems, we obtain sufficient conditions that guarantee the existence of a positive periodic solution. Moreover, we provide sufficient conditions that assure the global attractivity of the positive periodic solution. Numerical results corroborate the theoretical results in the sense that the solutions are positive and the periodic solution is a global attractor. This type of models are important, since they take into account the variability of the weather and the impact on some epidemics such as the current COVID-19 pandemic.

No abstract