Within the framework of a generalized Ising model, a one-dimensional magnetic of a finite length with free ends is considered. The correlation length exponent, dynamic critical exponent z of the magnet is calculated taking into account the next nearest neighbor interactions and the external field.
The magnetic critical exponent of the two-dimensional axial next-nearest-neighbor Ising model is estimated by means of the finite-size scaling of the order parameter. For a finite system without ordering field, this magnitude is computed from the two leading eigenvectors of the transfer matrix. The ferromagnetic-paramagnetic transition temperatures estimated with this method exhibit a small lattice-size dependence. Moreover, they are in good agreement with those obtained using phenomenological renormalization, and also with analytical approximations. The correlation-length critical exponent is calculated by means of phenomenological renormalization. The results found with these numerical techniques indicate that the ferromagnetic-paramagnetic transition of the model seems to be continuous, in contradiction with a previous result obtained using the persistence-length criterion, which indicated that the transition is always first order. We found that the phase transition is first order only when the ratio of the next-nearest-neighbor and nearest-neighbor interactions equals one-half.
In recent decades, one-dimensional (quasi-one-dimensional) Ising magnetic compounds have been synthesized, on which new promising materials are based. Here, percolation effects are considered in the model of a one-dimensional Ising magnet of finite nanometer size with boundary conditions “dangling ends”. The model takes into account the interaction with an external magnetic field, nearest-site interaction of nodes, interaction of second and third neighbors, as well as four-particle interactions. To model the phase transition, the Metropolis algorithm was used. Two options for the localization of a nonmagnetic impurity are considered: with mobile impurities and with fixed impurities (frozen impurities). For mobile impurities, the Metropolis algorithm contains the possibility of moving non-magnetic nodes along the chain. In the second variant, when the initial configurations are formed, nonmagnetic impurities in the magnet take random constant equiprobable positions. It is shown that the presence of non-magnetic nodes leads to a weakening of the correlation inside the chain and the magnet breaks up into several parts unconnected by magnetic interaction. The fraction of nonmagnetic atoms, in which the magnet is divided into two non-correlating parts, is an analogue to the percolation threshold in the percolation site problem. The percolation radius corresponds to the farthest nonzero interaction. The paper shows the existence of a relationship between the percolation threshold and the dependences of the relaxation time of the ferromagnet — antiferromagnet phase transition and the dynamic critical exponent Z on the fraction of nonmagnetic impurities in the model of a one-dimensional Ising magnet with fixed (frozen) nonmagnetic impurities. In the case of mobile nonmagnetic impurities, the absence of a clear connection between the percolation threshold and the dependences of the relaxation time and the dynamic critical exponent is shown.
In this work the two-dimensional Ising model with nearest- and next-nearest-neighbor interactions is revisited. We obtain the dynamic critical exponents z and θ from short-time Monte Carlo simulations. The dynamic critical exponent z is obtained from the time behavior of the ratio [Formula: see text], whereas the non-universal exponent θ is estimated from the time correlation of the order parameter <M(0)M(t)> ~ tθ, where M(t) is the order parameter at instant t, d is the dimension of the system and <(⋯)> is the average of the quantity (⋯) over different samples. We also obtain the static critical exponents β and ν by investigating the time behavior of the magnetization.
We obtain an approximation of the phase diagram of an Ising model with both nearest-neighbor and next-nearest-neighbor interactions on the square lattice. We use the Fisher zeros of the partition functions of a sequence of finite-sized systems along with various extrapolation methods to obtain phase transition points. In addition, we obtain an approximate value of the correlation length critical exponent nu. Our results are compared to previous results for this system using a wide variety of other approaches.
The critical properties of an antiferromagnetic Ising model on a square lattice with interactions of the next-to-nearest neighbors are investigated by a replica Monte-Carlo method. Using the finite-size scaling theory the static critical exponents of specific heat, ordering parameter, susceptibility, correlation radius as well as the Fisher exponent are calculated. An analysis of data is performed both with and without taking into account a correction to the finite-size scaling. It was found that in the model under consideration the second order phase transition is observed. It is shown that this model belongs to the new class of universality of critical behavior.