Konstantin Nefedev

Concentration Phase Transitions in Two-Sublattice Magnets

Phase transitions and critical phenomena are investigated within a two-sublattice model. In the framework of the random fields with the exchange interaction method the system of equations is obtained. It allows us to establish conditions under which magnetic phase transitions are possible. Critical concentration of ferromagnetic atoms which is necessary for the existence of ferromagnetic, ferrimagnetic and antiferromagnetic phases is determined.

Parallel algorithm for calculation of the nanodot magnetization

The task of the calculation of the dipole-dipole interaction between cobalt nanodot and magnetic tip was formalized and the computing algorithm was implemented in parallel C++ code with using of the MPI standard. The parallelization is fulfilled by means of the dividing total cycle and the passing the part of cycle in one process. Our parallel program allows obtain Magnetic-Force-Microscopy (MFM) images and to establish the magnetic configuration which one correspond given experimental MFM picture.

Large-scale calculation of ferromagnetic spin systems on the pyrochlore lattice

Abstract We perform the high-performance computation of the ferromagnetic Ising model on the pyrochlore lattice. We determine the critical temperature accurately based on the finite-size scaling of the Binder ratio. Comparing with the data on the simple cubic lattice, we argue the universal finite-size scaling. We also calculate the classical XY model and the classical Heisenberg model on the pyrochlore lattice.

Effect of long- and short-range interactions on the thermodynamics of dipolar spin ice

Abstract The thermodynamic properties of dipolar spin ice on square, honeycomb and shakti lattices in the long-range and short-range dipole interaction models are studied. Exact solutions for the density of states, temperature dependencies of heat capacity, and entropy are obtained for these lattices with a finite number of point dipoles by means of complete enumeration. The magnetic susceptibility and average size of the largest low-energy cluster are calculated for square spin ice by means of Wang–Landau and Metropolis methods. We show that the long-range interaction leads to a blurring of the energy spectrum for all considered lattices. The inclusion of the long-range interaction leads to a significant change in the thermodynamic behaviour. An additional peak of heat capacity appears in the case of the honeycomb lattice. The critical temperature shifts in the direction of low or high temperatures; the direction depends on the lattice geometry. The critical temperature of the phase transition of square spin ice in the long-range model with frustrated ground states is obtained with the Wang–Landau and Metropolis methods independently.

Interplay of dilution and magnetic field in the nearest-neighbor spin-ice model on the pyrochlore lattice

We study the magnetic field effects on the diluted spin-ice materials using the replica-exchange Monte Carlo simulation. We observe five plateaus in the magnetization curve of the diluted nearest-neighbor spin-ice model on the pyrochlore lattice when a magnetic field is applied in the [111] direction. This is in contrast to the case of the pure model with two plateaus. The origin of five plateaus is investigated from the spin configuration of two corner-sharing tetrahedra in the case of the diluted model.

Magnetic States and Frustrations of Square Spin Ice in 2D XY Point Dipoles Model

In the discrete 2D XY model of finite number of point dipoles of square spin ice, the order parameter has been calculated exactly as the average size of the maximal cluster. The finite size and edge effects do not allow research the critical behavior of the phase transition for finite systems. It is found that frustrations in spin ice model with long-range interaction are possible only in presence of symmetry for the distribution of magnetic moments.

Approaches to numerical solution of 2D Ising model

Parallel algorithm of partition function calculation of two-dimensional Ising model for systems with a finite number of spins was developed. Within a method of complete enumeration by using MPI technology with subsequent optimization of a parallel code time of calculations was reduced considerably. Partition function was calculated for systems of 16, 25, 36 Ising spins. Based on the obtained results, main thermodynamic and magnetic values dependences (such as heat capacity, magnetic susceptibility, mean square magnetization) for ferromagnetic and antiferromagnetic interactions was investigated. The analysis of a different configurations contribution showed, that states with the minimum energy have essential influence on dependences of thermodynamic values. Comparison with the results obtained by the Wang Landau algorithm was performed.

The magnetic states and Labyrinth ordering in 2D lattices of ising spins with RKKY Interaction

The numerical simulation of 2D simple square lattice of Ising superspins, that interact via the Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange interaction is performed. It is found, that at low temperatures in the researched systems predominantly realized labyrinth domain structure. The modeled image of magnetization distribution does not coincide with the image of energy distribution. Hysteresis loops of the samples and labyrinth domain structures similarly obtained by Monte Carlo method, are also observed in physical experiments.

High Performance Calculation of Magnetic Properties and Simulation of Nonequilibrium Phenomena in Nanofilms

Images of surface topography of ultrathin magnetic films have been used for Monte Carlo simulations in the frame of the ferromagnetic Ising model to study the hysteresis and thermal properties of nanomaterials. For high-performance calculations, a super-scalable parallel algorithm was used for finding the equilibrium configuration. The changing of the distribution of spins on the surface during the reversal of the magnetization and the dynamics of the nanodomain structure of thin magnetic films under the influence of a changing external magnetic field were investigated.