Yazar "Erdem, R." seçeneğine göre listele

Listeleniyor 1 - 5 / 5
  • Küçük Resim Yok
    Öğe
    Exact Analysis of Free Energy Surfaces for the Spin-1 Mean-Field Ising Model in the Microcanonical Formalism Using Gamma Function
    (World Scientific Publ Co Pte Ltd, 2018) Erdem, R.; Yalcin, O.
    Exact free energy values of the spin-1 mean-field Ising model for finite systems that consist of N particles have been calculated in the microcanonical formalism which uses the gamma function. We have constructed the contour maps of the free energy surfaces in the phase space of dipolar (M) and quadrupolar (Q) order parameters for various values of N. From these maps, stable, metastable and unstable states are seen explicitly. We have analyzed these states in comparison with the free energy values obtained by using the approximated expression for the entropy using the Stirling approximation to deal with factorials. Moreover, the relative error between our exact values and the approximate one using the Stirling formula has been calculated and displayed as the contour map in the M - Q phase space.
  • Küçük Resim Yok
    Öğe
    Ferromagnetism of Nanosized Bilayer on Square Lattice Within Ising Model: Static and Dynamic Aspects with Size Effects Using Pair Approximation
    (Polish Acad Sciences Inst Physics, 2021) Gorguluer, O.; Yalcin, O.; Erdem, R.
    In this study, the pair approximation in the Kikuchi version is applied to study ferromagnetism of a nanosized bilayer system on a square lattice using the Ising model. Each layer is divided into the core, core-surface and surface parts as introduced for the monolayers by Ozum et al. (2015). Temperature, magnetic field and interlayer exchange energy variations of the long- and short-range order parameters are investigated. Some second-order phase transitions between ferromagnetic and paramagnetic phases with size effects are observed. By using the path probability method, the flow of the system towards some stable equilibrium states in the ferromagnetic phase is obtained via relaxation curves and flow diagrams. The results of the numerical calculations are in a fairly good agreement with the available theoretical works of other authors.
  • Küçük Resim Yok
    Öğe
    Martensitic and austenitic transformations in core-surface cubic nanoparticles
    (ELSEVIER SCIENCE BV, 2015) Ozum, S.; Yalcin, O.; Erdem, R.; Bayrakdar, H.; Eker, H. N.
    As a continuation of our recently published work, we have used the pair approximation in Kikuchi version to investigate martensitic and austenitic transformations in homogeneous (HM) and composite (CM) cubic nanoparticles (CNPs) based on the Blume-Emery-Griffiths model. A single cubic nanoparticle made of a core surrounded by a surface is considered as shaped in two dimensional (2D) square arrays instead of hexagonal array. From the phase diagrams of HM and CM-CNPs it has been observed that the martensitic-austenitic transformations (MT-AT) occurred. The influence of the exchange coupling and single-ion anisotropy parameters in the model Hamiltonian on the MT-AT is studied and analyzed in comparison with the results for hexagonal nanoparticles. Significant changes of the phase transition points and hysteresis behaviours depending upon the particle structure have been discussed. (C) 2014 Elsevier B.V. All rights reserved.
  • Küçük Resim Yok
    Öğe
    Nanosized Monolayer on Square Lattice within Spin-1 Ising Model: Hysteretic Properties with Odd Interactions
    (Polish Acad Sciences Inst Physics, 2021) Ozum, S.; Kavlak, R.; Erdem, R.; Yalcin, O.
    The hysteretic properties of a nanosized monolayer on the square lattice have been studied based on the spin-1 Ising model, including a single-ion anisotropy (D) and dipolar-quadrupolar (or odd, L) interaction parameters using the pair approximation. The nanosized monolayer is divided into the core, core-surface, and surface parts as introduced in J. Mugn. Magn. Mater. 373, 217 (2015). The dipolar (or magnetization M) and quadrupolar (Q) order parameters are calculated. We have investigated M vs. magnetic field H behaviors for different values of L, temperatures T, and monolayer sizes R. We also observed Q-D, M-D and Q-H hysteresis loops. These results are discussed in relation to other theoretical findings.
  • Küçük Resim Yok
    Öğe
    Static and dynamic properties of two-sublattice spin-crossover systems
    (Elsevier, 2022) Yalcin, O.; Veliu, V.; Ozum, S.; Erdem, R.
    We investigate the static and dynamic properties of spin-crossover systems in the presence of an external magnetic field using two-sublattice Ising-like Hamiltonian. At low temperatures both sublattices are in the LS state, and in the HS state at high temperatures. The new feature of spin-crossover is depicted for our model when we take the non-equivalent sublattices JA =/ JB, the two-step spin-crossover phase transition is obtained with the presence of the hysteresis loop. In order to study the properties of two-step spin-crossover, we use the lowest approximation of the cluster variation method for static properties. The dynamic properties of the system with bilinear interactions are studied by the path probability method. We present high-spin state fraction nHS vs temperature and magnetic field variations for constant values of the degeneracy ratio between high-spin and low -spin states. In the present model, we obtained the relaxation curves of the order parameters SA and SB using different temperature values. Finally, the flow diagram of the order parameters is depicted for different values of rate constant k1 and temperature, which displays the stable, metastable, and unstable properties. Static and dynamic self-consistent equations of the system were performed by using Newton-Raphson and Runge-Kutta methods.