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Öğe Hysteretic behavior of quadrupolar ordering in a 2D magnetic spin-1 Ising nanoparticle(Elsevier, 2021) Erdem, Riza; Yalcin, Orhan; Ozum, Songul; Sahin, Ayse; Demirer, NazliPair approximation technique has been used to study the quadrupolar ordering properties for the 2D magnetic spin-1 Ising nanoparticles, consisting of core and surface parts with an interface coupling. Similar to the magnetic hysteresis, it has been shown that the quadrupole order parameter (Q) as a function of single-ion anisotropy (D) has a 'hysteresis' character which is generally called quadrupole (or QD) hysteresis. The observed QD loops strongly depend on temperature (T), external magnetic field (H) and biquadratic exchange interaction (K). Shifted QD loops with an asymmetry are found when H and K values are increased. These behaviors are also discussed in relation to other theoretical findings.Öğe Hysteretic dipolar and quadrupolar properties of the spin-1 Blume-Emery-Griffiths model under pair approximation(World Scientific Publ Co Pte Ltd, 2021) Ozum, Songul; Erdem, Riza; Yalcin, Orhan; Gorguluer, OmerAs a continuation of previously published work [A. Erdinc and M. Keskin, Int. J. Mod. Phys. B 18, 1603 (2004), doi:10.1142/S0217979204024823] the hysteretic properties of magnetization (m) and quadrupolar moment (q) have been investigated for the spin-1 Blume-Emery-Griffiths model using pair approximation. The magnetic field (h) dependence of m is produced for different values of the lattice coordination number (z), biquadratic interaction (k) and single-ion anisotropy (d). The (m,h) loops show different and novelty properties when d < 0. We also observed the hysteresis cycles in the (q,d), (q,h) and (m,d) planes. These curves strongly depend on biquadratic interaction k. Our results are found to be in good agreement with the other theoretical findings.Öğe Nonequivalent Antiferromagnetically Coupled Sublattices Induce Two-Step Spin-Crossover Transitions: Equilibrium and Nonequilibrium Aspects(Mdpi, 2024) Veliu, Valon; Yalcin, Orhan; Ozum, Songul; Erdem, RizaAs a continuation to the previously published work (Yal & ccedil;& imath;n et al. (2022)), we investigate the equilibrium and nonequilibrium properties of the spin-crossover systems, with a specific focus on the nonequivalent sublattice, and compare these properties with those of the equivalent sublattices. We used the lowest approximation of the cluster variation method (LACVM) to derive the static equations for the order parameters of the two sublattices and determine high-spin fraction in relation to temperature and external magnetic field in a spin-crossover system. At a low temperature, the transition from stable high-spin (HS) state where nHS=1 occurs in the plateau region, where nHS=0.5 for nonequivalent sublattices. The order parameters for non-equivalent sublattices exhibit different states at the transition temperature. Also, we study the nonequilibrium properties of the order parameters and high-spin fraction using the path probability method (PPM). With the current model, we obtain and analyze the relaxation curves for the order parameters Sa, Sb, and high-spin fraction. These curves demonstrate the existence of bistability at low temperatures. At the end of this study, we present the flow diagram that shows the order parameters for different temperature values. The diagram exhibits states that are stable, metastable, and unstable.Öğe Origin of the martensitic and austenitic phase transition in core-surface smart nanoparticles with size effects and hysteretic splitting(AMER INST PHYSICS, 2014) Yalcin, Orhan; Erdem, Riza; Ozum, SongulBlume-Emery-Griffiths model is used to study the effects of the bilinear and biquadratic exchange interactions as well as crystal field interaction on the magnetic properties of core-surface smart nanoparticles. From the minimization of the free energy in pair approximation in Kikuchi version, a set of equations for the bond variables and magnetization are derived. Based on the numerical solutions of these equations, magnetization and hysteresis curves are obtained. Besides the first-and second-order phase transitions, martensitic and austenitic phase regions are observed in the phase diagrams of homogeneous and composite nanoparticles and the origin of martensitic transitions (MT)-austenitic transitions (AT) is investigated. It is found that MT-AT occurred for a nonzero biquadratic exchange parameter. On the other hand, nonzero single-ion anisotropy caused the hysteretic splitting in core-surface type nanoparticles. Martensitic and austenitic results of nanoparticles agree with the experimental results in literature. (c) 2014 AIP Publishing LLC.Öğe Origins of the Exchange-Bias Phenomenology, Coercivity Enhancement, and Asymmetric Hysteretic Shearing in Core-Surface Smart Nanoparticles(HINDAWI LTD, 2016) Erdem, Riza; Yalcin, Orhan; Ozum, Songul; Ciftci, NazireWe have used a spin-1 Ising model Hamiltonian with dipolar (bilinear, J), quadrupolar (biquadratic, K), and dipolar-quadrupolar (odd, L) interactions in pair approximation to investigate the exchange-bias (EB), coercive field, and asymmetric hysteretic shearing properties peculiar to core/surface (C/S) composite nanoparticles (NPs). Shifted hysteresis loops with an asymmetry and coercivity enhancement are observed only in the presence of the odd interaction term in the Hamiltonian expression and their magnitudes show strong dependence on the value of L. The observed coercivity and EB in C/S NPs originated from nonzero odd coupling energies and their dependence on temperature (T) and particle size (R) are also discussed in relation to experimental findings.Öğe Spin-lattice relaxation within a dimerized Ising chain in a magnetic field(AMER INST PHYSICS, 2014) Erdem, Riza; Gulpinar, Gul; Yalcin, Orhan; Pawlak, AndrzejA qualitative study of the spin-lattice relaxation within a dimerized Ising chain in a magnetic field is presented. We have first determined the time dependence of the deviation of the lattice distortion parameter delta Delta from the equilibrium state within framework of a technique combining the statistical equilibrium theory based on the transfer matrix method and the linear theory of irreversible thermodynamics. We have shown that the time dependence of the lattice distortion parameter is characterized by a single time constant (tau) which diverges around the critical point in both dimerized (Delta not equal 0) and uniform (Delta = 0) phase regions. When the temperature and magnetic field are fixed to certain values, the time tau depends only on exchange coupling between the spins. It is a characteristic time associated with the long wavelength fluctuations of distortion. We have also taken into account the effects of spatial fluctuations on the relaxation time using the full Landau-Ginzburg free energy functional. We have found an explicit expression for the relaxation time as a function of temperature, coupling constant and wave vector (q) and shown that the critical mode corresponds to the case q = 0. Finally, our results are found to be in good qualitative agreement with the results obtained in recent experimental study on synchrotron x-ray scattering and muon spin relaxation in diluted material Cu1-yMgyGeO3 where the composition y is very close to 0.0209. These results can be considered as natural extensions of some previous works on static aspects of the problem. (C) 2014 AIP Publishing LLC.Öğe The phase diagrams with influence of biquadratic exchange coupling on martensitic-austenitic transformations for core-surface nanoparticles(ELSEVIER SCIENCE BV, 2015) Yalcin, Orhan; Erdem, Riza; Ozum, Songul; Demir, ZaferRecently, origin of the martensite-austenite transitions in core-surface type magnetic nanoparticles has been investigated theoretically and it has been indicated that repulsive biquadratic exchange coupling (K < 0.0) causes the coexisting martensite and austenite phases. In the present paper, the phase diagrams of homogeneous and composite nanoparticles in the k(B)T/J(0)-D/J(0) plane are studied for the presence and absence of attractive biquadratic exchange interaction in addition to repulsive biquadratic exchange interaction. Significant changes in the phase diagram points are discussed in the presence of martensitic and austenitic transformations. Four regions in the phase diagrams are found as second-order, martensitic-austenitic, T-cid and first-order phase transition regimes. (C) 2015 Elsevier B.V. All rights reserved.
