Publications by authors named "Iliana Apostolova"

Using Green's function theory and a microscopic model, the multiferroic properties of Co4Nb2O9 are investigated theoretically. There are some discrepancies in the discussion of the electric and dielectric behavior of CNO with and without external magnetic fields. We try to clarify them.

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With the help of a microscopic model and Green's function technique, we studied the multiferroic and phonon properties of the recently reported new multiferroic PrFeAlO (PFAO) compound, which belongs to the double perovskite ABB'O family. The magnetization decreases with the increase in temperature and disappears at the ferromagnetic Curie temperature TCFM. The polarization increases with the application of an external magnetic field, indicating strong magnetoelectric coupling and confirming the multiferroic behavior of PFAO.

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In a first theoretical investigation of the multiferroic properties of Pb5Fe3F19 (PFF) and Pb5Cr3F19 (PCF), we analyze their magnetic, ferroelectric, and dielectric characteristics as functions of temperature, magnetic field, and ion doping concentration using a microscopic model and Green's function theory. The temperature-dependent polarization in PFF and PCF shows a distinctive kink at the magnetic Neel temperature TN, which vanishes when an external magnetic field is applied, indicating the multiferroic behavior of these two compounds. Ion doping effectively tunes the properties of PFF and PCF.

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Using a microscopic model and Green's function theory, we have investigated the co-doping effect on ferroelectric KNbO nanoparticles. Let us emphasize that while the doping with transition metal ions at the Nb site leads an increase in the ferromagnetism and a reduction the band gap, it also decreases the ferroelectricity. On the other hand, doping with La or Ba at the K site leads to enhanced polarization, but does not lead to the appearance of ferromagnetism and reduction in the band gap.

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Using a microscopic model and the Green's function theory, the size and co-doping effects on the multiferroic and optical (band gap) properties of BiFeO (BFO) nanoparticles are investigated. The magnetization increases, whereas the band gap energy decreases with decreasing nanoparticle size. The substitution with Co/Mn, Nd/Sm, Ce/Ni, and Cd/Ni is discussed and explained on a microscopic level.

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Using a microscopic model, the temperature dependence of two phonon modes, ω0 = 32 cm and 72 cm, and their damping of the ferroelastic LiCsSO compound, are calculated within Green's function technique. It is observed that the first mode increases whereas the second one decreases with increasing temperature . This different behavior is explained with different sign of the anharmonic spin-phonon interaction constant.

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The film thickness, temperature, substrate and doping dependence of the phonon energy ω and damping γ, as well as the electrical conductivity, of pure and Sr-doped LaMnO thin films near the phase transition temperature TN are investigated using a microscopic model and the Green's function technique. Due to the strong spin-phonon interaction, there appears a kink at TN in the temperature dependence of ω(T) and γ(T). The softening and hardening of the ω = 495 cm (A) and ω = 614 cm (B) modes is explained by the different sign of the anharmonic spin-phonon interaction constant .

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Using a microscopic model and Green's function theory, we calculated the magnetization and band-gap energy in ion-doped LiPO (LPO), where = Fe, Ni, Co, Mn. Ion doping, such as with Nb, Ti, or Al ions at the Li site, induces weak ferromagnetism in LiFePO. Substituting Li with ions of a smaller radius, such as Nb, Ti, or Al, creates compressive strain, resulting in increased exchange interaction constants and a decreased band-gap energy, Eg, in the doped material.

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Electric, dielectric, and optical (band gap) properties of pure multiferroic as well as La- and Ni-doped SrFe12O19 (SFO) (at different sites) are investigated using a microscopic model and Green's function technique. The concentration dependence of the polarization is considered for substitution of rare earths ions on the Sr sites. For a small La ion doping concentration, = 0.

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Using a microscopic model and Green's function theory, we have calculated the band gap energy and the polarization of LiNbO, KNbO, AgNbO, and NaNbO. The effects by substitution of different ions at A or/and B sites for doping concentration = 0-0.1 are studied.

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Based on the proposed microscopic model, we investigate the multiferroic characteristics of VOX (X = Cl, Br, I) monolayers using a Green's function method. The dependence of the microscopic parameters of the ferroelectric system (pseudo-spin arrangement and flipping rate) on the magnitude and sign of the exchange magnetic interaction along the -axis and the value of the Dzyaloshinskii-Moria vector have been investigated and qualitatively explained. The possibility of observing a spin-reorientation transition with a change in the character of spin ordering from antiferromagnetic to ferromagnetic is investigated.

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The magnetic, electric, dielectric, and optical (band gap) properties of ion doped multiferroic KBiFe2O5 (KBFO) have been systematically investigated utilizing a microscopic model and the Green's function theory. Doping with Co at the Fe site and Ru at the Bi site induces changes in magnetization, coercive field, and band gap energy. Specifically, an increase in magnetization is observed, while the coercive field and band gap energy decrease.

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The influence of size and doping effects on the magnetization , phonon ω and band gap energy Eg of MgO nanoparticles is studied using a microscopic model. The room-temperature ferromagnetism is due to surface or/and doping effects in MgO nanoparticles (NPs). The influence of the spin-phonon interaction is discussed.

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The energy gap Eg between the valence and conduction bands is a key characteristic of semiconductors. Semiconductors, such as TiO2, SnO2, and CeO2 have a relatively wide band gap Eg that only allows the material to absorb UV light. Using the s-d microscopic model and the Green's function method, we have shown two possibilities to reduce the band-gap energy Eg-reducing the NP size and/or ion doping with transition metals (Co, Fe, Mn, and Cu) or rare earth (Sm, Tb, and Er) ions.

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The surface, size and ion doping effects on the magnetic, phonon and optical properties of ZnS nanoparticles are studied based on the s-d model including spin-phonon and Coulomb interaction, and using a Green's function theory. The changes of the properties are explained on a microscopic level, due to the different radii between the doping and host ions, which cause different strains-compressive or tensile, and change the exchange interaction constants in our model. The magnetization increases with increasing small transition metal (TM) and rare earth (RE) doping concentration.

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The magnetic, electric, phonon and optical properties of pure and ion-doped orthorhombic YFeO3 nanoparticles are studied for the first time theoretically. The spontaneous magnetization Ms in YFeO3 decreases with decreasing particle size. Ms is also shape dependent.

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