J. M. Wesselinowa

On the theory of thin ferroelectric films

A Greens function formalism is used to calculate self-consistently the temperature dependence of the layer polarization and the thickness dependence of the Curie temperature of thin ferroelectric transverse Ising films in the ferroelectric phase, T < T c .

Origin of ferromagnetism in transition metal doped BaTiO3

We have calculated the temperature, magnetic field, and ion doping dependence of the magnetic and electric properties in Fe-doped BaTiO3 using a microscopic model and the Greens function technique. It is shown that the ferromagnetic and multiferroic properties observed at room temperature in Fe doped BaTiO3 could be due to the super exchange interactions between Fe3+ ions in different occupational sites associated with oxygen vacancies and to the exchange coupling of Fe ions with mixed valence, Fe3+ and Fe4+. There is a multiferroic region which depends strongly on the Fe-doping concentration.

Size and doping effects on the coercive field of ferroelectric nanoparticles : A microscopic model

A microscopic model for describing ferroelectric nanoparticles is proposed which allows us to calculate the polarization as a function of an external electric field, the temperature, the defect concentration and the particle size. The interaction of the constituents of the material, arranged in layers, depends on both the coupling strength at the surface and that of defect shells in addition to the bulk values. The analysis is based on an Ising model in a transverse field, modified in such a manner to study the influence of size and doping effects on the hysteresis loop of the nanoparticles. Using a Green’s function technique in real space we find the coercive field, the remanent polarization and the critical temperature which differ significantly from the bulk behavior. Depending on the varying coupling strength due to the kind of doping ions and the surface configuration, the coercive field and the remanent polarization can either increase or decrease in comparison to the bulk behavior. The theoretical results are compared with a variety of different experimental data.

Theoretical study of multiferroic BiFeO3 nanoparticles

Multiferroics, which exhibit both ferroelectricity and ferromagnetism, are currently under intensive investigation. Their spontaneous polarization and saturation magnetization are very low in comparison to many standard ferroelectrics and ferromagnets. In order to explain the experimentally observed enhanced polarization and magnetization in multiferroic nanoparticles, we investigate the influence of surface and particle size on ferromagnetic and ferroelectric properties. The studies are based on two microscopic models: the modified Heisenberg and the transverse Ising model and a multiferroic coupling term. A Green’s function technique beyond the random phase approximation allows the calculation of static and dynamic properties in dependence of temperature, particle size, and different model parameters. It is demonstrated that magnetization, polarization, phase transition temperatures, spin-wave energies, and their damping are very sensitive to the exchange interaction constants on the surface and to the ...

Size, anisotropy and doping effects on the coercive field of ferromagnetic nanoparticles

The influence of size, anisotropy and doping effects on the hysteresis loop of ferromagnetic nanoparticles is studied, based on the modified Heisenberg model. A Greens function technique in real space allows the calculation of the dependence of the magnetization on the temperature, magnetic field, anisotropy, defects and particle size. It is demonstrated that the coercive field H(c) is very sensitive to the surface single-ion anisotropy, and to the exchange interaction constant on the surface J(s) and in the defect shells J(d). With respect to the strong surface single-site anisotropy D(s), we observe at small particle size, N = 4 shells, a maximum in the size dependence of the coercive field, whereas for the small surface anisotropy there is no maximum. Taking into account that J can be different in the defect shells compared to the case without defects, we have obtained for the first time that the coercive field H(c), the permanent magnetization M(r) and the Curie temperature T(C) can increase or decrease for different kinds of doping ions. The dependence on the particle size is discussed, too. The results are in accordance with the experimental data.

MO.Fe2O3 nanoparticles for self-controlled magnetic hyperthermia

Using a model Hamiltonian and the Green’s function technique for the Zn doped Mn-ferrite, Mn1-xZnxO.Fe2O3, and the Gd doped Zn-ferrite, ZnGdxFe(2-x)O4, nanoparticles of different compositions x were studied. The phase transition temperature, TC, and the coercive field, Hc, for different samples dependent upon composition, particle size, and shape were investigated. An attempt was made to enhance or to lower the TC of the nanoparticles to the optimum temperature required in magnetic hyperthermia (42–43°C).

A possibility to obtain room temperature ferromagnetism by transition metal doping of ZnO nanoparticles

Based on the s-d model and using a Green’s function technique, we have studied the influence of transition metal doping effects on different properties such as magnetization M, Curie temperature TC, and coercive field Hc of ZnO nanoparticles. We have shown that the experimentally obtained room temperature ferromagnetism is an intrinsic property and can be due to doping effects in ZnO nanoparticles. In dependence of the radii and the magnetic anisotropy of the dopants, we obtain a decrease or increase in M and Hc with increase in the Ni, Cu, Fe, Mn, V, and Co ion concentration.

Anharmonic effects in ferromagnetic semiconductors

A Green function technique is used to study the anharmonic spin - phonon and phonon - phonon interaction effects on optical phonon modes and spin - wave excitations in ferromagnetic semiconductors. The cubic spinels have been investigated because the magnetostriction of these compounds is small and the direct contribution of spin ordering to the phonon modes can be clearly observed. The phonon and spin-wave energy and damping are evaluated for the first time beyond the random-phase approximation. The temperature dependence of these quantities is discussed and is found to be in good agreement with the experimental data.

Size and anisotropy effects on static and dynamic properties of ferromagnetic nanoparticles

Based on the modified Heisenberg model we analyse the influence of size and anisotropy effects on static and dynamic properties of ferromagnetic nanoparticles. A Greens function technique in real space enables us to calculate the excitation energy and its damping as well as the magnetization depending on the temperature and the size of the particles. The critical temperature is also determined by the size of the particles. With decreasing particle size the spin-excitation energy can decrease or increase for different surface exchange interaction constants, whereas the damping always increases. Additionally, we consider the influence of surface anisotropy and external magnetic field on the excitation spectrum. The theoretical results are in reasonable accordance with experimental data.

Dynamical Properties of Thin Ferroelectric Films Described by the Transverse Ising Model

A Greens function formalism is used to calculate the temperature dependence of the soft modes of thin ferroelectric transverse Ising films including the effects of damping. It is shown that the soft-mode frequencies of thin ferroelectric films are smaller, whereas the damping effects are larger in comparison to the bulk.