A.T. Apostolov

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.

Temperature and layer number dependence of the G and 2D phonon energy and damping in graphene

We have studied the temperature and size dependence of the G and 2D phonon modes in graphene. It is shown that in a graphene monolayer the phonon energy decreases whereas the phonon damping increases with increasing temperature. The electron-phonon interaction leads to hardening whereas the fourth-order anharmonic phonon-phonon processes lead to softening of the phonon energy with increasing temperature. We have shown that the electron-phonon interaction plays an important role also by the dispersion dependence of the phonon G mode, by the observation of the Kohn anomaly. The G mode frequency decreases and damping increases, whereas the 2D phonon frequency and damping increase with increasing layer number. The temperature and size effects of the 2D mode are much stronger than those of the G mode.

Theoretical study of the phonon spectra of multiferroic BiFeO3 nanoparticles

The phonon properties of multiferroic BiFeO(3) (BFO) nanoparticles are studied using a Greens function technique on the basis of the Heisenberg and the transverse Ising models, taking into account anharmonic spin-phonon and phonon-phonon interaction terms. The phonon spectrum is obtained for different exchange, magnetoelectric, and spin-phonon interaction constants. The influence of temperature, surface and size effects on the phonon energy and damping is discussed. The phonon energy and damping in BFO nanoparticles are greater in comparison to those in bulk BFO. The strong spin-phonon interactions lead to anomalies in the phonon spectrum around the magnetic and ferroelectric phase transitions. The influence of an applied magnetic field is studied, too. The predictions are consistent with experimental results.

Microscopic approach to the magnetoelectric coupling in RCrO3

A microscopic model is proposed to describing the multiferroic properties in RCrO3, where R is the magnetic rare earth ion. Using the Green’s function theory, the weak ferromagnetism and the coercive field are calculated by a balance between the Dzyaloshinskii–Moriya interaction (DMI), the single-ion anisotropy and the exchange interaction. We have discussed the magnetic rotational spin-reorientation (SR) transition between Γ4 and Γ2 phases in SmCrO3 and the abrupt one between Γ4 and Γ1 in ErCrO3 calculating the energies in the corresponding phases. The type of the phase transition in RCrO3 is determined by the sign of the second magnetic anisotropy constant. In order to investigate the origin of the extraordinary ferroelectricity in RCrO3, we have studied the different contributions in the polarization due to the antisymmetric exchange DMI and the magnetostriction arising from the Cr-ordering. It is shown that the polarization is due to the interaction between the magnetic R- and Cr-ions. The influence o...

Room temperature ferromagnetism in pure and ion-doped SnO2 nanoparticles

Using a microscopic model taking into account the spin–phonon interactions we have studied the magnetic properties of pure and ion-doped SnO2 nanoparticles (NPs). The magnetization M in pure SnO2 NPs is due to surface oxygen vacancies. By doping with magnetic Co ion we observe a maximum in M for small Co-concentration, x = 1%, whereas for nonmagnetic Cu ion M increases with x. By Co-doping there is a local distribution for small Co-concentration, whereas by Cu this is not the case. It is shown that there is a strong connection between the lattice and M. The results are in good agreement with the experimental data.

Influence of spin–phonon interactions and spin-reorientation transitions on the phonon properties of RCrO3

Using a microscopic model and a Green’s function technique we calculate the renormalized phonon energy in multiferroic RCrO3 (R = Sm, Dy, Er, Pr, Gd and Y) compounds as a function of temperature, magnetic field and R-ionic radius. We explain the observed anomalies in the temperature dependence of the phonon spectra based on a detailed analysis of the influence of the magnetic sublattices, the interaction between them and the spin-reorientation (SR) transition on lattice vibrations via spin–phonon interactions. When the rare earth ions are magnetic we investigate their essential role for the anomalies around the SR temperature. For the case when R is nonmagnetic, for example YCrO3, we propose a new microscopic model. We define an induced Dzyaloshinskii–Moriya (IDM) vector as a consequence from the spontaneous polarization. This IDM interaction is responsible for the appearance of a temperature-driven SR transition, which itself is responsible for the phonon anomalies at low temperatures. The numerical calculations are in good qualitative agreement with the experimental data.

Multiferroic properties of S = 1∕2 chain cuprates LiCuVO4: Comparison with LiCu2O2

We propose a microscopic model in order to study the multiferroic (MF) properties of LiCuVO4 (LCVO) taking into account the competing nearest and next-nearest magnetic interactions, frustration and a linear magnetoelectric (ME) coupling. We obtain for α = |J2∕J1| = 0.76. The temperature and magnetic field dependence of the polarization Pa and Pc is observed. It is shown that the dielectric constant ϵa has a kink near the magnetic phase transition TN = 2.4 K which disappears with increasing of the external magnetic field. Some differences in the MF behavior between LiCu2O2 (LCO) and LCVO are discussed.

Microscopic Approach to the Magnetoelectric Coupling in RCrO3 (R = Y, La, Lu and Eu) Compounds

A microscopic theory of magnetoelectric (ME) effects in multiferroic RCrO3 compounds, where R is a nonmagnetic ion (R = Y, La, Lu and Eu) is presented. Taking into account the influence of polar lattice displacements on symmetric and antisymmetric exchange interactions, two types of coupling between the magnetic and the ferroelectric subsystems are defined. The first magnetoelectric interaction is biquadratic with respect to the spin and pseudo-spin operators. The second, called antisymmetric, is induced by the appearance of spontaneous polarization in RCrO3. The built-in microscopic model describes the occurrence of a temperature-dependent spin-reorientation (SR) transition and defines it as continuous (continuous rotation of the magnetic spins in the zx -plane within a given temperature interval). The emergence of additional polarization as a consequence of a magnetic phase transition has been proven theoretically. This is due to the induction of an antisymmetric magnetic interaction of type Dzyaloshinsky-Moriya (DM) interaction, which is a consequence of the occurrence of spontaneous polarization in these compounds. The influence of magnetoelectric interactions on the magnetic and ferroelectric subsystems has been investigated. The behaviour of magnetization at the application of an external electric field is qualitatively explained with the renormalization of the exchange symmetric and antisymmetric magnetic interactions by the spontaneous polarization PS. The dependence of spontaneous polarization on the direction of external magnetic field was explained by the occurrence of feedback between PS and magnetization. It has been established that the width of the temperature interval ΔТSR, in which the SR transition takes place, decreases with the increase of the value of the external magnetic field. The method of Green’s function is used for the numerical calculations.