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The multiferroic properties of ion-doped hexagonal and orthorhombic YFeO3 (YFO) nanoparticles (NPs) are studied theoretically. The magnetization M s in h-YFO NPs increases, whereas for o-YFO NPs it decreases with decreasing NP size. In the dielectric constant (DC) both h- and o-YFO have a peak around T C 1 ∼ 4 5 0 and 460 K, respectively, but only in h-YFO an anomaly appears at T C 2 ∼ 3 0 0 K in the DC and the polarization which could be connected with a possible P 6 3 m c – P 6 3 c m phase transition. The polarization in pure and Bi-doped o-YFO NPs increases with increasing magnetic field. M s (x) is studied by doping of a o-YFO NP with Ti 4 + ions at the octahedral Fe 3 + sites. M s (T) in undoped YFO shows a small kink at T C 1 ∼ 4 5 0 K, whereas in the doped YFO it shows at ∼ 480 K. By different ion doping on the Y or Fe sites in YFO there is a transformation from the h- to the o-phase or vice versa. In Mn-doped o-YFO a spin-reorientation transition appears. The bandgap of h-YFO is smaller compared to that of o-YFO. [ABSTRACT FROM AUTHOR]

Magnetization M, specific heat C p , polarization P and dielectric constant ϵ of DyFeWO 6 are investigated for the first time using a microscopic model and the Green's function technique. The magnetic field dependence of M for temperatures below and above T N is observed. C p shows an anomaly at the Neel temperature T N which disappears by applying an external magnetic field h. P decreases with increasing h. The antiferromagnetic transition in DyFeWO 6 is accompanied by a peak in ϵ . With increasing magnetic field the peak decreases, shifts to smaller T N values, and for strong magnetic fields vanishes. The magnetic field behaviour of P and ϵ is an evidence for the multiferroicity of DyFeWO 6 . The observed results are in good qualitative agreement with the existing experimental data. [ABSTRACT FROM AUTHOR]

The temperature and magnetic field dependence of the magnetic and electric properties of LiFeP2O7 (LFPO) and LiCrP2O7 (LCPO) are studied using a microscopic model and the Green's function technique. We have shown that LFPO is antiferromagnetic, but shows a weak ferromagnetism along the b axis which originates from the canted antiferromagnetic order. For LCPO, such a ferromagnetic order along the b axis is not observed. In the temperature dependence of the electrical polarization P b (T) along the b axis there is a kink at T N = 2 2 K which is an indirect evidence for the intrinsic magnetoelectric effect in LFPO. Applying an external magnetic field H b , the polarization P b increases, stronger for small temperatures and the kink at T N disappears. For LCPO, we do not obtain a kink at T N = 6 K. LCPO is polar, but not ferroelectric. We can conclude that the missing magnetoelectric properties in LCPO could be due to the differences in the magnetic orders between LFPO and LCPO. [ABSTRACT FROM AUTHOR]

The temperature and magnetic field dependence of the magnetic and electric properties of LiFeP2O7 (LFPO) and LiCrP2O7 (LCPO) are studied using a microscopic model and the Green's function technique. We have shown that LFPO is antiferromagnetic, but shows a weak ferromagnetism along the b axis which originates from the canted antiferromagnetic order. For LCPO, such a ferromagnetic order along the b axis is not observed. In the temperature dependence of the electrical polarization P b (T) along the b axis there is a kink at T N = 2 2 K which is an indirect evidence for the intrinsic magnetoelectric effect in LFPO. Applying an external magnetic field H b , the polarization P b increases, stronger for small temperatures and the kink at T N disappears. For LCPO, we do not obtain a kink at T N = 6 K. LCPO is polar, but not ferroelectric. We can conclude that the missing magnetoelectric properties in LCPO could be due to the differences in the magnetic orders between LFPO and LCPO. [ABSTRACT FROM AUTHOR]

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 Green's 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. [ABSTRACT FROM AUTHOR]

The temperature, magnetic field and ion doping dependence of the phonon properties of CuFeO2 are studied on the basis of a microscopic model and using Green's function technique. The phonon energy decreases with increasing temperature, whereas the phonon damping increases. There is a kink at the phase transition temperature T N 1 which shows the influence of the magnetic field on the phonon properties. The kinks vanish by applying an external magnetic field. By doping of ions with different radius compared to the Fe ion, the phonon energy in CuFeO2 can increase (Ga) or decrease (Sc) with increasing dopant concentration, whereas the damping is always enhanced. The results are compared with those of CuCrO2. Some discrepancies in the literature are discussed. The observed results are in qualitative agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The temperature, magnetic field and ion doping dependence of the phonon properties of CuFeO2 are studied on the basis of a microscopic model and using Green's function technique. The phonon energy decreases with increasing temperature, whereas the phonon damping increases. There is a kink at the phase transition temperature T N 1 which shows the influence of the magnetic field on the phonon properties. The kinks vanish by applying an external magnetic field. By doping of ions with different radius compared to the Fe ion, the phonon energy in CuFeO2 can increase (Ga) or decrease (Sc) with increasing dopant concentration, whereas the damping is always enhanced. The results are compared with those of CuCrO2. Some discrepancies in the literature are discussed. The observed results are in qualitative agreement with the experimental data. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The multiferroic properties of bulk CuO are manifested in the dielectric function which can be triggered by an external magnetic field h and by the temperature T. Within a microscopic model and a Green's function technique we have calculated the dielectric function \varepsilon ({\bf k};T,\;h). At the magnetic phase transition temperature T_{{\rm N}2} the dielectric function offers a pronounced anomaly. This kink disappears when the magnetic field is enhanced and \varepsilon ({\bf k};T,\;h) decreases with increasing h -field. Both properties are indications for a strong magnetoelectric coupling in this material. The observation of multiferroism in CuO within an analytical approach is achieved by considering frustration and a linear magnetoelectric coupling. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: In order to demonstrate the coupling between electric and magnetic polarizations in multiferroic BiFeO3 nanoparticles, the effect of an external magnetic field on different ferroelectric properties is investigated on the basis of a microscopic model — the modified Heisenberg and transverse Ising model. The Green’s function technique allows the calculation of static and dynamic properties. They are discussed for different temperature, particle size and magnetic field. The polarization, the Curie temperature and the pseudo-spin excitation energy increase, whereas the damping decreases with increasing applied magnetic field. [Copyright &y& Elsevier]

Abstract: Based on the spin–phonon model we analyze the influence of surface and size effects on the phonon properties of ferromagnetic nanoparticles. A Green''s function technique in real space enables us to calculate the renormalized phonon energy and its damping depending on the temperature and the anharmonic spin–phonon interaction constants. With decreasing particle size the phonon energy can decrease or increase for different surface spin–phonon interaction constants, whereas the damping increases always. The influence of an external magnetic field is discussed, too. The theoretical results are in reasonable accordance to experimental data. [Copyright &y& Elsevier]