中观察到的 1.7 和 2.3 eV 两个光学吸收峰都源于 Mn 3+ 离子 3d 与平面内 O(3, 4) 2p 形成的杂化态与 Mn 3d 3z 2r 2 之间的电子跃迁, 而 Ho 5d 空轨道与平面上的 O(3, 4) 2p 轨道之间在 z 方向的强烈杂化驱动 HoMnO3产生垂直 于平面方向的铁电极化.

The anomalies in both the dielectric constant and loss tangent occur at fixed temperature close to the magnetic ordering transition, which have been observed experimentally in multiferroic pervoskite YMnO3 and BiMnO3, indicating coupling between the magnetism and dielectric properties. In this paper, by analyzing the magnetoelectric coupling between magnetic and ferroelectric subsystems, an appropriate coupling term related to a combination of electric polarization and spin correlation is added to system Hamilton expression, Then apply the soft-mode theory for electric subsystem and mean-field approximation under Heisenberg model for magnetic subsystem,magnetoelectric properties have been investigated, including the change of polarization, dielectric constant induced by external magnetic field, and the change of magnetization induced by external electric field. We also make some qualitative comparison and analysis between our results and the experimented ones and give a reasonable interpretation of magnetoelectricity phenomenon in some multiferroic pervoskite materials.

Because of the strong coupling between the magnetic and dielectric properties, the study of quantum paraelectric EuTiO3 has attracted more and more attention in both theoretical and experimental research recently. In this paper, the first principles based on the density functional theory within the generalized gradient approxiamtion is used to investigate the magnetic and electronic structure of quantum paraelectric EuTiO3, and to analyze the effects of the strain on the magnetic and strutural phase transition, in turn to discuss the possible magnetoelectric coupling mechanism of this material. The calculations show that EuTiO3 with the strain-free is in a paraelectric cubic and G-type antiferromagnetic state at low temperature, while appling either compressive or tensile strain along the c-axis to it, the balance of hybridization between Ti 3d and O 2p orbit will be breaken and EuTiO3 will transite from paraelectric and G-antiferromagnetic to ferroelectric-ferromagnetic structure as the strain is increased to a certain value. All those indicate the strong spin-lattice coupling effect in EuTiO3.

Researches on electrocaloric effects of ferroelectric materials and their applications in solid-state refrigeration have attracted great interest in recent years. EuTiO3 is a new multiferroic material with many special physical properties, such as high dielectric constant, low dielectric-loss, as well as their responses to tunable external electric field and temperature. With EuTiO3 ferroelectric thin films, their polarization size and phase transition process not only can be changed by regulating external electric field and temperature applied, but also can be controlled by adjusting the external stress applied and the lattice mismatch with the substrate in a large scale. Accordingly, in this paper a phenomenological Landau-Devonshire thermodynamic theory is used to investigate the ferroelectric properties and electrocaloric effects of EuTiO3 ferroelectric films under different external tensile stresses (σ3 > 0) perpendicular to the film surface and different in-plane compressive strains. We have calculated the electric polarizations, electrocaloric coefficients and adiabatic temperature differences as a function of temperature for EuTiO3 ferroelectric films with a biaxial in-plane misfit strain um =-0.005 under different applied stresses. Results demonstrate that the changes of the electric polarization, the electrocaloric coefficient and the adiabatic temperature differences conform with the regulation of externally applied stresses. With the enhancement of applied tensile stress perpendicular to the film surface, the phase transition temperature and adiabatic temperature change of EuTiO3 thin film increase, and the operating temperature corresponding to the maximum adiabatic temperature difference moves toward high temperature region. For the thin films with a biaxial in-plane misfit compressive strain um =-0.005 and the external tensile stress σ3 = 5 GPa, when the change of electric field strength is 200 MV/m, the adiabatic temperature differences at room temperature can be over 14 K, and the maximum electrocaloric coefficient may approach 1.75×10-3 C/m2·K. In the meantime, the working temperature range, when the adiabatic temperature differences go beyond 13 K, is over 120 K. Then we investigate the effect of in-plane compressive strains on the changes of adiabatic temperature, showing that with the increase of compressive strain um, the adiabatic temperature change will also increase and the peak of the curve of adiabatic temperature change versus temperature will shift toward high temperature zone far away from room temperature. Therefore, the above results show that we can not only have relatively bigger adiabatic temperature differences in epitaxially grown EuTiO3 thin films through the regulation of external stresses and in-plane lattice misfit strain, but also a sound application prospect of ferroelectric EuTiO3 thin film in solid-state refrigeration at room temperature.

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