Effect Of The Magnetoelectric Coupling On Thermodynamic Properties In Ferroelectromagnets

Ferroelectromagnets are compounds in which the ferroelectric (or antiferroelectric) and ferromagnetic (or antiferromagnetic) orders coexist simultaneously in a certain temperature range. The coexistence of the two order parameters may result in the inherent magnetoelectric (ME) effect. In detail, the ferroelectric polarization may change the magnetic property by redistributing the spin order, correspondingly, the fluctuation of the spin order may induce the dielectric anomaly or the ferroelectric relaxation through the magnetostrictive effect or electron-phonon interaction. The dielectric anomaly at the magnetic transition temperature observed in experiment is indicative of the inherent magnetoelectric coupling in ferroelectromagnets. At present, a common material of ferroelectromagnets is the perovskite rare-earth manganite. For R3+ cations with large ionic size (R=La,Pr,Nd,Sm,Eu,Gd and Tb), RMnO3 oxides crystallize in a peroviskite-type structure, with orthorhombic symmetry (space group Pbnm). For rare-earth cations with small ionic size (R=Y,Lu,Sc,Ho,Er and Tm), the peroviskite structure becomes metastable and a new hexagonal polytype stabilizes(space group P63cm). In the hexagonal phase, the centers of gravity of the positive and negative electrons deviate from each other and then a spontaneous ferroelectric behavior occurs, which has been described to coexist with magnetic order at low temperature and gives rise to the inherent magnetoelectric coupling effect. In technologic area, the magnetoelectric coupling plays a remarkable role in thermodynamic properties of ferroelectromagnetic materials such as the lattice specific heat, magnetic specific heat, thermal conductivity and so on. A number of new device applications have been suggested for ferroelectromagnets, including non-volatile memory materials, gate ferroelectrics in field-effect-transistors and magnetoresistance. The investigation of inherent magnetoelectric coupling effect has important fundamental values and extensive technological applications.In this thesis, we have done the following work about magnetoelectric effect in ferroelectromagnets:1. Effect of magnetoelectric coupling on the magnetic correlations in the ferroelectromagnetic system.As far as the investigation of magnetoelectric coupling in ferroelectromagnetic system is concerned, much theoretical work on the magnetoelectric properties such as magnetization, polarization, magnetic susceptibility, electric susceptibility and so on has been produced. However, the previous work hasn't studied the influence of magnetoelectric coupling on the spin correlation which describes the magnetic property of the ferroelectromagnetic system. Therefore, in our work, we utilize the soft-mode theory and the mean-field theory on the basis of Heisenberg model to investigate the effect of magnetoelectric coupling on the magnetic property in a system with the coexistence of ferroelectric and ferromagnetic orders as well as the one with the coexistence of ferroelectric and antiferromagnetic orders.2. Effect of magnetoelectric coupling on the lattice specific heat in ferroelectromagnets.For ferroelectromagnets, the spontaneous magnetization will induce the dielectric anomaly, which indicates the existence of inherent magnetoelectric coupling. Furthermore, anλ-type anomaly of the thermodynamic property such as specific heat at its magnetic phase transition temperature is observed. Hence, we need to utilize the Debye model including magnetoelectric coupling to investigate the lattice specific heat in ferroelectromagnets. Our research results show that the magnetoelectric coupling has an explicit influence on the lattice specific heat in ferroelectromagnetic system and exhibits the same flipping behavior as the spin correlation. The lattice specific heat in the presence of magnetoelectric coupling deviates from the traditional Debye-T3 curve.3. Effect of magnetoelectric coupling on magnetic excitation and magnetic specific heat in the hexagonal ferroelectromagnet.The rare-earth manganite RMnO3 is one of the ferroelectromagnets. When R3+ cation is small, it crystallizes in a hexagonal lattice structure with the triangular antiferromagnetic geometrical frustration and the 1200 rotation between the nearest Mn ions in the basal plane. Given that the hexagonal phase of RMnO3 may be transformed in condition of a high temperature and pressure to the orthorhombic peroviskite-type structure, the magnetoelectric properties related with the external electric(or magnetic) fields have been studied by the model of A-type antiferromagnet with the ferromagnetic intraplane interaction and the antiferromagnetic interplane one in previous researches. Besides, the intraplane magnetic interaction is much larger than the interplane one, thus, for the case of a real situation, we'd better take the triangular magnetic structure in the basal plane into account. The influence of magnetoelectric coupling on polarization, magnetic excitation spectrum as well as magnetic specific heat in the basal plane of hexagonal ferroelectromagnetic system have been studied by three-sublattice model for the peculiar magnetic structure.