The Investigation On The Magnetocapacitance Effect In Ferroelectromagnet

Ferroelectromagnets (FEM) are compounds in which the ferroelectric (or antiferroelectric) and ferromagnetic (or antiferromagnetic) order coexist simultaneously in certain temperature range. The coexistence of the two order parameters may result in the coupling interaction between them. 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 (ME) coupling in ferroelectromagnet. Furthermore, the application of an external magnetic field will induce the dielectric change, which is named as the magnetocapaciance (MC) effect. The magnetocapaciance effect is another research focus for perovskite manganites except for the giant magnetoresistance effect. In technologic area, the ability to couple with either the magnetic or the elelctric polarization offers an extra freedom in the design of convention actuators, transducers, and storage devices. A number of device applications have been suggested for ferroelectromagnets, including multiple state memory elements, ferromagnetic resonance devices controlled by electric field, and variable transducers with either magnetically modulated piezoelectricity or electrically modulated pizeomagnetism. The investigation of the magnetocapacitance effect has important fundamental values and extensive technological applications. In this thesis, we have done the following work: 1. The further investigation on the magnetoelectric coupling. As far as the investigation of the magnetoelectric coupling mechanism is concerned, much theoretical work has been produced. However, the previous work hasn't considered the anisotropy of the magnetoelectric coupling resulting form the complex crystal and magnetic structure. We consider a three-dimensional ferroelectromagnet with A-type antiferromagnetic structure. A-type antiferromagnet shows a clear anisotropy in the magnetic structure, which will result in the anisotropy of magnetoelectric coupling. We investigate the influence of this kind of magnetoelectric coupling on the magnetic and dielectric properties. 2. The magnetocapacitance effect in EuTiO3. EuTiO3 is a quantum paraelectrics as well as a G-type antiferromagnet. Quantum paraeletrics is quite different from the normal ferroelectrics as far as the dielectric property is concerned. External factors may induce ferroelectric phase, which is the typical property for quantum paralelctrics. The sharp decrease of the dielectric constant at the néel temperature of EuTiO3 indicates the existence of the magnetoelectric coupling, which is another attractive property for EuTiO3. In our work, we investigate the magnetic influence on the dielectric property, including the polarization and the dielectric constant。We find the contribution of the magnetic field closely depends on the temperature and electric-field background. 3. The investigation on the rule of the magnetocapacitance effect. For ferroelectromagnet, the spontaneous magnetization will induce the dielectric anomaly, and the application of the magnetic field will further change the dielectric constant through the intrinsic magnetoelectric coupling, which is defined as the magnetocapacitance effect. Considering the spontaneous magnetic order may be ferromagnetic or antiferromagnetic, and the two kinds of the magnetic order may experience different fluctuation with the application of the magnetic field, we classify ferroelectromagnet as two kinds: ferroelectric-ferromagnetic (FE-FM) and ferroelectric-antiferromagnet (FE-AFM). We find for FE-FM, to obtain large MC, temperature around TC and a high magnetic field are two necessary conditions, for FE-AFM, temperature below TN and a high magnetic field are two necessary conditions. To obtain high magnetodielectric response, FE-FM and FE-AFM have different conditions: low magnetic field for the former and high magnetic field for the latter.