Theoretical Studies On The Magnetic And Electric Properties Of Multiferroic Materials

As the fast progress of modern technology, a great demand of device miniaturization has become more and more prevalent. Multiferroic material, which is a kind of new multifunctional material that simultaneously shows ferroelectricity, ferromagnetism and/or ferroelasticity, is for sure a promising candidate for device miniaturizing. The magnetic and ferroelectric materials permeate every aspect of modern technology, therefore, the coexistence of magnetic and ferroelectric properties in multiferroic material has important application value. Most importantly, the magnetoelectric couling in multiferroic material, such as controling magnetism with electric field, or controling ferroelectricity with magnetic field, can be exploited as new functional device. The multiferroic materials present opportunities for potential applications in information storage, the emerging field of spintronics, and sensors.The multiferroics materials can be divided into the composite or single-phase multiferroic materials. Beginning in2003, the discoveries of a series of famous multiferroics materials set off the revival of single-phase multiferroic materials. BiFeO3is the only known multiferroic materials by now which exhibits strong ferroelectricity and ferromagnetism above room temperature. The coupling between the ferroelectric polarization and antiferromagnetic plane provides great opportunities to achieve electrical control of ferromagnetism. Magnetically induced ferroelectric is another kind of prevalent multiferroic materials, which is promising to achieve magnetic field control of polarization.With the development of the experimental study of multiferroics, the mechanisms of multiferroic for some materials are not clear. Moreover, there are some defects in the existing multiferroic materials, such as small ferroelectric polarization, low Curie temperature and weak magnetoelectric coupling, preventing their application in device. Therefore, the theoretical study on multiferroics is necessary, which can explain the experimental phenomena, identify the mechanism. Meanwhile, the theoretical calculation such as first principles can predict the properties of new materials, as a guide to find and design multiferroic materials.In this thesis, the first-principles calculations, combined with model analysis and phenomenological approach were used to study the ferroelectric, magnetic properties, and magnetoelectric couplings of multiferroic materials. Meanwhile, the structural, electric and magnetic properties of a designed magnetic double-perovskite materials were also investigated. The main conclusions are as follow:1. We studied the magnetic properties of BiFeO3. We have shown that the single-ion anisotropy and the anisotropic superexchange coupling are the two origins of the easy-magnetization-plane anisotropy in BiFeO3. The presence of long period spiral spin order can be attributed to the competition between the isotropic superexchange and anisotropic DM interactions of NN spins. The resulting magnetization is sinusoidally modulated under the spiral spin structure. Then we simulated experimentally observed size effect of macroscopic magnetization.2. We studied the magnetic properties and the origins of the ferroelectric polarization of recently discovered multiferroics CaMn7O12. The calculations of superexchange interactions indicate that the spin frustration is related to the strong antiferromagnetic couplings between Mnl ions. The final anisotropy induced by all the Mn1or Mn2ions is easy-magnetization ab plane, which is consistent with the experimentally observed in-plane orientations of magnetic moments. We explained the results of exchange couplings and magnetic anisotropy by the empirical rule of exchange interaction and the single-ion theory, respectively. The noncollinear-type exchange-striction and the p-d hybridization mechanism coexist for the origins of polarization in this material. The noncollinear-type exchange-striction mechanism operates in two kinds of Mn3+-Mn4+charge-order chains, in which the Mn2-Mn3 chain plays a big role in the giant polarization. The polarization induced by the two mechanisms follows the P (?) Si· Sj and P oc S;×Sj laws, respectively.3. We studied the structural, electronic, and magnetic properties of designed double-perovskite Ho2MnFeO6using first-principles calculations. Our results show that Mn, Fe ions form staggered arrangement and the ground state is ferrimagnetic with G-type antiferromagnetic. The calculations of electric structure indicate the system has half-metallic nature. A local Coulomb repulsion parameter U can significantly effect on the electric structure, the structural parameters, and the exchange coupling, but not change the half-metallic ferromagnetic property.