Magnetic And Electric Properties Of Perovskite-like Oxides

Multiferroic materials refer to the materials that have two or more ferroic orders. The most common materials are the ones with ferroelectricity and magnetic order. Electricity and magnetism are two basic physics phenomenon. According to the Maxwell’s equations, alternating electric field and magnetic field are coupled to each other by mutual induction. In solid state maters, electric dipole moment can be induced by electric field, while magnetization can be induced by magnetic field. However, the coupling between the two is difficult. Single phase magnetic materials and ferroelectric materials kept separate for long until the last decade after the huge breakthrough happened. BiFeO3 was found with giant ferroelectric polarization coexisting with magnetism at room temperature as a single phase material in 2003. At the same time, magnetic ferroelectricity and giant magnetoelectric coefficient was found in TmMnO3. After that, flourishing happens and a series of type Ⅰ and type Ⅱ multiferroic materials was found. The origin of type I multiferroic materials includes lattice distortion, lone pair electrons and charge ordering, all of which allows the coexistence of both ferroelectricity and magnetism. These materials have huge ferroelectric polarization and high Curie temperature, containing specimen that have ferroelectricity and magnetism at room temperature. On the other hand, novel physical mechanisms were found in type II multiferroic materials, including spin current, exchange striction and spin-dependent p-d hybridization. Strong magnetoelectric coupling was found in these materials, and cross reverse may also happen in some of the specimen.There are two major problems in multiferroic area. First one is that magnetization that can be reversed by electric field at room temperature is not found yet, which requests further research for new materials. The other one is that multiple origins exist in available materials, which needs further discussion. We carried out detailed research on these problems. The thesis consists four chapters, experimental results and discussions listing below.In chapter one, we stated the background for perovskites, multiferroics and exchange bias. We introduced basic classifications and typical materials for multiferroic materials, for reference of exploring new multiferroic materials. The phase diagrams of perovskites are good reference for the speculation of new perovskites, such as the Neel temperature of magnetism and Curie temperature of ferroelectricity. Some new single phase exchange bias materials are introduced at the end of the chapter.In chapter two, we studied the exchange bias phenomenon in single crystalline SmFeO3. Exchange bias behavior of SmFeO3 at different temperature and different field was measured primarily, finding that the exchange bias field can be enlarged by compensation of magnetism, which is in accordance of Heusler alloys. We found that there is no reliable reference in such system. According to the cluster interpretation of SmCrO3 and Heusler alloys, we proposed our interpretation and verified it in NdFeO3. For that mean field approximation is used, we explored the magnetism of SmFeO3 at low temperature, verifying our understanding that Sm dose not form long range order down to 0.4 K.In chapter three, we measured structure, magnetic and electric properties of new material Ba3Ti2MnO9 Structure test verifies that Mn and Ti orders along c axis which shows that the space group is noncentrosymmetric. Raman spectrum at different temperature shows that there is no obvious phase transition. Magnetic measurement shows that there is probably strong frustration in the system, verifying our understanding of quasi-two dimensional triangular arrangement of the spin structure. Further test shows that lack and excess spins will increase the magnetic susceptibility and the excess spins will condense as spin dimers. Ferroelectricity may originate due to the insufficient volume of the co-faced octahedron in the BaO cage.In chapter four, we studied the magnetic and electric properties of In doping GaFeO3. Measurement shows that polarization of GaFeO3 can be increased by In doping. The saturation magnetization can be optimized by the In doping and spin-phonon coupling still exists after doping. The decrease of the magnetic ordering temperature can be amended by verifying the ratio of Ga/Fe.