Relationship Between Multiferroicity And Weak Ferromagnetism

Multiferroicity and multiferroics have become one of the hottest topics of condensed matter physics and material science in recent years. The coexistence and coupling of ferroelectric order and magnetic order in multiferroics not only bring out abundant and intriguing physical properties, but also provide novel and promising potential applications in spintronics, such as magnetic sensor, data storage, logical operation and magnetoelectric controlling.Among the dozens of multiferroic materials, the coupling mechanisms are quite different. In Chapter I, we highlight the physical concepts of multiferroicity and the challenges to integrate the ferroelectricity and magnetism into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. In the sections about proper ferroelectrics, independent systems and ferroelectricity induced by lone-pair electrons are discussed, with em-phasis on ferromagnetic ferroelectric BiMnO3and room-temperature multiferroic material BiFeO3. The ferroelectric order and magnetic order of improper fer-roelectrics have the common physical origin or intrinsic causal relationship, e.g. ferroelectricity induced by magnetism, and therefore the coupling between them is strong. In geometric ferroelectrics, ferroelectricity and magnetism are both closely related with structural transition (hexagonal YMnO3and HoMnOs). In charge ordered systems, ferroelectricity originates from site-centered order and/or bond-centered order (LuFe2O4, Fe3O4), and the combination of charge order and exchange striction for RMnO5. Special attention is paid to the ferroelectrics whose ferroelectricity is induced by magnetism, for the intrinsic magnetoelectric causal relationship. The ferroelectricity induced by non-collinear spiral/helical order is observed in numerous compounds, with the microscopic mechanism ex-plained as DM model/spin current model and electric current cancellation model. E-type antiferromagnetic order is collinear, and leads to the ferroelectricity in-volving with exchange striction. In spite of the above, the ferrotoroidic systems also have intrinsic magneto electric coupling, and cause some interesting optical magnetoelectric effects. Regarding the application of multiferroics, we give an introduction to magnetic field sensor, electric field control of exchange bias, and four logical states realized in a tunnelling junction.Noting the importance of DM model in multiferroic researches and the deter-mining effect of DM interaction on weak ferromagnetism, we choose the relation-ship between multiferroicity and weak ferromagnetism as the research subject. In Chapter II, the origin of weak ferromagnetism, DM interaction, is deduced in detail, and two weak ferromagnetic compounds, Ca3Mn2O7and LuFeO3, is selected.On the one hand, Ca3Mn2O7has ferroelectricity and weak magnetism, which are both due to structural distortion. The magnetization-electric field measure-ments verify the multiferroicity of Ca3Mn2O7. We succeed in the explanation for a single domain. As for the polycrystalline multidomain, neither the rotating effect nor the stretching effect can singly explain all the experiments, so the coordination of them is expected. On the other hand, although LuFeO3has the same structure with BiFeO3and the same components with LuFe2O4, it does not possess ferro-electricity or multiferroicity. The structural and magnetic investigations reveal its large anisotropy and extremely high coercivity, and thus being a super hard magnetic material.The experiments manifest the truth of our initial conjecture that the ferro-electric weak ferromagnet should have magnetoelectric coupling. As the common origin of ferroelectricity and weak magnetism, structural distortion gives rise to strong magnetoelectric coupling and convenience for magnetoelectric modulation. Even though the magnetic transition point and the coupling temperature are both low, our work provides the foundation and a paradigm for the search of room-temperature multiferroics with strong magnetoelectric coupling.