Micro Area Electrical Property And Magnetoelectric Coupling Effect In Multiferroic Films

Multiferroic material is a functional material with two or more ferroic order parameters such as ferromagnetic, ferroelectric, ferroelastic and so on. The magnetoelectric coupling can be produced by these ferroic order parameters, and the electric polarization controlled by magnetic field or magnetic polarization controlled by electric field becomes possible. It makes electric device multi-functional, integration and miniaturization. In recent years, with the rapid development of microelectronics technology, the research of multiferroic materials comes to the mesoscopic level, and the related research becomes hotspot in condensed matter physics.Multiferroic material can be divided into two categories by the composition: single-phase multiferroic and composite multiferroic. In single-phase multiferroics, the magnetoelectric coupling is weak at room temperature, because of the limit of the symmetry in ferromagnetic and ferroelectric and the filling rule of electron orbit. BiFeO3 is a single-phase multiferroic compounds which has magnetoelectric coupling effect at room temperature, so it is favored by lots of researchers. But the leakage is large, the ferroelectric and ferromagnetic is weak, so BiFeO3 is highly restrict in application. In another hand, multiferroic composites are not restricted by these rules, the ferroelectric and ferromagnetic materials with transition temperature of ferroelectric and ferromagnetic higher than room temperature can be selected to assemble the composites with strong magnetoelectric coupling at room temperature. Now, the research of multiferroic composites in mesoscopic scale is ascendant, so the discovery of new multiferroic composites in nano-scale has been focused by researchers for a long time.In this thesis, we researched some multiferroic materials in mesoscopic scale based on early works. By using scanning probe techniques, we studied mono phasic compound BiFeO3 thin film and composite film system based on AFE ceramics (Pb0.99Nb0.02) (Zr0.85Sn0.13Ti0.02)0.9803 (PNZST) and ferromagnetic metal Co. The electric properties and magnetoelectric coupling have been systematically researched in nano scale. The process and the mechanism of magnetoelectric coupling was discussed. The main results are as follows,By using sol-gel method and magnetic sputtering techniques, the Co/PNZST composite film was successfully deposited on the Pt(111)/Ti/SiO2/Si(111) wafers. The AFE PNZST layer has the orientation along (100) direction while the FM Co layer has the orientation along (200) direction. The good AFE and FM appears in the composite film at room temperature. We observed the change of magnetic domain in zero magnetic field induced by applied electric field, and discovered that electric field has obvious regulation on magnetic domain. When the applied electric filed is smaller than the AFE-FE switching field of PNZST, the domain structure only had slight changes and when the applied electric filed is larger than the AFE-FE switching field the domain structure had big changes. The width of domain is up to 75%larger and the angle of domain has a change up to 5°. The results of MOKE measurement further shows that when the composite film was applied a small electric field, the magnetic anisotropy of Co layer was changed obviously. This behavior of magnetoelectric coupling is mainly generated from the strain when the PNZST has AFE-FE phase transition as electric field applied on it. The strain of PNZST layer is smaller in the longitudinal direction than the transverse direction, so the magnetic anisotropy of Co layer is changed and thus the domain structure has corresponding changes.By using sol-gel method, La (10%) doped BiFeO3 thin film was deposited on the Pt(111)/Ti/SiO2/Si(100) wafer. The doped Bi0.9La0.1FeO3 film shows obvious (012) orientation and good FE/FM, in addition, its leak current is improved largely. We use c-AFM and PFM mode of SPM to study the process and mechanism of local conductive status systematically. The results indicate that the conductivity in interior of grain and grain boundary shows different feature which changes with the applied electric field. In low voltage, the conductivity of the film is dominated by grain boundaries and closely related to the distribution of oxygen vacuum in grain boundary. When the voltage became high, the conductivity is mainly controlled by grain boundary and FE domain. When the voltage is higher, the conductivity spreads from grain boundary to the grain interiors, which shows that the conductivity in grain interior is closely related to the reversal of FE domain. In the meantime, we discovered that the film shows obvious switching resistivity effect. The switching field in grain boundaries is much smaller than that in the grain interiors, and the switching field became larger from grain boundary to grain interior which reached its maximum in the center of grain interior. In addition, we also found that the local conductivity of grain is highly related to the growth state of the grain. When the direction of FE domain has same direction with applied field, the grain will become conductive easily. In the contrast, when the FE domain direction is perpendicular to the field, the grain will be conductive when the field is large enough.By combining the ferroelectric testing system in microscale and c-AFM mode/ PFM mode of SPM, the localized magnetoelectric coupling of Bi0.9La0.1FeO3 film in low magnetic field was in situ observed and studied. The results show that the ferroelectric properties, the conductive properties and resistance switching effect of Bi0.9La0.iFeO3 can be obviously regulated by magnetic field. We obtained that, the ferroelectric polarization reached its maximum in the 400e magnetic field, and the resistance including related switching field became smaller in the same time. When fht magnetic field changes from 10000e to 40 Oe, the statured polarization, the conductivity and the switching field increase with the decreasing field, and their tendencies are opposite during the process from 400e to zero field. The magnetoelectric coupling in Bi0.9La0.1FeO3 film is mainly originated from the suppression from magnetic field to the magnetic helix, and the change of lattice structure under the applied magnetic field. The structure transforms between R (rhombohedral) phase and T (tetragonal) phase. The spontaneous polarization is changed by the stress which induced by the strain of the material under the applied magnetic field.