| With the development of modern science and technology,the trend of device miniaturization and multifunctional integration has promoted the research and development of new materials with coexisting two or more functions.Multiferroics,which combine magnetic(M)and ferroelectric(FE)properties,are one kind of such multifunctional materials.In multiferroic materials,the coupling between crystal structure,magnetism and ferroelectricity might induce rich interesting phenomena like magnetoelectric(ME)effects,etc.Multiferroics have great application potentials in spintronics like sensors,multi-state data storage and logic devices.It is noteworthy that the multiferroics with strong magnetoelectric coupling are especially prospective in high-speed and low-energy electric-write-magnetic-read high density information storage,etc.Multiferroics with perovskite structure have attracted wide attension due to the easier tunable physical properties.As a room temperature(RT)single-phase multiferroic material,BiFeO3(BFO)has become a hot point in the area of condensed matter physics due to the coexistence of high FE(TC～1100K)and antiferromagnetic(AFM)(TN～640K)order temperatures and mutual coupling between the two orders.The ferroelectric polarization at room temperature in BiFeO3 is very high(～90-100μC/cm2).Whereas the spatially modulated cycloid AFM spin structure does not allow a net magnetization and prominent linear magnetoelectric effect in BiFeO3.In addition,bulk BiFeO3 suffers from the existence of impurity phases and high leakage current,resulting in poor FE properties.Although various methods have been tried over the years,such as element doping,grain refinement and strain regulation,to improve the room-temperature ferromagnetism and magnetoelectric effects,the results(especially the electric control of magnetism)are still unsatisfactory.In this thesis,we proposed a new route to design room-temperature BiFeO3-based multiferroic materials with strong ferromagnetism and magnetoelectric effects,i.e.,on on hand,we chose rhombohedral BiFeO3 and other tetragonal ABO3-type ferroelectric perovskites to construct multiferroic solid solutions that reside near the morphotropic phase boundary(MPB);on the other hand,we used the so-called GoodenoughKanamori(G-K)rule as a guide to introduce suitable magnetic ions to further improve the ferromagnetism in the BiFeO3-based multiferroic MPB systems.Based on the above idea,we have designed and prepared BiFeO3-based multiferroics with strong ferromagnetism and magnetoelectric effects(especially the electric control of magnetism)at room temperature.We revealed the possible microscopic mechanism of the electric control of magnetism in these materials via a systematic investigation of the crystal structure,multiferroic and magnetoelectric properties.Our main research results are listed as below.(1)A series of Cr and Mn co-doped(1-y)BiFe1-xCrxO3-yBaTi1-xMnxO3(BFCBTM,0.15≤y≤0.3,0≤x≤0.1)solid solution samples were prepared by sol-gel method.Based on the construction of MPB,we further introduced simultaneously the magnetic Cr and Mn ions at the B site and tuned x and y.By doing so,we have successfully obtained a series of multiferroic samples near the MPB showing enhanced ferromagnetism.We determined the MPB by the characterization and analysis of the lattice structure of the samples via X-ray diffraction(XRD),Raman scattering methods,etc.The physical properties of all these samples were studied in details,such as ferroelectric,piezoelectric and dielectric properties.We find that the samples near the MPB shows enhanced ferroelectricity,ferromagnetism,piezoelectricity and magnetoelectric efects.Furthermore,the samples were found to change from the FE phase to the R-FE phase and then to the dipole glass by varing x while keeping y the same.The structure-physical-property phase diagram was finally obtained for the CrMn co-doped BFC-BTM system by analyzing all the experimental resutls.(2)On the basis of the above-mentioned work,we chose a series of samples(1y)BiFe1-xCrxO3-yBaTi1-xMnxO3(0.15≤y≤0.33,x=0.03)as research tagets,and prepared these samples by optimizing the synthesis conditions.We focused on the detailed study of the electric field control of magnetism for these samples.We investigated in detail the lattice structure and physical properties and found that the sample near MPB,i.e.,y=0.27 and x=0.03,showed the enhanced ferroelectric,ferromagnetic,piezoelectric properties,as well as the optimized electric control of magnetism at room temperature.The results of the magnetization M variation at in-situ applied electric fields E showed that the sample near the MPB exhibited a butterfly-type shape in M-E curves at room temperature.The time-dependent M at periodic square-wave electric field further unambiguously demonstrated a reproducible and stable E control of M for the sample near MPB,i.e.,y=0.27 and x=0.03.This sample showed a repeatable butterfly M-E curve even at 350 K,illustrating the temperature stability of magnetoelectric effect.In order to further understand the possible microscopic mechanism of the E control of M,in situ XRD and Raman scattering experiments at different applied E were performed at RT for the typical sample y=0.27 and x=0.03 near the MPB.The results show that E-induced lattice distortion and phase transition play a dominant role in the converse magnetoelectric effect in the system.(3)In the above work,we constructed samples near the MPB in the binary BiFeO3BaTiO3(BFO-BTO)system,and obtained enhanced multiferroic,piezoelectric and the expected magnetoelectric effect(especially the electric field control of magnetism)at room temperature.In contrast with the MPB in the two-member solid solutions,the MPB in the three-member systems contains more elements and shows more complicated and disordered structure.As a result,it is expected that the samples near the MPB in the three-member systems might possess crystal structures more sensitive to external perturbations and hence show stronger multiferroicity and magnetoelectric effects,especially the E control of M at room temperature.Therefore,we chose BFO,BTO and Bi0.5Na0.5TiO3(NBT)members to form a ternary solid solution,and constructed samples near MPB by introducing further magnetic Mn ions.We prepared a series of multiferroic Mn doped(1-y)(0.9BiFeO3-0.1Bi0.5Na0.5TiO3)-yBaTi1-xMnxO3(BF-BNT-BTM)ternary samples.We found a composition-induced phase transition from rhombohedral to pseudo-cubic near the MPB by detailed characterization of the crystal structure.We also found a significant enhancement of ferroelectric polarization,piezoelectricity and E control of M at RT in the samples near the MPB.In addition,insitu XRD diffraction at different applied electric fields also revealed that the lattice distortion and phase transition dominate the RT converse magnetoelectric effects in the samples near the MPB. |
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