The Design Preparation And Magnetoelectric Coupling Mechanism Of CFO/PMN-PT Multiferroic Composite Films

Multiferroic materials realize the multi coupling of force,electrostatics and spin in the system,which allows to regulate the ferroelectricity and ferromagnetism by the applied field.Compared with traditional magnetic recording and ferroelectric memory,multiferroic materials possess higher degree of freedom to control the performance providing the potential application in the information storage.Among the multiferroic system,the heterostructure and composite films meet the requirement of integration and miniaturization for modern devices,and exhibit well magnetoelectric coupling effect at room temperature.Therefore,it is of great significance to study the role of the phase combination in the multiferroic materials.In this work,the Co Fe₂O₄（CFO）and Pb(Mg_1/3Nb_2/3)O₃-Pb TiO₃（PMN-PT）were used as the research object to fabricate the magnetoelectric composite system by sol-gel and pulsed laser deposition（PLD）method respectively.The direct and converse magnetoelectric behavior,the origin of ferroelectricity and the evolution of magnetic anisotropy were studied systematically,in the meanwhile the magnetoelectric systems with different preparation methods and composite structures were compared.The magnetoelectric coupling property of CFO/PMN-PT composite film was predicted by average method,indicating the strong interaction among the system.The CFO/PMN-PT layered and particulate composite films were successfully prepared via sol-gel method on the Pt/Ti/Si O₂/Si substrate using pure and mixed solution as the precursor respectively.The ferroelectric and ferromagnetic properties test manifests that the annealing temperature and reaction atmosphere significantly influence the performance of particulate composite films.The direct magnetoelectric coefficient maximum of particulate composite films reached 2.7 m V/cm·Oe at730°C and air atmosphere while the mole ratio of CFO and PMN-PT is 3:1.The tendency of remanence ratio and remanent polarization varying with the external condition was similar,indicating the intrinsic relationship of ferroelectricity and ferromagnetism which was beneficial to strengthen the magnetoelectric coupling in composite films.The experimental results agreed with the theoretical prediction.The ferroelectric properties test of layered composite films showed that improving ferroelectric performance occurred when the bottom layer was PMN-PT film,and the remanent polarization maximum reaches 12.8μC/cm² at 9 V.The direct magnetoelectric effect test showed that stronger magnetoelectric coupling was acquired when the PMN-PT film was on the top,and the coupling will be weakened as the number of film layer increases.The direct magnetoelectric coefficient maximum of layered composite films was 0.52 m V/cm·Oe for the two-layer film where the top layer was PMN-PT.The PMN-PT film was grown on STO（100）single crystal substrate by PLD.The Sr Ru O₃（SRO）buffer layer was demonstrated to be beneficial for improving the ferroelectric properties of PMN-PT film.The growth mode of SRO evolved from frank-van der Merwe growth to Volmer-Weber growth,and the former was more favorable to the ferroelectricity of PMN-PT film.The structural changes and ferroelectric behavior of PMN-PT film under temperature field was analyzed,which suggested the phase transition from monoclinic to tetragonal and then cubic by the temperature.The change of ferroelectric properties below room temperature was associated with dielectric loss.The size effect of film ferroelectricity was studied by observing the electric domain which changes with film thickness,and the critical thickness（6 nm）of non-volatile ferroelectricity was obtained.The CFO film was grown on SrTiO₃（100）（STO）single crystal substrate by PLD method.The epitaxial thin film with（400）orientation and nonepitaxial thin film with（222）orientation was able to be acquired respectively under different conditions.The CFO/PMN-PT heterostructure was fabricated by growing CFO film on PMN-PT（110）single crystal substrate via PLD.The epitaxial thin film with（220）orientation was obtained easily under different conditions for the small mismatch between the film and the substrate.The CFO film exhibits in-plane anisotropy and vertical anisotropy suffering the stress from the PMN-PT substrate,and the in-plane coercive field of CFO film ranges from 791 to 1526 Oe by rotating the sample.The study of converse magnetoelectric behavior to the heterostructure shows that the coercive field and in-plane anisotropic field of CFO film changes with the external voltage,and the tunable range of anisotropic field is from 841 to 608 Oe.The relationship between the variation of CFO coercive field and the PMN-PT strain was demonstrated to be linear by the theoretical derivation and experimental result together.In addition,the in-plane magnetization of the CFO film was observed to be able to converse under the voltage.The results indicate that the magnetoelectric coupling mechanism in CFO/PMN-PT heterostructure based on the stress/strain transmission between the film and the substrate.The highly epitaxial CFO/PMN-PT composite film was grown successfully on STO（100）single crystal substrate by PLD method.As the thickness of CFO film increased,the magnetism changed from vertical isotropy to vertical anisotropy.The function of film thickness on the magneto-optic Kerr effect was studied and obvious converse magnetoelectric behavior was able to be observed by controlling the thickness.As the thickness increasing from 200 to 300 nm,the electric field tunable range of CFO coercive field increased from 60 to 100 Oe while the remanence ratio tunable range remained almost the same.The stress transfer mechanism and polarization charge mechanism can be observed in the CFO/PMN-PT composite film,where the former dominates and the latter weakens with the film thickness.