| With the rapid development of information technology,the miniaturization and multi-functionality of devices have become an inevitable trend in the future.More and more researchers are looking for materials that have two or more functionalities within one components.Multiferroic materials have been found to adequately meet this requirement,but in single-phase multiferroic materials,their application is limited due to the weak magnetoelectric coupling response at room temperature.The layered artificial composite materials can effectively solve the problems compared with the single-phase multiferroic materials.In order to improve the magnetoelectric coupling response of layered composites,relying on the automated magnetoelectric test system,thesis investigated the effects of strain,volume ratio of composite two phases and element doping on the ME coupling properties of the layered composites.The experimental results are as follows:1.Pb(Zr0.52Ti0.48)O3(PZT)ferroelectric thin films and Ni Fe2O4(NFO)magnetic thin films were prepared on(001)oriented Sr Ti O3(STO)single crystal substrates by PLD method,respectively.Through the exploration of thin film growth process,the optimal growth process parameters of the two thin films were as follows:PZT thin film growth temperature is 580°C,growth oxygen pressure is 30 Pa,target base distance is 50 mm,laser frequency is 3 Hz,laser energy is 250 m J,annealing oxygen pressure is 1000 Pa.NFO film growth temperature is670°C,growth oxygen pressure is 15 Pa,target base distance is 50 mm,laser frequency is 5Hz,the laser energy is 300 m J.The XRD results show that the films all grew along the(001)crystal plane orientation and have good crystallinity.The AFM results show that the surface of the film is smooth,and the grain size is uniform.The electrical properties and magnetic results show that the PZT film and the NFO film have good ferroelectricity and ferromagnetism.2.NFO/PZT heterostructures were epitaxially grown on(001)Sr Ti O3 substrates via PLD.The PZT layers with various thicknesses were designed to grow between the substrate and the NFO layer,which acted as a piezoelectric layer and effectively reduced the mechanical constraints imposed by the substrate to the magnetic layer.The misfit strain in both PZT and NFO layers decreases with the increase of the PZT thickness.The ferroelectric,magnetic and magnetoelectric properties of the heterojunctions exhibit a PZT thickness dependence and a large ME coefficient of 240 m V/cm?Oe with a PZT thickness of 170 nm was achieved.And through theoretical calculations,it is found that when the interface coupling coefficient k=0.2,the experimental data agrees well with the calculated results.The magnetoelectric coupling effects were further confirmed by the magnetic capacitance measurements and a magnetodielectric response of?10%was revealed in the heterojunction with a PZT thickness of 170 nm.This work will be of significance for tuning magnetoelectric response via thickness ratio between ferroelectric and ferromagnetic layers.3.The effects of zinc doping on the magnetoelectric coupling effect of NFO/PZT composite films were studied in detail.The structure,magnetic properties,ferroelectric properties and magnetoelectric coupling properties of Ni(1)-xZnxFe2O4/PZT composite films have been investigated.The results show that the doping of Zn has obvious regulation on magnetoelectric effect.When the zinc content is 0.15,the composite film system can reach the maximum magnetoelectric coupling coefficient of 328 m V/cm?Oe.The magnetodielectric measurement confirmed the occurrence of the coupling between the two phases,and further the influence of the DC magnetic field on the magnetic capacitance of the composite film were also explored,the results show that with the increase of DC magnetic,the magnetic capacitance shows a trend of increasing first and then decreasing. |
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