Charge Coupling Effect And Mechanism At The Interface Of Multiferroic Heterostructures

Due to their great application potential the multiferroic materials have attracted much attention and become a research focus for the fast development of informatization.Adjusting electric field to modulate magnetic properties in the multiferroic heterostructures is the research purpose of magnetoelectric coupling effect.Charge coupling effect has attracted much attention due to it's potential application in micro-storage devices with low energy consumption,high density and multi-function.Because of the complexity of interface interaction,the mechanism of interface magnetic properties controlled by charge coupling effect in heterostructures is still not clear.In this thesis,the mechanisms of charge coupling effect have been systematically studied to modulate the magnetic properties of multiferroic heterostructures,which is expect to provide guide lines for application of this kind of materials.Firstly,despite intensive investigations on the Verwey phase transition of Fe₃O₄ over half a century,the mechanism for this phase transition remains controversial and needs to be further studied.The modulation of the magnetization of the Fe₃O₄/SrTiO₃ multiferroic heterostructure by electric field reversal has been studied at the temperature ranging from 50K to 150K.It has been found that the charge screening effect not only influences the magnetization but also induces the temperature of Verwey phase transition shifting¹3K.Two models have been proposed to explain the change of Fe₃O₄magnetization and the shift of the Verwey phase transition temperature in the Fe₃O₄/SrTiO₃ multiferroic heterostructure caused by charge coupling effect,respectively.This work proves the existence of the phase transition mechanism in the charge coupling effect and preliminarily reveals the mystery of Fe₃O₄ Verwey phase transition.The modulation of magnetization by the electric field reversal has been studied as a function of the temperature in the Fe/SrTiO₃ heterostructure.It is found that the magnetization is strong when the SrTiO₃ layer is at the accumulation state,but when the SrTiO₃ layer is at the depletion state,the magnetization of the Fe layer is weak.To explain this phenomenon a qualitative model is proposed that the electric field reversing causes the change in the spin electron density of the Fe layer.Meanwhile,it is found that,with the increase of temperature,the magnetoelectric conversion coefficient and the change of the magnetization caused by electric field reversing gradually reduce.At room temperature the effect of electric field modulating magnetization almost vanishes,which can be attributed to the decrease in the dielectric constant of the SrTiO₃ layer and the disorder movement of the spin electrons of the Fe layer caused by the increases of the temperature.Finally,the magnetization modulated by electric field has been studied at the Fe₃O₄/BaTiO₃interface and a magnetoelectric coefficient of 0.72 Oe?cm/kV is obtained by ferroelectric polarization reversal at room temperature.This work experimentally proves the existence of the bond-length mechanism.It is found that the change of magnetization is approximately proportional to the BaTiO₃layer c axial electrostriction.It can be ascribed to the fact that the ferroelectric switching results in atomic displacements at the interface,which affect the Fe-Ti bond length and interface atomic orbital hybridization.