Multiferroicity And Magnetoelectric Coupling Properties Of Hexagonal Ferrite And Bismuth Ferrite Based Ceramics

With the rapid development of science and technology,the demand for miniaturization and versatility of devices is increasing.The research of functional materials with multiple physical properties is the key to meet this demand.Multiferroics is a new type of multifunctional material that has both ferroelectricity and magnetism.The magnetoelectric coupling effect in the multiferroics can realize the mutual regulation of ferroelectricity and magnetism.Multiferroics with magnetoelectric coupling effect are expected to be used to prepare high-speed memory devices of“electro-writing and magnetic reading”and other spintronic devices,which has a huge application prospect.The most widely studied single-phase multiferroics is Bi FeO₃,which possesses ferroelectricity and helically modulated weak magnetism at room temperature.The magnetoelectric coupling performance is weak in Bi FeO₃ and it needs to be further improved.In the hexagonal ferrite multiferroics,the ferroelectric polarization is generated by the magnetic structure,which has a strong magnetoelectric coupling effect.It can truly realize the regulation of the electric field on the magnetization and the regulation of the magnetic field on the electric polarization performance.It is the current research hotspot of magnetoelectric multiferroics.In this dissertation,hexagonal ferrite and bismuth ferrite-based multiferroic ceramics were studied.The ferroelectric property,magnetic property,property of ferroelectric polarization reversed by magnetic field and dynamic magnetoelectric coupling property are mainly explored.(1)The dielectric,ferroelectric and magnetic properties of Bi FeO₃-Ba TiO₃ based ceramics can be controlled by adjusting the types and concentrations of defects.Firstly,the difference between the traditional solid-state sintering method and the quenching sintering method is analyzed.The study found that the ceramic prepared by the quenching sintering method has a smaller concentration of defects,which can enhance dielectric and ferroelectric properties.It can also improve the magnetization.Then,in the 0.70Bi FeO₃-0.30Ba Ti_xO₃(0.85?x?1.15)samples,the defect concentration is adjusted by the non-stoichiometric ratio of Ti.In the Ti-rich sample,Ti mainly exists in the form of impurity phase,it has little effect on the structure,morphology,dielectric and ferroelectric properties.While the lack of Ti has a great adjustment effect on the performance.When x=0.90,the ceramic structure changes from pseudocubic phse to coexisting phase of rhombohedral and pseudocubic phases.The hysteresis loop of the ceramics tends to be more saturated.A larger built-in electric field appears in the ceramic.And the magnetic properties show a nearly linear increase trend with the increase of Ti content.The adjustment effect of the oxygen partial pressure of the sintering atmosphere on the ceramic properties is studied.The larger the oxygen partial pressure,the lower the oxygen defect concentration,which is beneficial to the growth of crystal grains.And it also reduces the dispersion degree of the ferroelectric phase transition and obtains excellent ferroelectric properties.But it will make the magnetization of ceramics slightly decrease.(2)The performances of magnetodielectric and electric polarization continuous reversed by H in Y-type Ba Sr Co₂Fe₁₁AlO₂₂(BSCFAO)ceramics were studied.In the Y-type BSCFAO ceramic,when the angle(?)between H and the electrode surface is different,different magnetodielectric curves will be formed.The shape of the magnetic dielectric curve with different?angles can be used to determine whether the magnetic structure phase transition occurs.Continuously circulated H can continuously flip the electric polarization,andthe polarization intensity will decrease exponentially with the increase of the number of cycles.It is found in the study of polarization conditions that the role of the poling magnetic field is to make the magnetic structure in a transverse cone and induce the electric polarization.The poling electric field makes the induced electric polarization poled.Both the poling electric field and the poling magnetic field are indispensable.Only when they act together can good polarization effect be obtained.(3)Employing hot-pressing method to prepare oriented BSCFAO ceramics to enhance the magnetoelectric coupling performance.The BSCFAO ceramics prepared by the hot-pressing sintering method exhibit c-axis orientation in the direction of applied pressure.The electric polarization in oriented ceramics can be continuously reversed by circulating H.In the P–H curve,the saturation polarization P_m is 32.82?C/m²,and the remanent polarization P_r is 13.83?C/m².Comparing with those in BSCFAO ceramics prepared by conventiona solide-state method,P_m is increased by 3 times,and P_r is increased by 3.2 times.(4)The dynamic magnetoelectric coupling effect of Z-type Sr₃Co₂Fe₂₄O₄₁(SCFO)ceramics at room temperature and above is studied and the Al-doped Sr₃Co₂Fe₂₂Al₂O₄₁(SCFAO)ceramics are used to increase the maximum temperature of the dynamic magnetoelectric coupling effect.In Z-type SCFO ceramics,the transverse conical magnetic structure can induce electric polarization,and the electric polarization direction does not change under the action of H.The first-order dynamic magnetoelectric coupling coefficient(?)has a strong response mainly in low H region with magnetic structural phase transition.Near H with the magnetic structural phase transition,the magnetic moment vector and the corresponding electric polarization vector are the most unstable at their equilibrium position,which is conducive to generating a strong?response.When H increases,the stability of the magnetic moment in the equilibrium position increases and the?response will decrease.When the angle between the DC H and the AC H is changed,the?response exhibits an anomaly with a period of 90°,and the second-order dynamic magnetoelectric coupling coefficient(?)exhibits an anomaly with a period of60°.The maximum temperature with dynamic magnetoelectric coupling response in Z-type SCFO ceramics is 370 K.For Al-doped Z-type SCFAO ceramics,the maximum temperature with dynamic magnetoelectric coupling response is 444 K,which is 74 K higher than that of the undoped sample.This can greatly improve the application range of multiferroic materials with magnetoelectric coupling effect.