Research On The Magnetoelectric Coupling Effect Of Hexaferrite And M₂Mo₃O₈（M:3d Transition Metal）

In recent years,multiferroic materials have attracted extensive attention due to magnetoelectric coupling effect and rich physical mechanisms,as well as their potential applications in logic electronic devices,sensors and memories.From the perspective of basic research,it is an urgent problem to realize the strong coupling of magnetic order and ferroelectric order and to explain the microscopic mechanism of their mutual coupling.From the perspective of practical application,multiferroic materials with room-temperature magnetoelectric coupling are expected to be used for"electric-write-magnetic-read"polymorphic memories.It is difficult to explore new strong coupling material systems due to the mutual exclusion of ferroelectricity and magnetism in the electronic structure of multiferroic materials.However,chemical doping and high-pressure regulation are effective experimental means to control the magnetic interaction,therefore,the magnetic,dielectric and magnetoelectric coupling properties of the hexaferrite and M₂Mo₃O₈（M:3d transition metal）were regulated by using these two experimental means in this paper.1.The polycrystalline sample of hexaferrite Ba_0.5Sr_1.5Co₂Fe₁₁Al O₂₂ was synthesized via solid-state reaction,and the magnetic structure and magnetoelectric coupling properties of the sample were systematically studied at different temperatures and magnetic fields.The magnetic ordering temperature of sample was much higher than room temperature.At the same time,the adjustable conical magnetic structure and excellent magnetoelectric coupling effect existed at room temperature,and the regulation of magnetic field was realized at room temperature.In addition,the dynamic and static magnetoelectric coupling properties exhibited linear magnetoelectric coupling effect at low temperature and nonlinear magnetoelectric coupling effect at high temperature.2.The single crystal sample of hexaferrite Ba_2-xSr_xCo₂Fe₁₁Al O₂₂（x=1.2,1.6）were synthesized by flux method,and the magnetic structures and magnetoelectric coupling properties of the two doped samples were systematically investigated at different temperatures and magnetic fields.The multiferroic magnetic structure（FE3）appeared near room temperature in the low Sr（x=1.2）doped sample,while the temperature of FE3 phase dropped to 100 K in the high Sr（x=1.6）doped sample.An odd function-like electric polarization with magnetic field（P-H）curve and linear variable magnetization with electric field（M-E）curve showed strong linear magnetoelectric coupling effect.With the rising of temperature,the magnetic structure gradually changed from FE3 phase to alternate longitudinal cone（ALC）phase under low-magnetic field due to the reduction of energy barrier.In the room-temperature range,the main magnetic structure of the two samples were ALC phase,and the butterfly-like P-H and M-E curves showed strong second-order magnetoelectric coupling effect.3.The single crystal sample of hexaferrite Ba_0.4Sr_1.6Mg₂Fe₁₂O₂₂ was synthesized by flux method,and the magnetic structures,dielectric properties and magnetoelectric coupling properties were systematically studied at different pressures,temperatures and magnetic fields.The magnetoelectric coupling performance of FE3 phase decreased gradually with pressure increasing.A new ferroelectric phase（FE’phase）induced by pressure appeared at0.7 GPa,and the magnetoelectric coupling performance of FE’phase was enhanced as increasing in pressure.However,further increasing the pressure to about 4.32 GPa,pressure enhanced easy plane anisotropy induce a possible proper screw structure with the elimination of magnetoelectric effect.4.The single crystal sample of Ni_2-xFe_xMo₃O₈（x=0,0.5,1.0,1.5）were synthesized by chemical vapor transport method,and crystal structures,magnetic structures and magnetoelectric coupling properties were systematically studied.With the doping amount of Fe,the primary structural characters still kept intact,however,the antiferromagnetic temperature was improved,antiferromagnetic exchange interaction was enhanced,the magnetic field of spin-flop transition was decreased,and the influence of magnetic field on the spontaneous ferroelectric polarization associated with the specific antiferromagnetic ordering gradually changes from inhibition to promotion.The clear magnetism-driven ferroelectricity behaviors have been demonstrated by the quadratic magnetoelectric response in the antiferromagnetic region and linear magnetoelectric response in the spin-flop region.At the same time,the M-E curve of the sample changed almost linearly below antiferromagnetic temperature,indicating strong linear inverse magnetoelectric coupling effect.