Structure And Multiferroicity Of Z-type Hexaferrites

Multiferroic materials,wherein magnetic and ferroelectric orders coexist,have attracted extensive research interests duo to their potential technological applications for next-generation devices such as a muitibit memory.Spin-driven mutiferroics have the potential to yield large magnetoelectric effect since the magnetic order and ferroelectric order are correlated,such as hexagonal ferrite.In the thesis,we studies the crystal structure,magnetism and magnetoelectricity of Sr₃Co₂Fe₂₄O₄₁-based hexaferrites.It can provides theoretical basis and ideas for finding materials with high magnetoelectric coupling performance at room temperature.The primary research contents and conclusions are as follows:Single-phase polycrystalline samples of Sr₃Co₂Fe₂₄O₄₁ were synthesized by a sol-gel method.The magnetic phase transitions in the temperature range of 10-720 K were investigated by temperature and magnetic-field（H）dependence of magnetization（M）and by specific heat capacity measurement.Above 300 K,three phase transition points were detected,at～640 K,～480 K and～350 K,which are corresponding to the ferrimagnetic phase along the c-axis,in-plane ferrimagnetic structure and transverse conical spin structure,respectively.Below 300 K,there are two broad peaks at～85 K and～230 K in the curve of the temperature dependence of magnetization.T dependence of specific heat capacity and the unique two-step increase symbol of the magnetization curves indicate that there is no normal magnetic phase transition near 85 K and 230 K.We studied the magnetic and magnetoelectric coupling properties of Z-type hexaferrites with Ca,Zn and Al substitutions.The MH curve increases up to saturation（the corresponding magnetic field H₂）in two steps for the transverse conical structure.The H dependence on the magnetocapacitance and M is correlated,which indicates there is magnetoelectric coupling.The temperature corresponding to the maximum value of the magnetocapacitance and the one corresponding to the H₂ maximum are the same.It suggests the stronger superexchange interaction can stabilize the noncollinear spiral spin structure and contributes to its higher magnetoelectric performance.The substitution of Ca can reduce the operating magnetic field,which induced ferroelectricity.With the Zn substitution amount increasing,the magnetic order temperature decreases.The saturation magnetization gradually increases for x<0.8 and decreases for x>0.8.These experimental results suggest the occupation site of Zn²⁺is spin-down site in the range of x=0-0.8,and changes at higher substitution.The magnetization increases from two-step to three-step up to the saturation magnetization.The magnetocapacitance also increases in three steps up to the saturation magnetocapacitance.The magnetic structure shows fine behavior,which corresponds to the“platform”effect on the magnetocapacitance curve.The magnetic phase transition temperature increases from～380 K to～500 K when the Al substitution amount is equal to 7%.The researches about crystal structure and magnetic properties demonstrate that it is duo to the change of the magnetic anisotropy of Z-type hexaferrites and Fe-O-Fe bond angle.We reported the magnetic,dielectric and magnetoelectric properties of Nd Cr O₃polycrystalline ceramics.Magnetization curves revealed two magnetic transitions at 227 K and 38 K,which corresponded to Cr³⁺canted antiferromagnetic ordering and Cr³⁺spin reorientation phase transition,respectively.At 11.5 K,a Schottky-type anomaly was observed,caused by Nd³⁺ground doublet Zeeman splitting.High-temperature dielectric relaxation exhibited a type of thermally activated relaxation process,which mainly resulted from the Maxwell–Wagner effect.The spin-reorientation of Cr³⁺ions and the Nd³⁺ground doublet splitting were observed to be accompanied by an electric polarization.The polarization could be induced by the presence of the antiferromagnetic-type domain walls,which led to spatial inversion symmetry breaking.