High Pressure Synthesis And Physical Properties Of Novel Multferroics

In magnetoelectric multiferroics,the long-range spin ordering and ferroelectric polarization coexist and even strongly couple with each other,leading to abundant physics and promising applications.In this thesis,two new A-site ordered perovskite multiferroics were prepared using high temperature and high pressure conditions.We characterize the crystal structure and measure the versatile physical properties in detail.In addition,related microscopic physical mechanisms are also studied.The main conclutions are shown as follows:（1）The A-site ordered perovskite BiMn₃Cr₄O₁₂ was prepared for the first time under high pressure and high temperature conditions.High-resolution synchrotron x-ray diffraction,neutron powder diffraction,magnetic susceptibility,dielectric constant,polarization,ferroelectric hysteresis loop（P-E loop）and so on were measured to investigate the intrinics physical properties thoroughly.The results indicate that this compound shows a type-I multiferroic phase at 125 K,where the ferroelectric polarization is driven by the 6s² lone pair electronic effects of Bi³⁺ions,whereas the magnetic order origins from the superexchange interaction of Cr³⁺ions.The type-I mutiferroic phase displays a considerable polarization as large as 1.4μC/cm² as characterized by the P-E loop measurements.Further cooling to 48 K,the special magnetic structure gives rise to a type-II mutiferroic phase with remarkable magnetoelectric coupling up to 71 ps/m.These results demonstrate that,Bi Mn₃Cr₄O₁₂presents a rare example of single-phase multiferroic system where both type-I and type-II multiferroic phases coexist.The large electric polarization and strong ME effect are compatible with each other.In addition,one can obtain four different polarization states（±P₁,±P₂）by applying different poling E precedures.Therefore,the current BiMn₃Cr₄O₁₂ provides an advanced candidate material for devloping multifunctional spintronic devices and multiple-state memories.（2）The A-site ordered perovskite CdMn₃Mn₄O₁₂ was prepared by using high pressure and temperature synthesis methods.The significant interactions between charge,orbital,spin,and lattice degrees of freedom induce a series of interesting phase transitions in this compound.Two antiferromagnetic transitions are found to occur at 88 K and 41 K,respectively.The higher temperature magnetic order phase shows linear magnetoelectric effect under magnetic field from 88 K to 41 K without spountanous ferroelectric polarization,indicating the nature of paraelectricity for this spin phase.The lower temperature magnetic phase below 41K is ferroelectric in nature,as confirmed by magnetic dielectric constant and magnetoelectric current measurements.In this phase,the ferroelectric polarization drived by spin order is about 0.02μC/cm².Meanwhile,CdMn₃Mn₄O₁₂ displays a huge magnetic dielectric effect with the value up to 23.5%.Neutron diffraction experiments indicat that the orbital ordering and spin ordering in the higher temperature phase are helical both.Moreover,their propagative periods show integral multiple relationship,and the magneto-orbital coupling is commensurate.However,on cooling down to the lower temperature phase,the intergral multiple relationship is broken,and the magneto-orbital coupling becomes incommensurate.It means that the ferroelectrity origins from the incommensurate magneto-orbital coupling.The present CdMn₃Mn₄O₁₂ provides the first material in which orbital can control the spin ordering and thereby induces ferroelectric polarization.