Study On The Complicated Magnetism And Magnetoelectric Coupling Of Multiferroic Transition-metal Oxides

With the rapid development of the information society,the storage ability per unit area of semiconductor device is improved quickly and has approached its physical limitation up to now.In order to get the storage medium with larger storage ability,faster read-write speed and lower energy consumption,the people are searching for the next generation storage medium,such as multiferroic materials exhibiting magnetoelectric?ME?coupling property,holographic image basing on the principle of laser interference,topologically protected magnetic skyrmion,and so on.Among them,the non-volatile multiferroic memory device presents sub-nanosecond switching speeds and ultra-low power loss,especially,it exhibits four-state memory property which can improve the storage ability significantly.Thus,the multiferroic material is an important research area in physics and material science.It turned into a hot topic for searching the multiferroic materials with huge ME coupling at room temperature.Z-type hexaferrites present huge ME coupling coefficient at room temperature,and Ruddlesden-Popper?RP?double-layer perovskite manganites with orthogonal crystal structure exhibit intrinsic electrically controlled magnetism.The two systems both exhibit stronger magnetocrystalline anisotropy?MCA?and obvious magnetic frustration behavior,in which the multiferroic is releated to the spin-orbit coupling?SOC?.While the observed ferromagnetism and electric polarization induced by magnetic field are too weak to be applied,moreover,the microscopic influence mechanisms of MCA,SOC,and magnetic frustration behavior on the properties of multiferroic,ME coupling,and electrical transport are still unclear.The associated studies of the physical properties mentioned above are helpful to understand the physical mechanism by which the SOC and magnetic frustration behavior affect the multiferroic.In the dissertation,we summarized the multiferroic properties of hexaferrites and RP double-layer perovskite manganites,and we introduced the experimental theories and methods related to this thesis.Then,we studied systematically the MCA,ME coupling and ferroelectric mechanism of Z-type hexaferrites Ba₃(Zn_1-xCo_x)₂Fe₂₄O₄₁ polycrystals and Sr₃Co₂Fe₂₄O₄₁ single crystal.Last,we studied the magnetic frustration and electrical transport properties in La_xCa_3-xMn₂O₇ polycrystals and Ca_3-xAl_xMn₂O₇ single crystals.The main points are as follows:1)We studied the MCA,ferroelectric mechanism,ME coupling properties,and electrical transport properties in Z-type hexaferrites Ba₃(Zn_1-xCo_x)₂Fe₂₄O₄₁?x=0.2,0.4,0.6,0.8?.The MCA field presents an enhanced tendency with Co²⁺increasing and temperature decreasing,which related to the spin-orbit coupling effect and the improvement of effective moment of Co²⁺,respectively.The ferroelectricity of Ba₃(Zn_1-xCo_x)₂Fe₂₄O₄₁ originates from both inverse Dzyaloshinskii Moriya?DM?interaction and p-d hybridization mechanism,while the ME coupling property is only dominated by p-d hybridization with spin-orbit coupling.Thus we can improve the ME coupling property of Z-type hexaferrites according to these results,which have reference significance for designing ME coupling materials applied at room temperature.2)We studied the MCA,magnetic frustration and ME coupling properties of Sr₃Co₂Fe₂₄O₄₁ single crystal.The spin glass state induced by magnetic frustration presents in the sample at 10?400 K,specially an applied magnetic field within the c plane can suppress the spin glass state and be beneficial to sustain transversal conical state for the sample at higher temperature.The MCA field along c axis increases quickly with the temperature decreasing,and it is larger obviously than that within the ab plane.The ferroelectricity originates from inverse DM interaction,and the ME coupling property is most remarkable at 200 K,which is attributed to the competition between SOC effect and spin glass state.At 200 K,electric polarization induced by magnetic field presents damping with increasing scan times of the field,which may be related to the interaction between the polarization and electromagnon.3)We studied the magnetic frustration and electric transport properties of La_xCa_3-xMn₂O₇?x=0,0.3,0.5,0.7,0.9?.The materials exhibit magnetic frustrate character and tend to intrinsically separate into antiferromagnetic matrix state,spin glass regions,and ferromagnetic?FM?cluster regions,specially the spin glass regions and FM cluster regions tend to be enhanced with increasing La³⁺doping.According to the molecular field mechanism,the magnetic critical temperature decreases and then increases with La substitution,which is related to the variation of magnetic exchange interaction and the SOC effect.All the samples present insulating behavior according to variable range hoping model,which indicates that the electrons transfer between band-tail states in a disordered system.4)We studied micro-mechanism of Néel temperature variety and magnetic frustrate character at lower temperature in Ca_3-xAl_xMn₂O₇?x=0,0.01?single crystal.With Al³⁺doping,both the molecular field coefficients of the nearest-neighbor and next-nearest-neighbor increase,and the Ca_2.99Al_0.01Mn₂O₇ sample exhibits canting antiferromagnetism,but the effective magnetic moment of Mn ions decreases,eventually resulting in the decrease of Néel temperature,which results from the enhancement of spin-orbit coupling as Al³⁺doping.We found the Ca_2.99Al_0.01Mn₂O₇sample presented reentrant spin glass state at 2?30 K,which could induce negative magnetization when a residual magnetic field existed in sample chamber.