Properties Modulation And Its Physical Mechanism In Single Phase Multiferroic Materials Of Rare Earth Iron Oxides

Rare earth iron oxide single-phase multiferroic materials have rich physical properties,such as magnetoelectric coupling effect,charge ordering transition and so on.Based on these physical properties,single-phase multiferroic materials have attracted the attention and favor of many researchers in the fields of new information storage devices,new multifunctional devices and high-frequency switching devices.Due to ferroelectric and magnetic properties are related to structural distortion,hexagonal LuFeO₃(h-LuFeO₃)based single-phase multiferroic materials exhibit strong magneto-electric coupling effects.Although the ferroelectric phase transition temperature of the system is higher than room temperature,there are some problems such as low magnetic transition temperature and weak magnetic properties,which are not conducive to the practical application of materials.The perovskite-type iron oxide La_1/3Sr_2/3FeO₃ has a charge order phenomenon,and there may also be a multiferroicity from charge ordering.However,the charge ordering phenomenon and the related mechanism of charge ordering need to be further studied.Therefore,in this thesis,we take h-LuFeO₃ and La_1/3Sr_2/3FeO₃,two single-phase multiferroic materials,as the research objects to study the magnetoelectric coupling effect and charge ordering transition of the two systems,and explain the related physical mechanisms.The specific research contents are divided into the following parts:Firstly,in this thesis,the doping method is used to study the method of controlling the magnetic transition temperature of hexagonal LuFeO₃-based single-phase multiferroic materials.In³⁺doped hexagonal Lu_0.5(Sc_1-xIn_x)_0.5FeO₃ single phase polycrystalline samples were prepared by solid-state sintering.It was found that all the samples have an antiferromagnetic transformation near 150 K,and also have a dielectric anomaly,showing the magneto-electric coupling effect.The c/a in the system increases with the increase of In³⁺,while the antiferromagnetic transition temperature T_N decreases,which is different from the regulation of magnetic phase transition in the Sc³⁺doped hexagonal system.The results showed that the ion radius of In³⁺is larger than that of Sc³⁺,the structural distortion decreases with the increase of In³⁺doped,and the magnetic interaction between iron ions weakens,resulting in the decrease of antiferromagnetic transition temperature T_N.It was proposed that the degree of structural distortion can be represented by the tilting angle?of the FeO₅ bipyramids,and T_N decreases with the decrease of tilting angle?.Besides,we also found another magnetic anomaly transition temperature T_A above the T_N,which can be regulated near room temperature.Secondly,the negative magnetization of hexagonal Lu_0.5Sc_0.5FeO₃ polycrystalline samples was studied.It was found that the magnetization direction of hexagonal Lu_0.5Sc_0.5FeO₃polycrystalline samples is affected by both the residual trapped field and a huge coercivity.The negative trapped field magnetizes the sample along the negative direction,and due to the existence of huge coercivity,the negative magnetization direction in the sample is not easily reversed to a positive value under a certain external magnetic field.The huge coercivity in the sample was related to the strong anisotropic field in the hexagonal system.Through the fitting calculation of paramagnetic data at high temperature,the intensity of positive/negative trapped field was found be several Oe.In addition,the magnetic switching effect was realized in hexagonal Lu_0.5Sc_0.5FeO₃ by adding a rectangular variable magnetic field of 0.5-3 kOe,which was expected to be applied to frontier fields such as magnetic storage.Finally,the charge ordering phenomenon of La_1/3Sr_2/3FeO₃ system was also studied.Ca²⁺/Ba²⁺doped La_1/3(Sr_1-xCa_x)_2/3FeO_3-?and La_1/3(Sr_1-xBa_x)_2/3FeO_3-?polycrystalline samples were prepared by solid-state sintering.The refined results showed that the degree of lattice distortion increases with the increase of Ca²⁺/Ba²⁺.Through the study of the structure,resistance and magnetic properties of a series of samples,the charge ordeing transition temperature T_CO was found to be gradually decreases with the increase of Ca²⁺/Ba²⁺.The increase of Ca²⁺/Ba²⁺leads to the increase of structural distortion,the decrease of p-d hybridization and the weakening of antiferromagnetic interaction between Fe³⁺spins,which results in the decrease of phase transition temperature.This indicates that the structural distortion has an important effect on the stability of charge ordering in La_1/3Sr_2/3FeO₃.