Magnetic And Structure Studies Of Doped Multiferroic YMnO₃

Multiferroic materials have attracted increasing attention due to coexistence of ferroelectric and magnetic transitions.The hexagonal RMnO₃（R=Y,Yb,Ho,Lu,Sc）is a potential single phase multiferroic material with ferroelectrics and antiferromagnetism,which has attracted much attention in recent years.It has a large potential in applications in multifunctional devices,such as data storages,capacitors,transducers and actuators.The hexagonal YMnO₃ shows antiferromagnetic（AFM）transition temperature about 75K,while their ferroelectric transition temperature is about 914K.The space group of hexagonal YMnO₃ is P6₃cm.In YMnO₃,the Mn³⁺ion is located in the position of the body center of the double cone of the MnO₅ triangle,and the distance of the apical oxygen ion to Mn³⁺is different from the distance of Mn³⁺to the oxygen ion which on the triangle biconical plane.One of the important requirements for the application of multiferroic materials is to have magnetic order and coupling at room temperature.Therefore,it is the key issue to improve the antiferromagnetic transition temperature of the material to study the YMnO₃ material.In order to improve its magnetic properties,doping is a very important method.In general,the reasons for the change in the properties of YMnO₃ materials are very complex,and there is not a final conclusion.For example,when the YMn_1-xFe_xO₃compound is doped with Fe element at B site,the lattice structure and space group don’t change at low concentration.But the lattice constants are larger,because the doping of the Fe element leads to the bending of the Y layer and at the same time leads to the increasing angle of the MnO₅,the length of the Mn-O bond is longer and the interaction of ferromagnetic and antiferromagnetic exists in the system.However,the YMn_1-xCr_xO₃ samples are found to increase in ferromagnetic transition temperature due to the enhancement of double exchange between Cr³⁺and Mn³⁺.Most of the doping in YMnO₃ are magnetic ions,the doping of non-magnetic ions to change the magnetism and electronic structure of YMnO₃ are relatively few.Therefore,in this thesis,we focus on hexagonal YMnO₃ to investigate the influence of Al³⁺ion doping on the elctronic structure,magnetic structure and physical properties.Different from the previous investigations on hegxagonal YMnO₃,we use the synchrotron radiation X-ray absorption spectroscopy（XAS）to explain the changes of magnetoelectric properties caused by the ion doping.In Chapter 1,in this paper,the history of the research on the multiferroic material is briefly introduced,and the basic properties of the crystal structure,magnetic structure and internal exchange function of the multiferroic material are analyzed and explained.Then the theory applied in this paper is explained.The main properties of the hexagonal YMnO₃ multiferroic materials are introduced.In Chapter 2.First,the experimental preparation method of polycrystalline material is introduced,and the material is made by sol-gel method.Then the X ray diffraction,scanning electron microscope（SEM）,ferroelectric analyzer,GSAS（General Structure Analysis Syste）,magnetic measurement and the X ray absorption experiment used in the experimental research are described in detail.In Chapter 3.In order to study the effect of Al³⁺ion doping on the lattice structure,electronic structure and magnetic properties of YMnO₃ samples,a series of YMn_1-xAl_xO₃（x=0,0.02,0.05,0.1）samples were prepared by the sol-gel method.It is found that the lattice structure of YMnO₃ is not changed with the doping of Al³⁺ions,and the space group is P6₃cm.However,due to the difference in the radius of doped ions,the local structure of samples will be changed,which will lead to the change of MnO₅ tilting and the buckling of Y³⁺ions.The doping of Al³⁺ions will affect the Mn³⁺-Mn³⁺antiferromagnetic interaction,destroy the triangular structure of the magnetic moments of Mn³⁺ions and enhance the magnetism.By measuring the L edge absorption spectrum of Mn,we confirmed the effect of the change of electronic structure on the magnetic results.The K edge absorption spectrum of O shows that,with the increase of Al³⁺concentration,the hybridization between Mn and O_P is weakened,and the length of Y-O_P bond decreases.The changes in the length of the Mn-O_P bond length and the length of the Y-O_P bond will cause a change in the structure of the MnO₅,which leads to the change in the magnetic order and the electric polarization of the YMn_1-xAl_xO₃（x=0,0.02,0.05,0.1）.In Chapter 4.Y_0.95Yb_0.05Mn_1-x Al_xO₃（x=0.05,0.1,0.15）samples were prepared by sol-gel method.The doping of Yb³⁺ions and Al³⁺ions did not change the lattice structure of YMnO₃,But the lattice has corresponding shrinkage.The magnetic measurement results show that the magnetization of the sample increases with the increase of Al³⁺ion concentration,while the absolute value of the Curie–Weiss temperature,the antiferromagnetic transition temperature and the magnetoresistance factor gradually decrease.Al doping causes the Mn³⁺ion magnetic moment of the original antiferromagnetic arrangement to tilt,resulting in the generation of the net magnetic moment,the Mn³⁺-Mn³⁺antiferromagnetic superexchange will weaken,which will affect the magnetic properties of the substances.The above factors lead to the enhancement of the magnetic properties of the samples.The analysis on the X-ray absorption spectra from the view of point of the changes in electronic structure and orbital hybridization strength confirms that the frustration destroyed.These results are in agreement with the measurements of ZFC and FC.Experiments show that the co-doping of Yb and Al can cause changes in the local microstructure and magnetic properties of the materials,thereby regulating the YMnO₃ multiferroic properties.