| Single-phase magnetoelectric multiferroic materials contain both ferroelectricity and ferromagnetism simultaneously, and exhibit coupling effect between the two orders. Magnetoelectric multiferroics are expected to have broad application prospects in spintronic device, magnetic sensor, storage and microwave technology area. However, the multiferroic materials are far from the practical applications because of the deficiencies of single-phase multiferroic materials, considerably low magnetic transition temeperature and the weak coupling between magnetic order and ferroelectric order. So it is the key issue to explore new type of room temperature single-phase multiferroic materials, to improve the magnetic transition temperature and to realize considerable magnetoelectric coupling effect. Hexagonal YMnO3 plays a very important role in single-phase multiferroic materials due to the novel properties which forebode great potential in practical applications. Therefore in this thesis, we focus on hexagonal YMnO3 to discuss the influence of 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.Firstly, to study the influence of Cu2+ ion doped YMnO3 on lattice structure, electronic structure and magnetism, polycrystalline YMn1-xCuxO3 (x=0,0.05,0.1) samples were prepared by sol-gel method. The lattice structure of hexagonal YMnO3 unchanged with Cu2+ doping, and could be indexed within the P63cm space group. Doping Cu2+ ions introduced Mn3+-Mn4+ double exchange interactions and Mn3+-Mn3+, Mn3+-Cu2+, Mn4+-Cu2+ antiferromagnetic interactions, and competition between different magnetic interactions which broke the frustrated Mn3+ triangular structure and improved the magnetism. Meanwhile Mn3+-Mn4+ double exchange interactions also made contributions to the increase of magnetization. By measuring the Mn L-edge XAS, the existence of Mn4+ ions in YMn0.95Cu0.05O3 and YMn0.9Cu0.1O3 samples was confirmed. O K-edge XAS showed that the hybridization between Mn and Op increased and the Mn-Op bond length decreased with increasing Cu2+ ions concentration, which corresponded to the transformation from Mn3+ to Mn4+. We also found the hybridization between Y and Op decreased, which meant the increase of Y-Op bond length. The variation of Mn-Op and Y-Op changed the MnO5 trigonal bipyramidal structure, which led to the change of magnetic order and electric polarization of YMn1-xCuxO3 (x=0,0.05,0.1).Secondly, polycrystalline Y0.95Yb0.05Mn1-xFexO3 (x=0,0.05,0.1,0.2) samples were synthesized via sol-gel method. The lattice structure of YMnO3 has not been changed by doping Yb3+, Fe3+ ions. It is still hexagonal structure. The magnetization of samples increased with increasing Fe3+ ions concentration, while the absolute value of Curie-Weiss temperature, antiferromagnetic transition temperature and magnetic frustration factor decreased. The theoretical magnetic moment of Fe3+(5.92 ?B) ions are larger than Mn3+(4.9?B) ions. The doping of Fe3+ ions made the magnetic moment of antiferromagnetic Mn3+ ions tilt, which resulted in the occurrence of net magnetic moment. And the competition among Mn3+-Mn3+, Mn3+-Fe3+, Fe3+-Fe3+ interactions weakened Mn3+-Mn3+ antiferromagnetic superexchange interaction. The factors above were the reasons for the increase of magnetization of the samples. Mn L-edge XAS spectra proved Mn3+ valence states. O K-edge XAS showed that the increase of Fe3+ ions doping enhanced O 2p-Mn 3d andY 4d-O 2p hybridization, which aroused the increase of tilting of MnO5 bipyramid, and led to the change of lattice structure and magnetic order. The experiments confirmed Yb and Fe co-doping YMnO3 could cause the change of local microstructure and magnetism, thus regulating the multiferroicity of YMnO3. |
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