Effects Of Nb、K Doping On The Crystal Structure,Magnetic And Optical Properties Of BiFeO₃

Multiferroic bismuth ferrite with magnetic and ferroelectric orders has a wide application prospect in the fabrication of spin electronic devices and other devices.However, the small magnetic moment as well as the large leakage current limits its further applications. Chemical doping can improve the multiferroic properties of Bi Fe O3 although the physical mechanism is still under debate. In this theses, the Nb and K doped Bi Fe O3 nanoparticles are prepared by the sol-gel method. The structure,the magnetic properties and the optical properties are investigated. Main results are listed as follow:1) Using sol-gel method to prepare the Bi(Fe1-x Nbx)O3 and Bi1-x Kx Fe1-x Nbx O3 samples without impurity phase. This provides a favorable precondition for the study on the crystal structure, magnetic and optical properties of the Bi Fe O3 doped with different valence ions.2) With a small mount of Nb doping, the Bi Fe O3 maintains R3 c structure while the lattice parameters a, c and the volume of the unit cell change slightly. The length of Fe-O bond increases and the length of Fe-Bi decreases with the increase of the doping concentration x, while the variation of the antirotation of the Fe-O6 octahedra is negligible. The XPS shows that both Nb+4 and Nb+5 ions coexists in the doped samples. The residual magnetization Mr enhances with the Nb doping, which could be attributed to the decreasing of particle size. The UV-Vis absorption spectra show that the band gap decreases with the increase of Nb doping concentration x.3) Similar to the Nb doped samples, the Bi1-x Kx Fe1-x Nbx O3 samples maintain R3 c structure and the lattice parameters a, c and the volume of the unit cell change slightly,although the the radium difference of Bi3+ and K+ is quite large. The length of Fe-O bond increases and the length of Fe-Bi decreases with the increase of the doping concentration x, while the variation of the antirotation of the Fe-O6 octahedra is negligible. Due to the valence compensation of Nb and K, the ratio of Fe2+ and Fe3+holds constantly, which is confirmed by XPS. It suggests that the variation of Bi-O bond is mainly due to the Nb dopant by comparing the Raman scattering spectra ofBi1-x Kx Fe1-x Nbx O3 samples with those of Bi1-x Kx Fe O3 samples. The magnetic properties of all samples enhance by doping. The residual magnetization Mr reaches its maximum value at the Nb and K doping concentration x = 0.01 and decreases as x > 0.01. The enhancement of the magnetic property at low doping concentration may be attributed to the existence of bound magnetic polaron in the doped samples.