Ferromagnetic Properties Of BiFeO₃ Nanoparticles Prepared By Sol-gel Method

Multiferroics are multifunctional materials which exhibit coexistence of ferroelectricity (or antiferroelectricity), ferromagnetism (or antiferromagnetism) and ferroelasticity. As a typical single-phase multiferroic, bismuth ferrite (BiFeO3) of perovskite structure is one of few candidates exhibiting room-temperature ferroelectric and G-type antiferromagnetic properties. The magnetic structure of BiFeO3 is antiferromanetic and the magnetic phase transition occurs at TN ~ 643K. However, with the help of neutron scattering measurement, the antiferromagnetic spin order of BiFeO3 has an incommensurate spiral spin structure (the period length of ~62 nm), which does not allow a net magnetization in bulk BFO.In this dissertation, BiFeO3 nanoparticles were prepared by sol-gel method with tartaric acid used as chelant. The structure, composition and magnetic properties of BiFeO3 nanoparticles were studied in details. The main contents in the dissertation are listed in the following:1) BiFeO3 nanoparticles have been synthesized by a sol-gel method with different gel-drying temperatures at 80oC and 150 oC respectively. X-ray diffraction (XRD) patterns showed that both samples formed similar single phase of perovskite structure after annealing at 550oC. However, much higher magnetization was observed in the sample derived from higher gel-drying temperature. Combined with transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) measurements, it was confirmed that the enhanced magnetization was attributed to the presence ofγ-Fe2O3 secondary phase, which was actually nucleated during the process of gel-drying and remained in the subsequent annealing process. Our results showed that the gel-drying temperature was critical for the formation ofγ-Fe2O3 nuclei during the sol-gel fabrication of BiFeO3 nanoparticles. This work has been submitted to Materials Chemistry and Physics.2) BiFeO3 nanoparticles was prepared by sol-gel method with assistance of ultrasonic radiation. The magnetization of such sample was higher than that of BiFeO3 nanoparticles synthesized without ultrasonic radiation. By annealing the samples in oxygen atmosphere and vacuum, it was confirmed that the enhancement of magnetization could be attributed to the combined effect of oxygen vacancies and particle sizes. This work was submitted to Nanotechnology.3) Bi1?xEuxFeO3 (BEFOx, x=0, 0.05, 0.075, 0.10 and 0.15) nanoparticles were prepared by a sol-gel method followed by rapid liquid-phase sintering process. XRD, TEM, Raman scattering spectra, Fourier transform infrared spectra and XPS were used to identify the effect of Eu dopant on structural and composition of the samples. It was found that the doping of Eu has caused noticeable lattice distortion of samples. An enhancement in magnetization for BEFOx samples was also observed with the increase of Eu doping concerntrations, which was ascribed mainly to lattice distortion and the contribution of the magnetically active characteristic of Eu3+ ions. This work was published in Applied physics letter and embodied by Virtual Journal of Nanoscale Science & Technology.