研究鐵酸鉍經由摻雜後的晶格與磁性結構 --- 塊材，奈米顆粒與薄膜

Abstract

In the dissertation, we investigate the mutiferroic bismuth ferrites Bi1−xDyxFeO3 in various forms, including bulk, nanoparticle, and thin film in order to understand the general relation between crystal structure and spin structure. In particular, the crystalline and spin structures are studied with X-ray powder diffraction (XRD), magnetization, scanning electron microscope (SEM) images, electron spin resonance (ESR), and neutron powder diffraction (NPD). The crystal structure for the bulk samples of bismuth ferrites Bi1−xDyxFeO3 with x = 0 ~ 0.05 are indexed based on the rhombohedral space group R3c in the XRD patterns. With x = 0.30 and 0.40, the structure further transforms to the orthorhombic group Pbnm. Weak magnetism of Bi1−xDyxFeO3 is studied via the electron spin resonance (ESR) of X-band (9.53 GHz) at various temperatures. The g-factor of pure BiFeO3 is 2.0, which originates from its cycloidal spin structure; while for the doped Bi1−xDyxFeO3 samples with x > 0.10, ESR spectra reveal a second phase with a different g-factor around 1, which is attributed to the homogeneous magnetized phase of Bi1−xDyxFeO3. Temperature dependent of ESR and neutron data further suggest a spin-reorientation at 140 and 200 K. For nanoparticles of Bi1−xDyxFeO3, it is evidential that Dy-doping can lead to suppression of grain size, and the diameter of particle d plays an important role in nanoparticles. The linearity between magnetization and 1/d indicates that the magnetic anisotropy constant Keff does not violate the magnetic anisotropy model. The data shows a great increase of magnetization M without following the linearity in M vs. 1/d at x = 0.4. There exists a critical size dc correlated with magnetic anisotropy constant and exchange constant. When dc is smaller than the cycloid spin wavelength (62 nm), the exchange constant in nanoparticle is enhanced. The ESR spectra of nanoparticles are observed and the resonance field (or g-factors) is different from that of bulk samples. In the ESR spectra of BiFeO3 thin film, there are six sharp lines observed for different resonance field (Hr). These lines are assigned to the five in-plane spin wave (SW) resonances, because the values of Hr have a linear relation with the square of spin wave index n. However, the in-plane mode of n = 5 only appears at 110 K. An unknown mode appears at temperatures of 110 K and 170 K which may be related to the spin reorientation temperatures of Fe3+ magnetic moment.