Local geometric and electronic structures and origin of magnetism in Co-doped BaTiO3 multiferroics

Abstract

We have prepared polycrystalline samples BaTi1−xCoxO3 (x = 0–0.1) by solid-state reaction. X-ray diffraction and Raman-scattering studies reveal the phase separation in crystal structure as changing Co-doping content (x). The samples with x = 0–0.01 are single phase in a tetragonal structure. At higher doping contents (x > 0.01), there is the formation and development of a secondary hexagonal phase. Magnetization measurements at room temperature indicate a coexistence of paramagnetic and weak-ferromagnetic behaviors in BaTi1−xCoxO3 samples with x > 0, while pure BaTiO3 is diamagnetic. Both these properties increase with increasing x. Analyses of X-ray absorption spectra recorded from BaTi1−xCoxO3 for the Co and Ti K-edges indicate the presence of Co2+ and Co3+ ions. They locate in the Ti4+ site of the tetragonal and hexagonal BaTiO3 structures. Particularly, there is a shift of oxidation state from Co2+ to Co3+ when Co-doping content increases. We believe that the paramagnetic nature in BaTi1−xCoxO3 samples is due to isolated Co2+ and Co3+ centers. The addition of Co3+ ions enhances the paramagnetic behavior. Meanwhile, the origin of ferromagnetism is due to lattice defects, which is less influenced by the changes caused by the variation in concentration of Co2+ and Co3+ ions.

Resistance-temperature characteristics of a new high-temperature thermistor ceramics of Mn-doping Ba–Ca–Zr–Ti–O system

“…The intensity and sharpness of the spectra have rather decreased in the sample of x = 0.100, which confirms that the phase transition is observed in the XRD results, also confirms that the different vibration bands in Raman spectra of the Mn-doped BCZT samples are in agreement with the changes in the phase contents of the corresponding XRD patterns [23]. Compared with pure BCZT ceramics, a significant shift exists at * 740 cm -1 in BCZTM (x = 0.010) with a sharp peak at * 700 cm -1 , which is probably a consequence of defects caused by Mn-doping [24].…”

Structural, optical and conductivity properties in tetragonal BaTi1−xCoxO3 (0≤ x ≤0.1)

“…They believed that the system of isolated paramagnetic centers of Co 4+ ions produced a paramagnetic state in the samples. It was in good agreement with the work reported by Liu et al 9 Meanwhile, Phan et al 10 observed the co-existence of paramagnetic and weak ferromagnetic behavior in BaTi 1− x Co x O 3 samples (0.0< x ≤0.1). Using X-ray absorption spectra data, they explained that the paramagnetic nature was due to isolated Co 2+ and Co 3+ centers, in which the Co 3+ ions played a dominant role in enhancing the paramagnetic behavior.…”

“…[14][15][16][17][18][19] Li et al 15 observed that the tetragonal structure of BaTi 1Àx Co x O 3 (0.01# x #0.20) samples only exist at x #0.03, and it is transformed gradually to a hexagonal structure when Co content increases up to the value of 0.05. Phan et al 10 observed that tetragonal-hexagonal transformation occurs at x ¼ 0.01 in BaTi 1Àx Co x O 3 (0.0< x #0.1). However, a few works paid attention investigating the optical, dielectric characteristics, and conductivity mechanism of Co-doped BTO.…”

Band gap narrowing and magnetic properties of transition‐metal‐doped Ba0.85Ca0.15Ti0.9Zr0.1O3 lead‐free ceramics

“…The modes around 530 cm −1 are mainly derived from the vibrations of oxygen displacements, and the modes beyond 700 cm −1 are known to correlate with the presence of oxygen vacancies. In BCZTM and BCZTC, the peaks at ~ 700 cm −1 are sharp, which may originate from the defects induced by Mn and Co doping . Moreover the asymmetry and disorder of the crystal structure can be inferred from the broadening and offset of Raman spectra, which are caused by the phase transition…”

“…In BCZTM and BCZTC, the peaks at ~ 700 cm −1 are sharp, which may originate from the defects induced by Mn and Co doping. 31 Moreover the asymmetry and disorder of the crystal structure can be inferred from the broadening and offset of Raman spectra, which are caused by the phase transition. 32 The SEM images of the sintered BCZT ceramics, as shown in Figure 3, further demonstrate the surface morphologies.…”