Oxygen octahedra distortion induced structural and magnetic phase transitions in Bi1−xCaxFe1−xMnxO3 ceramics

Abstract

The co-doping of Ca and Mn in respective Bi and Fe-sites of BiFeO3 lattice leads to structural transition from rhombohedral (R3c space group) to orthorhombic (Pbnm space group) crystal symmetry. The tilt angle for anti-phase rotation of the oxygen octahedra of BiFeO3 at room temperature is observed to be ∼13.8°. It decreases with the increase in the co-doping percentage which suggests the composition-driven structural phase transition. The remnant magnetization for sample with 15% of co-doping becomes about 16 times that of BiFeO3. It may be attributed to the suppression of cycloid spin structure and uncompensated spins at the surface of nanocrystallites. Further increase in co-doping percentage results in the sharp reduction of remnant magnetization due to the dominant contribution from the collinear antiferromagnetic ordering in the Pbnm space group. The Arrott plot analysis clearly indicates the composition-driven crossover from the antiferromagnetic to weak ferromagnetic ordering and vice versa. Electron spin resonance results provide the evidence for the composition-driven phase transitions from an incommensurate spin cycloidal modulated state to one with nearly homogeneous spin order. The band gap (2.17 eV) of BiFeO3 measured using UV-Vis spectra was supported by the resonance Raman spectra.

Saturation magnetization and band gap tuning in BiFeO3 nanoparticles via co-substitution of Gd and Mn

“…However, during annealing at 600°C the citrate complexing agents evaporate at above 300°C [14] and the ultra-fine BFO nanocrystals due to high surface energy form neck by solid state diffusion and evaporation-condensation process which leads to agglomeration and particle growth [20]. Nevertheless, both the crystallite hinder crystallite nucleation and thereby reduces particle size [21]. hν plots for the synthesized samples ( Fig.…”

Substitution driven structural and magnetic properties and evidence of spin phonon coupling in Sr-doped BiFeO3 nanoparticles

“…Using 514.5 nm (2.41 eV) laser source, which is around 0.2 eV higher than the band gap value, the intensity of the second order phonon modes at 1255 cm À1 is of the same order as that of the corresponding fundamental modes. 20,42 The intensity of the second order modes around 1255 cm À1 , observed by using the laser source of 785 nm (1.58 eV). Laser source, is negligible because it does not have enough energy for direct and indirect electronic transitions.…”

“…It is important to mention that the intensity variations observed in some vibrational modes are exclusively related to the atoms taking part in the electronic transition in the resonance Raman phenomenon. 42 In BiFeO 3 ceramics, the top of the valence band is composed of strong hybridization among O 2p and Bi 6p and also the bottom of the conduction band comprises of Fe 3d states. The optical band gap of the BiFeO 3 ceramics is due to the hybridization between the Fe 3d and O 2p states.…”

“…These facts could mean that when the energy of the laser source is sufficient to promote an-electronic transition from the valence band to the lower conduction band in BiFeO 3 , a perturbation occurs in the electronic density of the 3d orbitals of the Fe atoms. 42 It consequently changes the polarizability of iron atoms resulting in enhancement of the intensity of 218 cm À1 mode. These results conrm that electronic transitions take place under excitation with 514.5 nm laser radiations, allowing the resonance effect.…”

Structural transition and magnetic properties of Mn doped Bi0.88Sm0.12FeO3 ceramics

“…6,7,9,19 Moreover, the enhancement of magnetization at the MPB of BFO-based compounds can also be fullled by considering the contribution of PB ferromagnetism. 13,[21][22][23] To date, numerous attempts have been made to study the effects of Sm and Mn substitution on the crystal structure and on the ferroelectric and magnetic properties of BFO multiferroic. [24][25][26][27][28] However, these reports do not provide a clear understanding of the correlation between the coexistence phase and the ferroelectric/ferromagnetic properties of Mn doping on Bi 1Ày Sm y FeO 3 compounds.…”