The structural and magnetic properties of dual phase cobalt ferrite

The bismuth (Bi³⁺)-doped cobalt ferrite nanostructures with dual phase, i.e. cubic spinel with space group Fd3m and perovskite with space group R3c, have been successfully engineered via self-ignited sol-gel combustion route. To obtain information about the phase analysis and structural parameters, like lattice constant, Rietveld refinement process is applied. The replacement of divalent Co²⁺ by trivalent Bi³⁺ cations have been confirmed from energy dispersive analysis of the ferrite samples. The micro-structural evolution of cobalt ferrite powders at room temperature under various Bi³⁺ doping levels have been identified from the digital photoimages recorded using scanning electron microscopy. The hyperfine interactions, like isomer shift, quadrupole splitting and magnetic hyperfine fields, and cation distribution are confirmed from the Mossbauer spectra. Saturation magnetization is increased with Bi³⁺-addition up to x = 0.15 and then is decreased when x = 0.2. The coercivity is increased from 1457 to 2277 G with increasing Bi³⁺-doping level. The saturation magnetization, coercivity and remanent ratio for x = 0.15 sample is found to be the highest, indicating the potential of Bi³⁺-doping in enhancing the magnetic properties of cobalt ferrite.

Band filling effect on polaron localization in La1-x(Ca(y)Sr1-y)xMnO3 manganites

We report on a ìSR and 55 Mn NMR investigation of the magnetic order parameter as a function of temperature in the optimally doped La(5/8)(CaySr(1.y))(3/8)MnO3 and in the underdoped La1.xSrxMnO3 and La1.xCaxMnO3 metallic manganite families. The study is aimed at unravelling the effect of lattice distortions, implicitly controlled by the Ca-Sr isoelectronic substitution, from that of hole doping x on the Curie temperature TC and the order of the magnetic transition. At optimal doping, the transitions are second order at all y values, including the y = 1 (La(5/8)Ca(3/8)MnO3) end member. By contrast, they are first order in the underdoped samples, which show a finite (truncated) order parameter at the Curie point, including La(0.75)Sr(0.25)MnO3 whose TC is much higher than that of La(5/8)Ca(3/8)MnO3. The order parameter curves, on the other hand, exhibit a very minor dependence on x, if truncation is excepted. This suggests that the effective exchange interaction between Mn ions is essentially governed by local distortions, in agreement with the original double-exchange model, while truncation is primarily, if not entirely, an effect of under- or over-doping. A phase diagram, separating in the x.y plane polaron-driven first order transitions from regular second order transitions governed by critical fluctuations, is proposed for the La(1.x)(CaySr(1.y))xMnO3 system.