Effect of Co substitution on the magnetic properties of BiFeO3

Bismuth ferrite is doped with a dilute concentration of cobalt, _{BiFe1−XCoXO3}${\mathrm{BiFe}}_{1-X}{\mathrm{Co}}_X{\mathrm{O}}_3$; X=0, 0.01, 0.02 is prepared by sol-gel auto combustion technique. X-ray diffraction data refined via Reitveld method shows single phase and shrinkage in cell volume for Co doped BiFeO₃. Various magnetic ground states viz. superparamagnetic, glassy antiferromagnetic and glassy ferrimagnetic behavior is observed for X=0, 0.01, 0.02, respectively. A first-order magnetic transition is observed in the Arrott plot data of cobalt doped BiFeO₃. Possibility of thermally induced magnetic transition is also seen in the magnetization data of cobalt doped BiFeO₃. A model based on the existing spinoidal cyclic arrangement of spins is proposed to explain the observed data. Finally, a very dilute concentration of Co⁺³${\mathrm{Co}}^{+3}$ in BiFeO₃ is found sufficient to tailor the magnetic properties.     

Electrical behavior of high resistivity Ce-doped BiFeO3 multiferroic

Bi_1+xCe_xFeO₃ (Ce–BFO) for x=0, 0.05, 0.1, and 0.15 monophasic ceramic samples were successfully synthesized by conventional solid-state reaction routes. The influences of Ce doping on structural, dielectric, ferroelectric, leakage current and capacitive properties of BiFeO₃ ceramics were investigated intensively. At higher concentrations of x (x=0.1 and 0.15) the samples showed good crystallinity with almost impurity free phases. No structural phase transformation took place after partial doping of Ce ions and all ceramic bulk samples remain in their rhombohedral structure with space group R3c. The dielectric behavior of the samples improved significantly and the ferroelectric hysteresis loops changed their shape from rounded to a strongly nonlinear typical ferroelectric feature mainly originating from the domain switching and became enhanced with increase in doping concentration of cerium (Ce). Experimental results also suggested that partial doping of higher valence, smaller ionic radius Ce ions in BiFeO₃ forces the reduction of oxygen vacancies, resulting in a great suppression of leakage current. It is found that the sharp capacitance peak/discontinuity present in the C–V characteristics of Ce–BFO for different Ce doping concentrations is directly associated with the polarization reversal. Incorporation of excess bismuth in the presence of Ce in BiFeO₃ is expected to compensate Bi loss during high temperature sintering and caused structural distortion which also favors enhancement of ferroelectric properties in Ce-doped BFO.     

Enhancement of ferromagnetism and multiferroicity in Ho doped Fe rich BiFeO3

Crystalline solid solution of BiHo_XFe_1+XO₃ (X=0, 0.05, 0.1, and 0.15) ceramics have been successfully synthesized by slow step sintering schedule. At particular value of X (X=0.1) the sample showed good crystallinity with almost impurity free phases. It was observed that at particular concentration of X (X=0.1), the sample showed enhanced M–H loop by Ho doping in presence of excess Fe. Furthermore, the M–H loop gets reduced with the increase in value of X (X=0.15). Compared to the pure BFO, the dielectric properties for X=0.1 composition are improved due to the decreased oxygen vacancies by the stabilized oxygen octahedron. Raman spectra over the frequency range of 100–1500 cm^?1 have been systematically investigated with different concentrations of X. Besides the changes of the peak position and the line width of the all modes, the prominent frequency shift, the line broadening and variation of the intensity for the two-phonon mode between 1270 and 1280 cm^?1 were observed with increase in value of X. All these results indicate the existence of strong spin–phonon coupling in Ho doped Fe rich BFO.     

Total syntheses of 28,29-diepi-arenamide A, 29-epi-arenamide A, and 28-epi-arenamide A

This communication describes the synthesis of stereochemical analogs of arenamide A, a 19-membered cytotoxic depsipeptide isolated from the fermentation broth of a marine bacterial strain Salinispora arenicola. The key steps are diastereoselective aldol reaction, Mitsunobu reaction, and HATU mediated macrolactamization.     

Some Magnetized Bulk Viscous String Cosmological Models in Cylindrically Symmetric Inhomogeneous Universe with Variable ∧-Term

Some cylindrically symmetric inhomogeneous viscous fluid string cosmological models with magnetic field and cosmological term A varying with time are investigated. To get the deterministic solution, it has been assumed that the expansion （θ） in the model is proportional to the eigen value σ^11 of the shear tensor σ^ij. The value of cosmological constant for the model is found to be small and positive, which is supported by the results from recent supernovae Ia observations. The effect of bulk viscosity is to produce a change in perfect fluid and hence exhibits essential influence on the character of the solution. The physical and geometric properties of the models are also discussed in presence and absence of magnetic field.     

Cyclodextrin-mediated regioselective photo-Fries reaction of 1-naphthyl phenyl acylates

1-Naphthyl phenyl acylates upon irradiation in solution yield eight products via β-cleavage process. However, excitation of these molecules as included in γ-cyclodextrin results in a single product (>95%). This medium dependent product selectivity is attributed to conformational and translational restrictions enforced on the reactant as well as intermediates by the cyclodextrin cavity.     

Cylindrically symmetric inhomogeneous cosmological models with viscous fluid and varying Λ

Cylindrically symmetric non-static cosmological models representing a bulk viscous fluid distribution have been obtained which are inhomogeneous and anisotropic. Without assuming anyadhoc law, we obtain a cosmological constant as a decreasing function of time. Various physical and geometrical features of the models are also discussed.     

Geometry effect on the nonlinear magnetic response of YBa2Cu3O7−y

Results are presented for study of nonlinear magnetization of a sintered YBa₂Cu₃O_7−y pellet of different thickness of a sample subjected to very low magnetic field. On cooling the sample belowT _c in zero field a change, in the oscillatory structure of harmonics in increasing dc field is observed in very low ac magnetic field. The effect of finiteness of the sample on the oscillatory structure and on the hysteresis of harmonics is also studied. The results are explained qualitatively.