Md. Motin Seikh

Synthesis and characterization of silicon carbide, silicon oxynitride and silicon nitride nanowires

Several methods have been employed to synthesize SiC nanowires. The methods include heating silica gel or fumed silica with activated carbon in a reducing atmosphere, the carbon particles being produced in situ in one of the methods. The simplest method to obtain β-SiC nanowires involves heating silica gel with activated carbon at 1360 °C in H2 or NH3. The same reaction, if carried out in the presence of catalytic iron particles, at 1200 °C gives α-Si3N4 nanowires and Si2N2O nanowires at 1100 °C. Another method to obtain Si3N4 nanowires is to heat multi-walled carbon nanotubes with silica gel at 1360 °C in an atmosphere of NH3. In the presence of catalytic Fe particles, this method yields α-Si3N4 nanowires in pure form.

An infrared spectroscopic study of the low-spin to intermediate-spin state (1A1–3T1) transition in rare earth cobaltates, LnCoO3 (Ln=La, Pr and Nd)

Low-spin (LS) to intermediate-spin (IS) state transitions in crystals of LnCoO3 (Ln=La, Pr and Nd) have been investigated by variable temperature infrared spectroscopy. The spectra reveal the occurrence of the transition around 120, 220 and 275 K, respectively, in LaCoO3,PrCoO3 and NdCoO3, at which temperatures the intensities of the stretching and the bending modes associated with the LS state decrease, accompanied by an increase in the intensities of the bands due to IS state. The characteristic frequencies of both the spin states decrease with increase in temperature, showing anomalies around the transition.

Crystalline silica nanowires

α-Cristobalite nanowires of 50-100 nm diameter with lengths of several microns have been synthesized for the first time by the solid-state reaction of fumed silica and activated charcoal. The nanowires have been characterized by X-ray diffraction, electron microscopy, photoluminescence, and Raman scattering. The nanowires are single crystalline as revealed by high-resolution electron microscope images. The crystalline nanowires are clad by an amorphous silica sheath when the carbon to fumed silica ratio in the starting mixture is small. Use of hydrogen along with Ar helps to eliminate the amorphous sheath.

Spectacular switching from ferrimagnetism to antiferromagnetism by zinc doping in “114” orthorhombic CaBaCo4O7

We report a spectacular switching from ferrimagnetism to antiferromagnetism in orthorhombic “114” CaBaCo4O7 by doping with a diamagnetic cation, Zn2+. Magnetic hysteresis loops, together with M(T) and χ′(T) curves at various frequencies, show that ferrimagnetism is abruptly converted to antiferromagnetism below T ∼ 80 K for less than 3% Zn doping, with the simultaneous appearance of local magnetic frustration. A model is proposed based on the ordered doping of Zn2+ at Co2+ sites in the ferromagnetic zig-zag chains of CaBaCo4O7, leading, by domino effect, to ferrimagnetic domains distributed at random and ordered at 180°. The boundaries of the junction of these domains contain Zn2+ and are magnetically frustrated. The crucial role of such ferromagnetic zig-zag chains for designing and tuning magnetic properties in oxides with a triangular metallic lattice is emphasized.

The ordered double perovskite PrBaCo2O6: Synthesis, structure, and magnetism

The stoichiometric layered perovskite cobaltite PrBaCo2O6 has been synthesized using an oxidative reaction of PrBaCo2O5.80 by sodium hypochlorite. The ferromagnetic properties of this oxide, which exhibits the highest TC of 210 K among the “112” layered cobaltites, are interpreted by double exchange mechanism. In contrast, the creation of oxygen vacancies in this framework leads for the oxides PrBaCo2O5+δ (0.80 ≤ δ < 1) to a strong competition between ferromagnetism and antiferromagnetism due to the appearance of superexchange Co3+—O—Co3+ antiferromagnetic interactions.

Complex magnetic phase separation induced by Li-doping in multiferroic CaBaCo4O7

The doping of lithium at the cobalt sites in the multiferroic orthorhombic cobaltite CaBaCo4O7 has been investigated. The oxides CaBaCo4-2xLixGaxO7 and CaBaCo4-xLixO7 keep the same polar space group as the parent phase. In contrast, they show a spectacular decrease of their ferrimagnetic properties for very low doping levels (0 < x ≤ 0.10), with the appearance of antiferromagnetism below TN ∼ 80 K and magnetic frustration at around Tf ∼ 20 K, which should have a great impact upon multiferroism in this system. This behavior is different from that of the Ga-doped phases CaBaCo4-xGaxO7, but remarkably similar to that of the Zn-doped phases CaBaCo4-xZnxO7. The actual role of the valence of the diamagnetic cation, Li+ is interpreted from its ability to sit in the Co2+ zig-zag chain, breaking the ferromagnetism along those chains in both series of oxides. Importantly, the Li-doping in CaBaCo4-xLixO7 oxides, introduces an excess Co3+ with respect to the other two series, CaBaCo4-xZnxO7 and CaBaCo4-2xLixGaxO7. Th...

Quintuple perovskites Ln2Ba3Fe5−xCoxO15−δ (Ln = Sm, Eu): nanoscale ordering and unconventional magnetism

Quintuple perovskites Ln2Ba3Fe5−xCoxO15−δ, with Ln = Sm, Eu have been synthesized for x values varying from 0 to 2. The HRTEM and HAADF investigations show that these oxides are ordered at the nanoscale with a tetragonal “ap × ap × 5ap” lattice, corresponding to an ordered fivefold stacking of the Ba and Ln layers. These structures are in fact chemically twinned, so that only cubic or pseudo-cubic symmetry is detected by XRD investigation. The detailed magnetic study shows the existence of short range antiferromagnetic ordering, canted in nature in the temperature range from 5 to 330 K. This particular behavior is interpreted on the basis of local antiferromagnetic M–O–M (M = Fe/Co) interactions within the nanodomains that are limited by pinning of the Co/Fe spins at the boundaries.

Closely related magnetic and dielectric transitions in the “114” magnetoelectric Zn-doped CaBaCo4O7

We report on the impact of zinc doping upon the magneto-electric properties of CaBaCo4O7.We show that the presence of less than 1% Zn at the Co sites decreases TC from 64 K to 48 K and generates two magnetic transitions, concomitant with changes of slope of dielectric permittivity, at 54 K and 77 K, respectively. Powder neutron diffraction and magnetic data show the existence of an incommensurate magnetic phase and a spin-flop transition in this temperature range. The crossover of the magnetodielectric effect of this oxide, from positive below 50 K to negative above this temperature emphasizes the primordial role of the incommensurate magnetic phase in the evolution of these properties.

Incoherent effect of Fe and Ni substitutions in the ferromagnetic-insulator La0.6Bi0.4MnO3+δ

A comparative study of the effect of Fe and Ni doping on the bismuth based perovskite La0.6Bi0.4MnO3.1, a projected spintronics magnetic semiconductor has been carried out. The doped systems show an expressive change in magnetic ordering temperature. However, the shifts in ferromagnetic transition (TC) of these doped phases are in opposite direction with respect to the parent phase TC of 115 K. The Ni-doped phase shows an increase in TC ∼200 K, whereas the Fe-doped phase exhibits a downward shift to TC∼95 K. Moreover, the Fe-doped is hard-type whereas the Ni-doped compound is soft-type ferromagnet. It is observed that the materials are semiconducting in the ferromagnetic phase with activation energies of 77 & 82 meV for Fe & Ni-doped phases, respectively. In the presence of external magnetic field of 7 T, they exhibit minor changes in the resistivity behaviors and the maximum isothermal magnetoresistance is around −20% at 125 K for the Ni-phase. The results are explained on the basis of electronic phase s...

Magnetic and electronic properties of Eu0.9Ca0.1BaCo2O5.5+δ with the variation of oxygen stoichiometry

The effect of the oxygen content on the magnetic and transport properties of the ferromagnetic Eu0.9Ca0.1BaCo2O5.5+δ has been investigated. Unlike the increase in TC with calcium content, paradoxically the TC value decreases as the oxygen content increases, as observed in the undoped phase. Assuming the Kanamori-Goodenough rules apply to this very complex magnetic system, these experiments suggest that the Co3+ disproportionation into Co4+ and Co2+, plays an important role in the appearance of ferromagnetism. The magnetic phase separation scenario of ferromagnetic domains embedded in an antiferromagnetic matrix play an important role in the ferromagnetism of the calcium doped “112” cobaltites. All the samples covering a wide range of oxygen content, exhibit p-type conductivity.