M. Firoz Pervez

Cu₂OSeO₃ is a chiral magnet with a rich phase diagram. The positions of the phase transitions are sensitive to chemical substitution, however the exact mechanism by which dopants influence magnetic behavior remains elusive. In this work, Cu₂OSe_1 − xTe_xO₃ with (0 ≤ x ≤ 0.1) is synthesized with the lattice expanding upon Te inclusion. Our SANS and magnetometry data indicate that doping lowers the paramagnetic to helical ordering temperature and the critical field required for the conical to field polarized phase transition. Furthermore, the stability region of the skyrmion lattice also expands. We present a universality class analysis on the materials using the Heat-map Modified Iteration Method to evaluate the critical behavior and show that the Heisenberg to Ising transition is robust to chemical doping. Additionally, we demonstrate that the change to both critical temperature and critical field to be due to decreases in the strengths of the Dzyaloshinskii-Moriya and Symmetric Exchange Interactions upon doping.

Magnetic relaxation in a nanoparticles system depends on the intra-particle interactions, reversal mechanism, the anisotropy field, easy axis distribution, particle volume, lattice defects, surface defects, materials composite, etc. Here we report the competing magnetic states between superparamagnetic blocking and Néel transition states in 14 nm core–shell NiO nanoparticles. A crossover temperature of 50 K was observed for both these states from the zero field cooled/field cooled magnetization curves taken at different fields. At crossover temperature, an interesting M – H loop splitting is observed which is attributed to the slow spin relaxation. This anomalous M – H loop splitting behaviour was found to be particle size dependent and suppressed for diameters above and below 14 nm which indicates a critical size for these competing magnetic states. Additional neutron diffraction experiments confirmed this observation. This experimental study provides a new insight for the understanding of intra-particle interactions in fine antiferromagnetic nanoparticles and obtained results are an important step towards deeper understanding of the competing/non-competing modes between superparamagnetic blocked and Néel transition states.

Use of Degussa P25 titanium-dioxide nanopowder in dye-sensitized solar cell (DSSC) photoanode improves efficiency of the DSSC cell. Annealing of titanium dioxide is required for fabrication of crystalline mesoporous thin film photoanode on transparent conducting glass using doctor blade method. Different annealing temperatures provide different structural, morphological, and optical properties of the photoanode, which may influence the efficiency of the cell. In this paper, energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and UV-Vis-NIR spectroscopicanalysis have been carried out to investigate annealing temperature effect on various structural parameters, mole-fraction, phase-content, and optical bandgap of the TiO₂film photoanode. It was observed that depending on annealing temperature, theratio of polymorphs of Degussa P25 changed substantially. For the change in annealing temperature from 350 °C to 600 °C, variations occurred in crystallite size from 11.9 nm to 24.9 nm, strain from 0.006 to 0.014, specific surface area from 62.77 m²·g^-1to 125.74 m²·g^-1, morphology index from 0.49 to 0.64, dislocation density from 5 × 10¹³line/m²to 8 × 10¹⁵line/m², crystallite per unit surface area from 2 × 10¹³m^-2to 2.5 × 10¹⁴m^-2, and optical bandgap from 2.4 eV to 3.1 eV.

Zinc oxide thin films with different thicknesses were prepared on microscopic glass slides by sol-gel spin coating method, then hydrothermal process was applied to produce zinc oxide nanorod arrays. The nanorod thin films were characterized by various spectroscopic methods of analysis. From the images of field emission scanning electron microscope (FESEM), it was observed that for the film thickness up to 200 nm the formed nanorods with wurtzite hexagonal structure were uniformly distributed over the entire surface substrate. From X-ray diffraction analysis it was revealed that the thin films had good polycrystalline nature with highly preferred c-axis orientation along (0 0 2) plane. The optical characterization done by UV-Vis spectrometer showed that all the films had high transparency of 83 % to 96 % in the visible region and sharp cut off at ultraviolet region of electromagnetic spectrum. The band gap of the films decreased as their thickness increased. Energy dispersive X-ray spectroscopy (EDS) showed the presence of zinc and oxygen elements in the films and Fourier transform infrared spectroscopy (FT-IR) revealed the chemical composition of ZnO in the film.