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(iii). Magnetic Nanostructures:
1. Generation and characterization of 1-D multiferroic BiFeO3 nanotubes (e.g., 100 and 200 nm diameter) using a pressure-filter variation of a template synthesis involving the sol-gel technique. Ref.: Chem. Commun., (23), 2708 (2004).
2. A systematic series of single-crystalline BiFeO3 nanoparticles, prepared using a sol-gel synthesis and ranging from ~14 to >300 nm in diameter, show strong size-dependent magnetic properties that correlate with: (a) increased suppression of the known spiral spin structure (period length of ~62 nm) with decreasing nanoparticle size and (b) uncompensated spins and strain anisotropies at the surface. Zero-field-cooled and field-cooled magnetization curves exhibit spin-glass freezing behavior due to a complex interplay between finite size effects, interparticle interactions, and a random distribution of anisotropy axes in our nanoparticle assemblies. Ref.: Nano Letters, v.7, 766 (2007).
3. Single-crystalline, submicron-sized Bi2Fe4O9 cubes of reproducible shape were prepared using a facile, large-scale molten salt reaction in the presence of a nonionic surfactant. The role of surfactant as well as alterations in the molar ratio of Bi3+ to Fe3+ precursors were correlated with the predictive formation of different shapes of Bi2Fe4O9 products. Ref.: J. Mater. Chem. (Hot Article; inside cover), v.15, 2099 (2005).
4. Monodisperse nanocrystalline rhombohedral composites of Fe and Fe3O4 magnetic materials have been obtained employing a reduction reaction, in a flowing gas mixture of H2 and N2, of single-crystalline, submicron-sized α-Fe2O3 rhombohedral precursors. This synthesis is significant in that we were able to create a nanocomposite with hard and soft magnetic phases juxtaposed within one discrete, anisotropic structure. In turn, the precursor hematite rhombohedra of reproducible shape were successfully prepared using a facile, large-scale molten-salt reaction. Rhombohedra represent a high-surface-area, anisotropic formulation of an industrially important material (iron oxide) which is an active component of gas sensors, photocatalysts, and other types of catalytic materials. Ref.: Chem. Mater., v.18, 5289 (2006).
5. We have reported on the Mössbauer spectra and magnetization properties of single crystalline (BiFeO3)x-(BaTiO3 )1-x solid solution nanostructures in the form of nanocubes, measuring approximately 150 nm to 200 nm on a side, prepared by a molten salt solid-state reaction method in the compositional range wherein 0.5 ≤ ‘x’ ≤ 1. Powder X-ray diffraction (XRD) and monochromatic synchrotron XRD studies indicate products of high purity, which undergo gradual, well-controlled structural transformations from rhombohedral to tetragonal structures with decreasing ‘x’. For all solid solution products, room temperature magnetization studies exhibit hysteretic behavior with remnant magnetization values of Ms ≥ 0.32 emu/g, indicating that the latent magnetization locked within the toroidal spin structure of BiFeO3 has been released. Room-temperature Mössbauer spectra show composition-dependent characteristics with decreasing magnetic hyperfine field values and increasing absorption line widths due to a decrease in the magnetic exchange interaction strength with decreasing ‘x’. For the lowest ‘x’ = 0.5 composition studied, the Mössbauer spectra show paramagnetic behavior, indicating a Néel temperature for this composition below 300 K. However, room temperature magnetization studies with applied fields of up to 50 kOe show hysteretic behavior for all compositions, including the ‘x’ = 0.5 composition, presumably due to field-induced ordering. Furthermore, hysteresis loops for all compositions exhibit smaller coercivities at 10 K than at 300 K, an observation that may suggest the presence of magnetoelectric coupling in these systems. Ref.: Phys. Rev. B, 82(2), 024431/1-024431/10 (2010).
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