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We report the structural and optical properties of xSnO2–yFe2O3 nanocrystalline composite thin films. SnO2 and Fe2O3 exhibit strong phase separation instability and their particle size and crystallinity are tunable by changing their composition and annealing temperature. The bandgap for these composites continuously increases from 2.3 to 3.89 eV. We discuss the increasing bandgap values in terms of the quantum confinement effect manifested by the decreasing size of Fe2O3 crystallites. The method provides a generic approach for the tuning of the bandgap in nanocomposite systems.

Rate of heat generated by magnetic nanoparticles in a ferrofluid is affected by their magnetic properties, temperature, and viscosity of the carrier liquid. We have investigated temperature dependent magnetic hyperthermia in ferrofluids, consisting of dextran coated superparamagnetic Fe 3 O 4 nanoparticles, subjected to external magnetic fields of various frequencies (188-375 kHz) and amplitudes (140-235 Oe). Transmission electron microscopy measurements show that the nanoparticles are polydispersed with a mean diameter of 13.8 6 3.1 nm. The fitting of experimental dc magnetization data to a standard Langevin function incorporating particle size distribution yields a mean diameter of 10.6 6 1.2 nm, and a reduced saturation magnetization ($65 emu/g) compared to the bulk value of Fe 3 O 4 ($95 emu/g). This is due to the presence of a finite surface layer ($1 nm thickness) of non-aligned spins surrounding the ferromagnetically aligned Fe 3 O 4 core. We found the specific absorption rate, measured as power absorbed per gram of iron oxide nanoparticles, decreases monotonically with increasing temperature for all values of magnetic field and frequency. Using the size distribution of magnetic nanoparticles estimated from the magnetization measurements, we have fitted the specific absorption rate versus temperature data using a linear response theory and relaxation dissipation mechanisms to determine the value of magnetic anisotropy constant (28 6 2 kJ/m 3) of Fe 3 O 4 nanoparticles. V

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We report an experimental investigation of time dependent anisotropic light scattering by an aqueous suspension of tetramethyl ammonium hydroxide coated Fe3O4 nanoparticles (approximately 6 nm) under the ON-OFF transient of an external dc magnetic field. The study employs the synchronized recording and measurement of the two magnetic-field-induced light-scattering patterns produced by two identical orthogonal He-Ne laser beams passing through the ferrofluid sample and propagating parallel and perpendicular to the applied field, respectively. From these patterns, we extract the time dependence of the induced optical anisotropy, which provides a measure of the characteristic time scale and kinematic response for field-induced structure formation in the sample. We propose that the time evolution of the scattering patterns, which is very fast at short times and significantly slower at long times, can be explained using a model based on a two-stage chain formation and coarsening processes.

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We have investigated the ac magnetic susceptibility and magnetic heating of aqueous suspensions of γ-Fe2O3 nanoparticles embedded in alginate hydrogel matrix and isolated γ-Fe2O3 and Fe3O4 nanoparticles coated with tetramethyl ammonium hydroxide. All three ferrofluids were characterized by measuring the dc magnetization, ac susceptibility, and magnetic heating. We found that significant Néel relaxation is present in all samples, but only the isolated nanoparticle ferrofluids show any significant feature associated with Brownian relaxation near the freezing temperature of the carrier liquid. The heating rate of the ferrofluids varies systematically with the magnitude of the Brownian relaxation peak, despite similar values of the absolute magnetization. These results highlight the importance of the Brownian relaxation for heating applications incorporating magnetic nanoparticles.