Arthur J. Viescas

Size- and composition-dependent radio frequency magnetic permeability of iron oxide nanocrystals.

We investigate the size- and composition-dependent ac magnetic permeability of superparamagnetic iron oxide nanocrystals for radio frequency (RF) applications. The nanocrystals are obtained through high-temperature decomposition synthesis, and their stoichiometry is determined by Mössbauer spectroscopy. Two sets of oxides are studied: (a) as-synthesized magnetite-rich and (b) aged maghemite nanocrystals. All nanocrystalline samples are confirmed to be in the superparamagnetic state at room temperature by SQUID magnetometry. Through the one-turn inductor method, the ac magnetic properties of the nanocrystalline oxides are characterized. In magnetite-rich iron oxide nanocrystals, size-dependent magnetic permeability is not observed, while maghemite iron oxide nanocrystals show clear size dependence. The inductance, resistance, and quality factor of hand-wound inductors with a superparamagnetic composite core are measured. The superparamagnetic nanocrystals are successfully embedded into hand-wound inductors to function as inductor cores.

Electronic and Magnetic Characterization of in vivo Produced vs. in vitro Reconstituted Horse Spleen Ferritin

Magnetic nanophases nucleated within horse spleen apoferritin nanotemplates under in vivo physiological conditions and in vitro reconstitution were characterized by Mossbauer spectroscopy in lyophilized form. Mossbauer spectra recorded at 80 K indicate that for the in vivo produced ferritin the presence of phosphates within the ferritin biomineral core results in larger quadrupole splittings, both at interior and surface sites, 0.62 mm/s and 1.06 mm/s, respectively, as compared to 0.56 mm/s and 0.75 mm/s for the reconstituted ferritin. Data collected at lower temperatures give blocking temperatures of 55 and 40 K for in vitro and in vivo samples. At 4.2 K, both samples give similar saturation hyperfine field values for the interior (495 kOe) and surface (450 kOe) iron sites. The temperature dependence of the reduced hyperfine magnetic fields at the interior iron sites is consistent with the collective magnetic excitations model, due to the particles magnetization precession about the anisotropy axis. In contrast, a marked decrease in the reduced hyperfine field at surface sites with increasing temperature indicates a more complex spin excitation energy landscape at the surface.