Single-step chemical synthesis of ferrite hollow nanospheres

ENIO LIMA JR.; J M VARGAS; R D ZYSLER; H R RECHENBERG; R COHEN; J ARBIOL; G F GOYA; A IBARRA; M RICARDO IBARRA

Rome, Italy

Congreso; IX National Meeting Nanophase Materials; 2009

We report a single-step chemical synthesis of iron oxide hollow nanospheres of 9.3 nm in diameter. Monodisperse organically passivated (oleic acid) ferrite nanoparticles are prepared by high-temperature organic-phase synthesis. The nanoparticles have a narrow size distribution with = 9.3. The broadening of the x-ray patterns reflects the existence of crystallite size of 2 nm, much lower than the 9.3 nm observed from HRTEM. Detailed analyis of magnified HRTEM revealed the polycrystalline nature of the nanostructures with the absence of any preferential orientation. High angular annular dark field and STEM analysis reveal that our system is composed by hollow nanoparticles. The large fractions of atoms existing at the inner and outer surfaces gives rise to a high magnetic disorder with unusual magnetic behavior. An enhancement of the magnetization at low temperatures, and shift in the field cooling hysteresis loops (exchange bias shift) are observed. M(H;T) measurements and in-field Mössbauer (120 kOe) spectroscopy clearly show that magnetic contribution of the disordered shell play the fundamental rule in the magnetic properties. The overall magnetic behavior can be explained considering the coexistence of a soft superparamagnetic phase and a hard phase corresponding to the highly frustrated cluster-glass phase at the surface regions.