Magnetic properties of magnetite hollow nanospheres

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

Dublin

Congreso; IV Joint European Magnetic Symposia (JEMS´08); 2008

JEMS

We report unusual magnetic behavior in iron oxide (magnetite) 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 mean diameter of <d> = 9.3 nm. The broadening of the x-ray patterns reflects the existence of crystallite sizes of 2 nm, much lower than the 9.3 nm observed from HRTEM. Detailed analysis 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 fraction of atoms existing at the inner and outer surfaces gives rise to a high magnetic disorder. An enhancement of the magnetization at low temperatures, and shift in the field cooling hysteresis loops (exchange bias effect) are observed.M(H,T) and In-field Mössbauer (120 kOe) measurements clearly show that magnetic contribution of the disordered shell plays 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 like phase at the surface regions.