Magnetic properties improvement by interface exchange interaction in CoO core/ CoFe2O4 shell nanoparticles

ELIN WINKLER; ENIO LIMA JR.; DINA TOBIA; HORACIO TROIANI; MARTÍN E. SALETA; ROBERTO ZYSLER; ELISABETTA AGOSTINELLI; DINO FIORANI

Rhodas

Conferencia; XI International Conference on Nanostructured Materials; 2012

Several nanostructured technological devices, as magnetic storage media and permanent magnets, require high stability of the magnetization. However, the ways towards miniaturization found a lower threshold in the grain size to maintain the magnetization thermal stability, the so-called ?superparamagnetic limit?. It has been shown that the superparamagnetic threshold can be pushed to a lower grain size in nanostructures with ferromagnetic (FM)/ antiferromagnetic (AFM) interfaces. This fact, together with the new size-controlled synthesis methods, has driven in the last years the research on bi-magnetic core/shell nanoparticles. In this work, we present a magnetic properties study of a novel bi-magnetic CoO core/CoFe2O4 shell nanoparticles. The nanoparticles were synthesized by seed-mediated high temperature decomposition of organo-metallic precursors. The structure and morphology were characterized by x-ray, electron diffraction and TEM microscopy. The nanoparticles are formed by a CoO antiferromagnetic core coated with a 2 nm CoFe2O4 ferrimagnetic shell. A Gaussian nanoparticle size distribution was obtained by TEM imaging obtaining an average diameter of ~ 7 nm and sigma= 2 nm of dispersion. The magnetic measurements have shown that the system presents coercivity enhancement and high squareness with respect to the single phase counterpart. At T=5 K the coercive field (HC) results HC = 27.8 kOe and the squareness, defined as the remanence (Mr) normalized to the saturation (MS), is Mr/MS =0.79. In order to analyze the origin of the CoO /CoFe2O4 effective anisotropy enhancement ZnO core/CoFe2O4 shell nanoparticles of ~ 7. 5 nm were synthesized. This system presents a blocking temperature at TB~ 200 K and HC = 7.8 kOe at T=5 K. The effective anisotropy is analyzed taking into account the surface and the interface exchange coupling effects.