Magnetic Properties Improvement by Exchange Interaction in CoO core/ CoFe2O4 shell Nanoparticles

E. WINKLER; E. LIMA JR.; D. TOBIA; H.E. TROIANI; R.D. ZYSLER; E. AGOSTINELLI; D. FIORANI

Rodas

Congreso; XI International Conference on Nanostructured Materials NANO2012; 2012

International NANO

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) interface. This fact, together with the new size-controlled synthesis methods, has driven in the last years the research on bimagnetic core/shell nanoparticles. In this work we present a magnetic properties study of a novel bimagnetic CoO core/CoFe2O4 shell nanoparticles system. The nanoparticles were synthesized by seed-mediated high temperature decomposition of organometallic precursor. 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 2 nm CoFe2O4 ferrimagnetic shell. A Gaussian nanoparticle size distribution were obtained by TEM image with an average diameter of <> ~ 7 nm and = 2 nm of dispersion. The magnetic measurement has 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.