Critical dimension for magnetic exchange-spring coupled core/shell CoFe2O4/CoFe2 nanoparticles

Core/shell nanoparticles of CoFe2O4/CoFe2 were prepared by reducing nanoparticles of CoFe2O4 under hydrogen atmospheres. The structure, morphology and room temperature magnetization of the core/shell nanoparticles were analyzed by X-ray diffraction, transmission electron microscopy and magnetometry, respectively. The sample preparation procedure allowed the diameter of the CoFe2O4 core to be varied from 67.7 to 2.8 nm. From the magnetic data it was found that the core couples to the shell through the exchange-spring mechanism while the critical thickness of the soft phase (shell) was estimated using a current theoretical model to be 8.0 nm. This value is very close to the one (7.8 nm) obtained for nanoparticles with same composition but obtained using different synthesis route and having an average particle size (32 nm) less than half of the value estimated (73 nm) for the samples investigated in the present work. (C) 2012 Elsevier B.V. All rights reserved.