Synthesis and magnetic properties of CoFe2/CoFe2O4 nanoparticles diluted in the MgO matrix

Well-dispersed CoFe2O4 nanoparticles were synthesized and coated by MgO, followed by reduction in a H-2/N-2 gas mixture to prepare CoFe2/MgO. The CoFe2/MgO was oxidized in the open air at 500 degrees C and 900 degrees C to prepare CoFe2(core)/CoFe2O4(shell)/MgO nanostructures with the CoFe2/CoFe2O4 mass ratios of 25.4% and 4.7%, respectively. It was found that the magnetization of the nanostructure with CoFe2(core)/CoFe2O4(shell) was not the simple sum of individual magnetizations of CoFe2 and CoFe2O4 due to the dipolar interaction (interparticle) and the coupling between CoFe2 and CoFe2O4 (intraparticle). Furthermore, the remanence (M-r) to saturation magnetization (M-S) ratio M-r/M-s, mainly determined by the magneto-crystalline anisotropy in the bulk materials, is sensitive to the surface spins and dipolar interaction in the nano-sized materials. It was observed that M-r/M-s is proportional to the inverse of the logarithm of maximum dipolar field H-dip, i.e. M-r/M-s proportional to 1/1gH(dip), independent to the moment and concentration of magnetic particles. (C) 2017 Elsevier Ltd. All rights reserved.