Tuneable magnetic properties of hydrothermally synthesised core/shell CoFe_2O_4/NiFe_2O_4 and NiFe_2O_4/ CoFe_2O_4 nanoparticles

摘要

Core/shell hetero-nanostructures of hydrothermally synthesised cobalt and nickel ferrites are shown to exhibit novel magnetic properties. The compositions and phase distributions of homogeneous Co_(0.5)Ni_(0.5)Fe_2O_4, and core/shell NiFe_2O_(4-Core)/ CoFe_2O_(4-Shell) and CoFe_2O_(4-Core)/NiFe_2O_(4-Shell) nanoparticles (NPs) are confirmed using high-resolution transmission electron microscopy and electron energy loss spectroscopy. SQUID magnetometry investigations demonstrate that, at room temperature, homogeneous Co_(0.5)Ni_(0.5)Fe_2O_4 NPs (~8 nm in diameter) are in the super-paramagnetic state, the magnetisation of NiFe_2O_(4-Core)/CoFe_2O_(4-Shell) NPs (~11 nm in diameter) is partially blocked, whilst CoFe_2O_(4-Core)/NiFe_2O_4- Shell NPs (~11 nm in diameter) are in a blocked state. In particular, NiFe_2O_(4-Core)/CoFe_2O_(4-Shell) NPs exhibit twice the out-of-phase v00 susceptibility of CoFe_2O_(4-Core)/NiFe_2O_(4-Shell) NPs, being dominated by the magnetisation of the core ferrite phase. Hence, when exposed to a high-frequency magnetic field, it is considered that the high v00 susceptibility of NiFe_2O_(4-Core)/CoFe_2O_(4-Shell) NPs will promote large magnetically induced heating effects,making these core/shell NPs strong candidates for hyperthermia applications.