Magnetic Anisotropy and Switching Behavior of Fe3O4/CoFe2O4 Core/Shell Nanoparticles

A uniform core/shell nanoparticle system composed of a soft magnetic core (Fe3O4) and a hard magnetic shell (CoFe2O4) was synthesized and characterized to understand how the shell influences the magnetism and exchange coupling of the system. In the case of Fe3O4(8 nm)/CoFe2O4(2 nm) core/shell nanoparticles, DC and AC susceptibility measurements revealed three features associated with the blocking temperatures of the core/shell system (TB-cs similar to 300K), the CoFe2O4 shell (TB-s similar to 200K), and the Fe3O4 core (TB-c similar to 50K). Radio-frequency transverse susceptibility gave a direct probe of the effective magnetic anisotropy field (H-K) and switching field (H-S), as well as their temperature evolutions. Interestingly, we found that H-K of the core/shell structure increased with decreasing temperature. H-S was observed only below TB-s, which first decreased drastically with lowering temperature and then increased sharply below TB-c. This is attributed to the effect of a coercive field of CoFe2O4 on the spin flipping of Fe3O4 in the superparamagnetic state (TB-c < T < TB-s) and the blocked state (T < TB-c), respectively. Our study sheds light on the magnetic exchange coupling mechanism in core/shell nanoparticle systems and demonstrates the possibility of controlling the nanomagnetism of a soft magnetic core to which the hard magnetic shell is coupled in such systems.