Magnetic Anisotropy and Switching Behavior of Fe3O4/CoFe2O4 Core/Shell Nanoparticles
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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 ∼ 300 K), the CoFe2O4 shell (TB-s ∼ 200 K), and the Fe3O4 core (TB-c ∼ 50 K). Radio-frequency transverse susceptibility gave a direct probe of the effective magnetic anisotropy field (HK) and switching field (HS), as well as their temperature evolutions. Interestingly, we found that HK of the core/shell structure increased with decreasing temperature. HS 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.
KeywordsCore/shell nanoparticle magnetic anisotropy magnetic switching
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Research at the University of South Florida was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-07ER46438. Joshua Robles acknowledges the financial support provided by the NSF Florida-Georgia Louis Stokes Alliance for Minority Participation (FGLSAMP) through Award No. HRD #1612347.