Exchange-coupled Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ nanoparticles for advanced magnetic hyperthermia

Iron oxide nanoparticles especially Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$, $\gamma $-Fe$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$have been extensively studied for magnetic hyperthermia because of their tunable magnetic properties and stable suspension in superparamagnetic regime. However, their relatively low heating capacity hindered practical application. Recently, a large improvement in heating efficiency has been reported in exchange-coupled nanoparticles with exchange coupling between soft and hard magnetic phases. Here, we systematically studied the effect of core and shell size on the heating efficiency of the Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ core/shell nanoparticles. The nanoparticles were synthesized using thermal decomposition of organometallic precursors. Transmission electron microscopy (TEM) showed formation of spherical shaped Fe$_{\mathrm{3}}$O$_{\mathrm{4\thinspace }}$and Fe$_{\mathrm{3}}$O-/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ nanoparticles. Magnetic measurements showed high magnetization ($\cong $70 emu/g) and superparamagnetic behavior for the nanoparticles at room temperature. Magnetic hyperthermia results showed a large increase in specific absorption rate (SAR) for 8nm Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ compared to Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ nanoparticles of the same size. The heating efficiency of the Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ with 1 nm CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ (shell) increased from 207 to 220 W/g (for 800 Oe) with increase in core size from 6 to 8 nm. The heating efficiency of the Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ with 2 nm CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ (shell) and core size of 8 nm increased from 220 to 460 W/g (for 800 Oe). These exchange-coupled Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ core/shell nanoparticles can be a good candidate for advanced hyperthermia application.