Structural and magnetic properties of core-shell Au/Fe$_3$O$_4$ nanoparticles

Abstract

We present a systematic study of core-shell Au/Fe$_3$O$_4$ nanoparticles produced by thermal decomposition under mild conditions. The morphology and crystal structure of the nanoparticles revealed the presence of Au core of $d$ = (6.9 ± 1.0) nm surrounded by Fe$_3$O$_4$ shell with a thickness of ~3.5 nm, epitaxially grown onto the Au core surface. The Au/Fe$_3$O$_4$ core-shell structure was demonstrated by high angle annular dark field scanning transmission electron microscopy analysis. The magnetite shell grown on top of the Au nanoparticle displayed a thermal blocking state at temperatures below $T_\text{B}$ = 59 K and a relaxed state well above $T_\text{B}$. Remarkably, an exchange bias effect was observed when cooling down the samples below room temperature under an external magnetic field. Moreover, the exchange bias field ($H_\text{EX}$) started to appear at $T$~40 K and its value increased by decreasing the temperature. This effect has been assigned to the interaction of spins located in the magnetically disordered regions (in the inner and outer surface of the Fe$_3$O$_4$ shell) and spins located in the ordered region of the Fe$_3$O$_4$ shell.