Structural and magnetic properties of core-shell Au/Fe3O4 nanoparticles

TL;DR

This study investigates the synthesis, structure, and magnetic properties of Au/Fe3O4 core-shell nanoparticles, revealing a temperature-dependent exchange bias effect linked to spin interactions within the magnetite shell.

Contribution

It provides a detailed characterization of Au/Fe3O4 core-shell nanoparticles, demonstrating the presence of exchange bias and thermal magnetic behavior, which are novel insights for such nanostructures.

Findings

Au core size approximately 6.9 nm with a 3.5 nm Fe3O4 shell

Observation of exchange bias effect below room temperature

Thermal blocking state at T_B = 59 K and exchange bias onset at ~40 K

Abstract

We present a systematic study of core-shell Au/Fe_3O_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\pm 1.0) nm surrounded by Fe_3O_4 shell with a thickness of ~3.5 nm, epitaxially grown onto the Au core surface. The Au/Fe_3O_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_B = 59 K and a relaxed state well above T_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_{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_3O_4 shell) and spins located in the ordered region of the Fe_3O_4 shell.