  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.
