Stabilization of surface spin glass behavior in core/shell-Fe67Co33/CoFe2O4 nanoparticles

Magnetic properties of Co33Fe67--CoFe2O4 (core-shell) nanoparticles are presented. Both dc magnetization and ac susceptibility measurements indicate a spin glass (SG) like transition occurring at $T$F $\sim $175 K. The SG nature of the transition is also confirmed by the field dependence of the freezing temperature $T$F(H) following the well known Almeida--Thouless line, $\delta T$F$\sim $H$^{2/3}$. Additionally, the particles exhibit a large exchange bias ($H$EB $\sim $ 1357 Oe) arising from the core-shell (ferromagnetic-SG) coupling. The unusually high SG transition temperature and large exchange bias effects are attributed to a combination of several factors including the thickness of the amorphous oxide shell and large values of the exchange and anisotropy constants associated with the CoFe2O4 shell.