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 similar to 175 K. The SG nature of the transition is also confirmed by the field dependence of the freezing temperature (T-F similar to(H)) following the well known Almeida-Thouless line, delta T-F similar to H-2/3. Additionally, the particles exhibit a large exchange bias (H-EB similar to 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. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3478738]