Core/shell bimagnetic nanoparticles: magnetic interactions and magnetization reversal.

Novel bimagnetic nanoparticles (NPs) have been recently proposed for the development of new magnetic materials and comprehensive studies are required in order to understand their complex properties. In this work we compared two core/shell magnetic systems: one formed by non-magnetic ZnO NPs surrounded by ferrimagnetic (FiM) CoFe2O4 shell and the other formed by CoO antiferromagnetic (AFM) NPs surrounded by FiM CoFe2O4 shell. As both systems present analogous size and morphology (spherical NPs formed by a ~4 nm core and a ~2 nm thick shell), some general issues about core/shell systems can be discussed. The results showed that CoO/CoFe2O4 NPs present a mean blocking temperature (TB) of ~270 K, higher than the ~110 K observed for the ZnO/CoFe2O4 system. The field dependence of TB and the temperature variation of HC were explained through the Stoner-Wohlfarth model for ZnO/CoFe2O4, but deviations were found for CoO/CoFe2O4 NPs due to intra-particle interactions. A higher thermal stability of MS up to room temperature and an HC enhancement up to ~27.8 kOe at 5 K were observed for the bimagnetic sample. Magnetic remanence studies revealed that, although weak dipolar inter-particle interactions are observed, they play a minor role in determining the magnetic behavior of the materials. Relaxation experiments clarified that the magnetization reversal process of CoFe2O4 is strongly dependent on the magnetic order of the core. At 10 K, activation volumes of 46 and 96 nm3, representing ~20 and 40% of the total shell volume, were found for CoO/CoFe2O4 and ZnO/CoFe2O4 NPs respectively. While the main characteristic of ZnO/CoFe2O4 NPs is the high surface-to-volume ratio, the exchange coupling at the CoO/CoFe2O4 interface rules the magnetization reversal and the NPs' thermal stability by inducing a larger energy barrier and promoting a smaller switching volume.