Magnetic Interactions and Energy Barrier Enhancement in Core/Shell Bimagnetic Nanoparticles

In this work, we studied the dynamic and static magnetic properties of ZnO-core/CoFe2O4-shell and CoO-core/CoFe2O4-shell nanoparticles. Both systems are formed by a core of ~4 nm of diameter encapsulated in a shell of ~2 nm of thickness. The mean blocking temperature changes from 106(7) to 276(5) K when the core is diamagnetic or antiferromagnetic, respectively. Magnetic remanence studies revealed the presence of weak dipolar interparticle interactions, where Hint is approximately -0.1 kOe for ZnO/CoFe2O4 and -0.9 kOe for CoO/CoFe2O4, playing a minor role in the magnetic behavior of the materials. Relaxation experiments provided evidence that the magnetization reversal process of CoFe2O4 is strongly dependent on the magnetic order of the core. At 10 K, activation volumes of ~46(6) and ~69(5) nm³ were found for CoO/CoFe2O4 and ZnO/CoFe2O4 nanoparticles, respectively, corresponding to one-third and one-fifth of the total shell volume. While the magnetic behavior of ZnO/CoFe2O4 nanoparticles is strongly affected by the surface disorder, the exchange coupling at the CoO/CoFe2O4 interface rules the magnetization reversal and the nanoparticles' thermal stability by inducing a larger energy barrier and promoting smaller switching volume.