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/CoFe<inf>2</inf>O<inf>4</inf>-shell and CoO-core/CoFe<inf>2</inf>O<inf>4</inf>-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 H<inf>int</inf> is approximately -0.1 kOe for ZnO/CoFe<inf>2</inf>O<inf>4</inf> and -0.9 kOe for CoO/CoFe<inf>2</inf>O<inf>4</inf>, playing a minor role in the magnetic behavior of the materials. Relaxation experiments provided evidence that the magnetization reversal process of CoFe<inf>2</inf>O<inf>4</inf> 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/CoFe<inf>2</inf>O<inf>4</inf> and ZnO/CoFe<inf>2</inf>O<inf>4</inf> nanoparticles, respectively, corresponding to one-third and one-fifth of the total shell volume. While the magnetic behavior of ZnO/CoFe<inf>2</inf>O<inf>4</inf> nanoparticles is strongly affected by the surface disorder, the exchange coupling at the CoO/CoFe<inf>2</inf>O<inf>4</inf> interface rules the magnetization reversal and the nanoparticles' thermal stability by inducing a larger energy barrier and promoting smaller switching volume.