Magnetic Interactions and Energy Barrier Enhancement in Core/Shell Bimagnetic Nanoparticles - The Journal of Physical Chemistry C (ACS Publications)

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

Gabriel C. Lavorato, Davide Peddis, Enio Lima , Jr., Horacio E. Troiani, Elisabetta Agostinelli, Dino Fiorani, Roberto D. Zysler, and Elin L. Winkler

J. Phys. Chem. C 119, 15755 (2015) 

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 H_int 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.