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; ELIN L. WINKLER

JOURNAL OF PHYSICAL CHEMISTRY C

AMER CHEMICAL SOC

Lugar: Washington; Año: 2015

1932-7447

In this work, we studied the dynamic and static magneticproperties of ZnO-core/CoFe2O4-shell and CoO-core/CoFe2O4-shell nanoparticles.Both systems are formed by a core of ∼4 nm of diameter encapsulatedin a shell of ∼2 nm of thickness. The mean blocking temperature changes from106(7) to 276(5) K when the core is diamagnetic or antiferromagnetic,respectively. Magnetic remanence studies revealed the presence of weak dipolarinterparticle interactions, where Hint is approximately −0.1 kOe for ZnO/CoFe2O4 and −0.9 kOe for CoO/CoFe2O4, playing a minor role in the magneticbehavior of the materials. Relaxation experiments provided evidence that themagnetization reversal process of CoFe2O4 is strongly dependent on the magneticorder of the core. At 10 K, activation volumes of ∼46(6) and ∼69(5) nm3 werefound for CoO/CoFe2O4 and ZnO/CoFe2O4 nanoparticles, respectively, corresponding to one-third and one-fifth of the totalshell volume. While the magnetic behavior of ZnO/CoFe2O4 nanoparticles is strongly affected by the surface disorder, theexchange coupling at the CoO/CoFe2O4 interface rules the magnetization reversal and the nanoparticles? thermal stability byinducing a larger energy barrier and promoting smaller switching volume.