Exchange bias and surface effects in bimagnetic CoO-core/Co0.5Ni0.5Fe2O4-shell nanoparticles

GABRIEL LAVORATO; ELIN WINKLER; ALBERTO GHIRRI; ENIO LIMA, JR; DAVIDE PEDDIS; HORACIO E. TROIANI; DINO FIORANI; ELISABETTA AGOSTINELLI; DANIELE RINALDI; ROBERTO D. ZYSLER

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

American Physical Society

Año: 2016

0163-1829

Bimagnetic nanoparticles have been proposed for the design of new materials with controlled properties,which requires a comprehensive investigation of their magnetic behavior due to multiple effects arising fromtheir complex structure. In this work we fabricated bimagnetic core/shell nanoparticles formed by an ∼3-nmantiferromagnetic (AFM) CoO core encapsulated within an ∼1.5-nm ferrimagnetic (FiM) Co0.5Ni0.5Fe2O4 shell,aiming at studying the enhancement of the magnetic anisotropy and the surface effects of a ferrimagnetic oxideshell. The magnetic properties of as-synthesized and annealed samples were analyzed by ac and dc magnetizationmeasurements. The results indicate that the magnetic response of the as-synthesized particles is governedby the superparamagnetic behavior of the interacting nanoaggregates of spins that constitute the disorderedferrimagnetic shell, whose total moments block at TB = 49 K and collectively freeze in a superspin-glass-typestate at Tg = 3 K. On the other hand, annealed nanoparticles are superparamagnetic at room temperature andbehave as an exchange-coupled system below the blocking temperature TB = 70 K, with enhanced coercivityHC(10 K) ∼ 14.6 kOe and exchange bias field HEB(10 K) ∼ 2.3 kOe, compared with the as-synthesized systemwhere HC(10 K) ∼ 5.5 kOe and HEB(10 K) ∼ 0.8 kOe. Our results, interpreted using different models forthermally activated and surface relaxation processes, can help clarify the complex magnetic behavior of manycore/shell and hollow nanoparticle systems.