Exchange bias and surface effects in bimagnetic CoO − core / Co 0.5 Ni 0.5 Fe 2 O 4 -shell nanoparticles

摘要: Bimagnetic nanoparticles have been proposed for the design of new materials with controlled properties, which requires a comprehensive investigation their magnetic behavior due to multiple effects arising from complex structure. In this work we fabricated bimagnetic core/shell formed by an $\ensuremath{\sim}3$-nm antiferromagnetic (AFM) CoO core encapsulated within $\ensuremath{\sim}1.5$-nm ferrimagnetic (FiM) ${\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ shell, aiming at studying enhancement anisotropy and surface oxide shell. The properties as-synthesized annealed samples were analyzed ac dc magnetization measurements. results indicate that response particles is governed superparamagnetic interacting nanoaggregates spins constitute disordered whose total moments block $\ensuremath{\langle}{T}_{B}\ensuremath{\rangle}=49$ K collectively freeze in superspin-glass-type state $\ensuremath{\langle}{T}_{g}\ensuremath{\rangle}=3$ K. On other hand, are room temperature behave as exchange-coupled system below blocking $\ensuremath{\langle}{T}_{B}\ensuremath{\rangle}=70$ K, enhanced coercivity ${H}_{C}(10\phantom{\rule{4.pt}{0ex}}\text{K})\ensuremath{\sim}14.6$ kOe exchange bias field ${H}_{EB}(10\phantom{\rule{4.pt}{0ex}}\text{K})\ensuremath{\sim}2.3$ kOe, compared where ${H}_{C}(10\phantom{\rule{4.pt}{0ex}}\text{K})\ensuremath{\sim}5.5$ ${H}_{EB}(10\phantom{\rule{4.pt}{0ex}}\text{K})\ensuremath{\sim}0.8$ kOe. Our results, interpreted using different models thermally activated relaxation processes, can help clarify many hollow nanoparticle systems.