Strong magnetic coupling between antiferromagnetic and ferromagnetic phases in polycrystalline hollow nanoparticles composed of spinel solid solution

Abstract

Exchange bias is an interfacial phenomenon that is used to increase the magnetic anisotropy in order to surpass the superparamagnetic limit. Exchange bias has been effectively manipulated through variation of local arrangement of ferromagnetic and antiferromagnetic nanocrystals in an unusual morphology and for very specific compositions. For this purpose, polycrystalline hollow nanoparticles with spinel structure composed of Co3-δFeδO4 (with δ > 0) and Co1+γFe2-γO4 (γ > 0) crystallites, were prepared using = CoFe metal nanoparticles as precursors. These particles were oxidized to obtain the desired mixed-oxide nanoparticles with various compositions using the well-known Kirkendall effect. Spinel nanoparticles were composed of an antiferromagnetic phase, which was predominantly Co3-δFeδO4, and a ferromagnetic phase, Co1+γFe2-γO4. The antiferromagnetic phase showed a relative increase in its concentration with the increase in Co content. The magnetic response includes high field thermoremanent magnetization in antiferromagnetic phase, ferromagnetic crystal size, increased anisotropy due to antiferromagnetic and surface spin glass phases, etc. Antiferromagnetic phase has been demonstrated to be robust in comparison with the surface spin glass phase to generate exchange bias. Finally, a phase diagram of the magnetic phases as a function of Co concentration is constructed to identify ferromagnetic, antiferromagnetic and spin glass regimes in this system.