Highlights

• The magnetic ground states of chemical synthesized multi-grain Fe₃O₄ nanorings.
• The formation of the multi-grain structure that disfavors the formation of a shape-induced magnetic vortex-domain structure and causes the reduction of the heating efficiency in magnetite nanorings.
• Our study demonstrated the possibility of using Fe₃O₄ nanorings for dual-purpose applications in localized magnetic hyperthermia and targeted drug delivery because of its high effective anisotropy and hollow morphology.