Preparation, magnetization, and microstructure of ionic ferrofluids based on γ-Fe2O3/Ni2O3 composite nanoparticles

► A novel ionic ferrofluid, which is based on γ-Fe2O3/Ni2O3 composite nanoparticles is prepared, and the magnetization and microstructure are investigated. ► It is found that the aggregation cannot form only by the magnetic interaction, and nevertheless, the chain aggregates can still appear which is proved by optical relaxation. ► We advance that the chainlike aggregates can be attributed to the action of the non-magnetic “depletion force” in polydispersed colloidal. ► These results presented in the paper are helpful to stimulate further the investigation and application for ferrofluids field.