The anomalous memory effect related to the relaxation of surface and core moments observed in well-dispersed CoFe2O4 nanoparticles

• Well-dispersed uniform CoFe2O4 nanoparticles were synthesized.
• Surface spins were proved by many phenomena.
• The anomalous memory effect was observed for CoFe2O4 nanoparticles.

Well-dispersed uniform cobalt ferrite nanoparticles were synthesized by thermal decomposition of a metal–organic salt in organic solvent with a high boiling point, and characterized by XRD, TEM and detailed magnetic measurements. The moments of CoFe2O4 nanoparticles consist of the core and canted surface moments below 200 K, and the exchange-coupling between the surface and core spins enhanced the remanence (Mr) to saturation (Ms) magnetization ratio (Mr/Ms) at the temperature of 10 K. Interestingly, the anomalous memory effect was observed in a broad temperature range which can be attributed to the relaxation of surface spins below 200 K and the one from the moments of magnetically ordered entity larger than one particle above 200 K.