Unravelling the onset of the exchange bias effect in Ni(core)@NiO(shell) nanoparticles embedded in a mesoporous carbon matrix

Ni(core)@NiO(shell) nanoparticles (NPs) were synthesized through the pyrolysis of an inorganic precursor taking place within the pores of an active carbon matrix at different temperatures between 673 and 1173 K, and a subsequent oxidation in air. For the lowest temperature (673 K), the smallest average size of the NPs (9 nm) and the largest percentage of NiO (82%) are observed. Upon increasing the temperature up to 1173 K, an average diameter of 23 nm is observed while the NiO percentage decreases below 20%. We found that each NP consists of a Ni core surrounded by a structurally disordered NiO shell with a constant thickness of similar to 2 nm, regardless of the core size. The spins inside the NiO shell freeze into a spin glass (SG)-like state below T-f similar to 40 K. The magnetic exchange coupling between the Ni core and the NiO shell spins gives rise to the occurrence of the exchange bias (EB) effect, whose temperature dependence follows a universal exponential trend in all samples. The SG nature of the shell spins yields a vanishing EB above T-f. This is far below the Neel temperature of bulk antiferromagnetic NiO (T-N similar to 523 K) that usually determines the onset of the EB effect in Ni/NiO interfaces.