Spin Disorder and Particle Size Effects in Cobalt Ferrite Nanoparticles with Unidirectional Anisotropy and Permanent Magnet-like Characteristics

Spinel ferrites, especially cobalt ferrite, have the capability of preserving sufficient magnetization ( M s ) and coercive field ( H c ). However, precise nanostructuring is needed to sustain the steady ferrimagnetic (FiM) response with high H c and remnant magnetization ( M r ). By a two-step process involving co-precipitation and inert calcination, CoFe 2 O 4 nanoparticles (NPs) with inherent lattice strain were achieved with appreciable magnetic anisotropy. The disordered spins on the NP surface are found to be the key for maintaining the anisotropy and squareness ( M r / M s ) of hysteresis loops, the extent of which is a function of the processing temperature and NP size. The freezing of surface spins results in 2.8 and 1.2 nm thick spin glass (SG) layers for the as-prepared and 350 °C-calcined CoFe 2 O 4 NPs, respectively, analyzed at 5 K. The frozen surface spins pin the core FiM spins, resulting in the exchange bias ( H eb ) coupling. At room temperature, 20 nm NPs exhibit permanent magnet-like features with a maximum energy product, BH max, of 2.4 MGOe, which is quite promising considering CoFe 2 O 4 as an environmentally friendly alternative to the hazardous rare-earth-based permanent magnets. Our CoF 2 O 4 NP ensemble delivers both intriguing low-temperature magnetic properties having academic and technological interests and worthwhile room-temperature features that have direct translational relevance.