Size-dependent magnetic hardening in CoFe₂O₄ nanoparticles: effects of surface spin canting

Magnetic cobalt ferrite CoFe₂O₄ is rich with physical phenomena, owing to its crystalline and magnetic structures. When such a ferrite is produced in a modulated nanoscale size, the increased specific surface area gives rise to even more complex behavior in its magnetism, particularly in relation to magnetic hardening. By correlating nanoparticle size (from 3.5 nm to 80 nm) with crystallite size and magnetic properties, we can observe interesting relations between particle size and magnetic coercivity. On exceeding the superparamagnetic limit of about 10 nm, room-temperature coercivity and remanence values are found to increase with increasing nanoparticle size, up to a maximum value of 4.1 kOe and 52 emu g⁻¹, respectively, at a size of approximately 45 nm. Above this critical size, the nanoparticles are comprised of multiple crystallites, and demonstrate the existence of a cooperative phenomenon, the so-called interaction domains, which leads to a decrease in coercivity and remanence values. More interestingly, the ultrasmall-sized CoFe₂O₄ nanoparticles (3.5–16 nm) show an anomalous coercivity enhancement and irreversible behavior at low temperatures, as compared to the large-sized nanoparticles, which may be ascribed to enhanced effective magnetic anisotropy due to the surface spin-canting effect. Furthermore, training behavior in the exchange bias field, together with field-dependent blocking behavior, indicate that ultrasmall CoFe₂O₄ nanoparticles possess highly frustrated surface spins, which rearrange much more slowly than their interior spins, resulting in magnetic hardening at low temperatures. (paper)