Superspin glass behavior of self-interacting CoFe2O4 nanoparticles

Abstract

Low-temperature magnetic properties of CoFe2O4 nanoparticles (3–16nm) have been investigated by AC and DC magnetic measurements. The saturation magnetization (MS) of ultra-small CoFe2O4 nanoparticles (3–9nm) sharply increases at low temperature (10K) compared to room temperature (RT) MS value. For example, the increment in MS value for 3nm CoFe2O4 nanoparticle is 22emu/g and is null for 12nm and larger sized nanoparticles. A similar trend of increment in MS is also seen in ultra-small size Fe3O4 and MnFe2O4 nanoparticles. However, the MS enhancement in ultra-small CoFe2O4 nanoparticles is found much higher as compared to Fe3O4 and MnFe2O4 nanoparticles. The ultra-small sized nanoparticles arrange with a high packing density to induce a strong exchange as well as dipolar interactions, which renders the enhanced low temperature MS with superspin glass (SSG) state. The exchange coupling strongly depends on magnetic anisotropy energy, which increases in the order Mn2+<Fe2+<Co2+ and thus the ultra-small CoFe2O4 nanoparticles show a large enhancement of MS at low temperature due to strong exchange coupling. A noticeable enhancement of spin glass temperature (Tg) for ultra-small sized CoFe2O4 nanoparticles also confirms the presence of strong exchange coupling in this case. Fitting of the ac susceptibility χ′(T, f) data to a power-law scaling and Vogel–Fulcher model shows a satisfactory fit and the dynamic critical exponent takes value between 8.9 and 11.9 which are in a range typical for the spin-glass systems. Memory behavior in ultra-small CoFe2O4 nanoparticles suggest that the frequency dependent blocking process of ultra-small sized nanoparticles can be better described by power law model, while the interaction regime present in the 12nm and above sized nanoparticles is ascribed to a Vogel–Fulcher model.