Superspin glass behavior of self-interacting CoFe2O4 nanoparticles

Low-temperature magnetic properties of CoFe2O4 nanoparticles (3-16 nm) have been investigated by AC and DC magnetic measurements. The saturation magnetization (M-S) of ultra-small CoFe2O4 nanoparticles (3-9 nm) sharply increases at low temperature (10 K) compared to room temperature (RT) MS value. For example, the increment in M-S value for 3 nm CoFe2O4 nanoparticle is 22 emu/g and is null for 12 nm and larger sized nanoparticles. A similar trend of increment in MS is also seen in ultra-small size Fe3O4 and MnFe2O4 nanoparticles. However, the M-S 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 M-S 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 M-S at low temperature due to strong exchange coupling. A noticeable enhancement of spin glass temperature (T-g) for ultra-small sized CoFe2O4 nanoparticles also confirms the presence of strong exchange coupling in this case. Fitting of the ac susceptibility chi'(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 12 nm and above sized nanoparticles is ascribed to a Vogel-Fulcher model. (C) 2015 Elsevier B.V. All rights reserved.