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MnFe2O4 nano-particles with an average size of about 7 nm were synthesized by the thermal decomposition method. Based on the magnetic hysteresis loops measured at different temperatures the temperature-dependent saturation magnetization (M-S) and coercivity (H-C) are determined. It is shown that above 20 K the temperature-dependence of the M-S and H-C indicates the magnetic behaviors in the single-domain nano-particles, while below 20 K, the change of the M-S and H-C indicates the freezing of the spin-glass like state on the surfaces. By measuring the magnetization-temperature (M-T) curves under the zero-field-cooling (ZFC) and field-cooling procedures at different applied fields, superparamagnetism behavior is also studied. Even though in the ZFC M-T curves peaks can be observed below 160 K, superparamagnetism does not appear until the temperature goes above 300 K, which is related with the strong inter-particle interaction. (C) 2012 Elsevier B.V. All rights reserved.