In this study MnFe 2 O 4 nanoparticles at different pH (2-8) have been prepared by sol – gel method. The results of TEM images and scherrer formula indicated that by increasing pH up to 6 average particles size increases and by further pH increasing, size of nanoparticles decreases. Also magnetization measurements showed that the saturation magnetization (M S ) and coersive field (H C ) values related to size of particles. The M S value of the sample with pH=2 is smaller than other samples (due to smaller particle) and H C for this sample is zero which indicated that this sample is in superparamagnetic state

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