Coprecipitation and microemulsion were used to prepare nanosized magnetite. The average particle sizes of samples A and B were calculated by Brunauer–Emmett–Teller analysis and are 8.16 and 2.24 nm, respectively. The saturation magnetization of sample A at 300 K was 57.2 emu/g, but sample B was not saturated even 7.0 T. The blocking temperature for sample B was 17 K and the crystalline anisotropy energy was 1.2×105J/m3. At 5 K, sample B is ferrimagnetic with σs=7.62 emu/g. This value is much lower than the σs=65.4 emu/g of the crystallized sample A at 50 K. Mössbauer spectra showed that sample A was ferrimagnetic at room temperature, but sample B was superparamagnetic at T>Tb (critical blocking temperature).

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