A comparison of coprecipitation with microemulsion methods in the preparation of magnetite

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 x 10(5) J/m(3). At 5 K, sample B is ferrimagnetic with sigma(s) = 7.62 emu/g. This value is much lower than the sigma(s) = 65.4 emu/g of the crystallized sample A at 50 K. Mossbauer spectra showed that sample A was ferrimagnetic at room temperature, but sample B was superparamagnetic at T>T-b (critical blocking temperature). (C) 1999 American Institute of Physics. [S0021-8979(99)24808-6].