iv) NO 3 ± + H 2 O +2e ± ® NO 2 ± + 2OH ± v) Fe 2+ + 2Fe 3+ + 8OH ± ® (Fe 2+ , 2Fe 3+ )(OH) 8 vi) (Fe 2+ , 2Fe 3+ )(OH) 8 ® Fe 3 O 4 + 4H 2 O At elevated temperature of 333 K, the following reactions will be dominant
Figure shows magnetization curves for the Fe 3 O 4 films prepared at 293 and 313 K. The magnetization curves were recorded with a vibrating sample magnetometer at room temperature. The Fe 3 O 4 film prepared at 293 K had a coercive force of 11.9 kA m ±1 and saturation magnetization (4pM s ) of 0.61 T. These values are good agreement with those for bulk Fe 3 O 4 . [13] Any magnetic anisotropy could not be observed for the Fe 3 O 4 film. This will be attributed to the polycrystalline granular structure with random orientation.
The Fe 3 O 4 film prepared at 313 K showed decreased saturation magnetization (4pM s ) of 0.51 T and a coercive force of 11.9 kA m ±1 . The decrease in saturation magnetization is evidence that the film contained some Fe 3+ oxide and hydroxide, as pointed out in Figure . No magnetization could be observed for the Fe 3+ oxide and hydroxide mixture film prepared at 333 K.
The 500 nm-thick Fe 3 O 4 film prepared at 293 K showed a resistivity as high as 1.48 10 3 Xcm. The resistivity was calculated from the film thickness and the sheet resistivity measured with a conventional four-point probe method with a probe spacing of 1 mm at room temperature. The value was much higher, by about five orders of magnitude, than that for bulk Fe 3 O 4 . [14] Although the stoichiometric O/Fe ratio of 1.33 could be obtained for the Fe 3 O 4 film, the film contained non-stoichiometry of Fe 3+ /Fe 2+ ions, as shown by the ESCA measurement. This will be the reason for the high resistivity of the Fe 3 O 4 film.
In conclusion, Fe 3 O 4 (magnetite) film with a spinel structure of 0.8390 nm in unit-cell parameter has been prepared on a non-conductive substrate only by immersing the substrate in an aqueous solution containing iron(III) nitrate and DMAB at 293 K. The Fe 3 O 4 film had a characteristic magnetic property of 11.9 kA m ±1 coercive force and 0.61 T in saturation magnetization (4pM s ), and high resistivity of 2.9 10 3 Xcm.