MAGNETIC-PROPERTIES OF SODIUM-MODIFIED IRON-OXIDE POWDERS SYNTHESIZED BY SOL-GEL METHOD

Spinel-type ferrites of the sodium-modified iron-oxide (Na-Fe-O) system were synthesized by using a sol-gel method. The maximum magnetic moment of 50 emu/g was obtained at the sodium concentration of C(Na) = 0.1 to 0.2 (the mol ration of sodium to the sum of sodium and iron), which is about 1.5 times larger than that of non-substituted ones. Monotropic phase transition temperature from the gamma-Fe2O3 phase to the alpha-Fe2O3 phase was shifted to higher temperature by substituting a small amount of sodium and the highest transition temperature reaches as high as about 600-degrees-C for C(Na) = 0.2, where the crystal structure of spinel was maintained except elongation of the lattice parameter, which results in the decreases of Curie temperature and magnetic hyperfine field. Improvement in thermal stability of gamma-Fe2O3 is considered to be due to the formation of sodium-modified gamma-Fe2O3.