The strontium ferrite ultrafine particles have been prepared using the microemulsion processing. The mixed hydroxide precursor was obtained via the coprecipitation of Sr(2+) and Fe(3+) in a water-in-oil microemulsion of water/CTAB/n-butanol/isooctane. According to the investigation on the thermochemical properties by TGA/DTA and the phase analysis by XRD, it was shown that the precursor could yield pure strontium ferrite after calcination at 700 degrees C for 5 h while using an appropriate molar ratio of Sr/Fe in microemulsions. From TEM measurement, the diameters of the precursor and calcined particles were 3.8+/-0.7 and 50-100 nm, respectively. The magnetic properties characterized by a SQUID magnetometer showed that the saturation magnetization, remanent magnetization, coercivity, and squareness ratio were 55 emu/g, 28 emu/g, 492 Oe, and 0.51, respectively. The magnetization was also observed to increase with the decrease of temperature at 5-400 K. Compared with those reported earlier, the quite low coercivity implies the potential application of final product in the high-density perpendicular recording media. Copyright 2001 Academic Press.