Ultrafine subdomain particles of NiFe2O4, Ni0.5Zn0.5Fe2O4, and CoFe2O4 were prepared under mild hydrothermal conditions by precipitating from metal nitrates. Magnetic and Mössbauer effect measurements revealed the superparamagnetic nature of these ferrite particles. The crystallite sizes were in the range 5.0–20.0 nm with a very narrow particle size distribution. The magnetic properties and Mössbauer hyperfine parameters of these subdomain ferrite particles are somewhat different from those reported for multidomain bulk materials. The hydrothermal ferrite powders were found to sinter to almost theoretical density with little or no intergranular porosity.

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Ultrafine and nearly spherical particles of zinc ferrite were synthesized under mild hydrothermal conditions by precipitating from metal nitrates. These particles exhibited antiferromagnetic ordering below 13 K. Mossbauer spectroscopic measurements revealed the subdomain superparamagnetic nature of the particles having a narrow particle size distribution. The hydrothermal ferrite powders were found to sinter to almost theoretical density with little or no intragranular porosity.

Subdomain size superparamagnetic particles of Mn-Zn ferrites, having a narrow particle size distribution (peaked around 10 nm) were prepared under mild hydrothermal conditions. High densities (≳99.5%) have been obtained upon sintering these green ferrite powders at relatively low temperatures (∼1000 °C), without the use of any sintering aids. Permeability spectra and other magnetic and electrical properties of the sintered ferrites were studied.

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