SIZE-DEPENDENT MAGNETIC-PROPERTIES OF MNFE2O4 FINE PARTICLES SYNTHESIZED BY COPRECIPITATION

Manganese ferrite, MnFe2O4, fine particles in the size range 5-15 nm h ave been prepared by an aqueous phase coprecipitation method. The fres hly prepared particles are in a metastable state with regard to the ca tion distribution between the A and B sires of the spinel lattice. The rmomagnetic, differential scanning calorimetry and Mossbauer spectrosc opy show that heat treatment under inert gas to 670 K irreversibly cha nges the cation distribution to a stable state. Heat treatment of fres hly prepared samples in air to 470 K both changes the cation site dist ribution somewhat and, most importantly, oxidizes the Mn2+ to Mn3+. Th e Curie-temperature is modified in association with these changes. Wit h this knowledge, we create a series of samples with various sizes but the same microstructure. We find the following. (1) The Curie tempera ture decreases relative to the bulk as size decreases. The decrease is consistent with finite size scaling. (2) The fit of the spontaneous m agnetization to the Bloch T-3/2 law yields a Bloch constant larger tha n the bulk increasing with decreasing size in proportion to the specif ic surface area of the particles. We find a better fit is obtained if the exponent of the temperature is increased to be in the range 1.6 to 1.8. (3) The saturation magnetization decreases with decreasing size also in proportion to the specific surface area of the particles.