Nanocrystalline MnFe₂O₄ produced by niobium doping

Nanosized MnFe₂O₄ phase with diameters in the range 13.7 to 100 nm were produced by calcination and sintering treatments in the system zNb₂O₅ · (50 – z)MnO · 50Fe₂O₃ with z having values between 0 and 20. Nb⁵⁺ ions are believed to give rise to vacancies in the Mn²⁺ sites, which break up the coupling of ferrimagnetically active oxygen polyhedra. The Curie temperature decreases as the size of the MnFe₂O₄ phase is reduced. This is explained on the basis of a decrease in the number of exchange pairs of the type Mn²⁺–Fe³⁺. The coercivity increases with a decrease in the size of the ferrimagnetic phase. This is believed to arise due to a decrease in saturation magnetization as the size of the MnFeO₄ phase is reduced.