Nanocrystalline MnFe 2 O 4 produced by niobium doping
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Nanocrystalline MnFe2O4 produced by niobium doping T.K. Kundu Indian Association for the Cultivation of Science, Jadavpur, Calcutta, 700 032, India
D. Chakravorty Indian Association for the Cultivation of Science, Jadavpur, Calcutta, 700 032, India, and Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, 560 064, India (Received 31 December 1998; accepted 3 August 1999)
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.
I. INTRODUCTION
Investigation of the properties of nanocrystalline materials has been an active area of research in recent years. Investigations of the effect of material dimension on the transition temperature of ferromagnetic systems have been reported.1,2 The Curie temperature in nanoscale MnFe2O4 particles has been found to increase as the particle size is reduced.2 However, some studies indicate an opposite effect.1 This has been explained as arising due to different cation distributions in materials synthesized by different techniques.3 Various physical and chemical methods have been used to prepare nanoscale particles of magnetic materials.1,2,4 In a recent work, formation of nanoscale domains of a ferroelectric phase was reported by a suitable doping of an aliovalent ion, which brings about vacancies in some cation sites. The latter appears to destroy the long range ferroelectric order.5,6 We have used this approach to prepare nanocrystalline MnFe2O4. The Curie temperature in the present series of samples shows a decrease as the particle size is reduced. The details are reported here.
II. EXPERIMENTAL
The starting materials used were MnCO3, Fe2O3 and Nb2O5. The target compositions can be represented by zNb2O5 ⭈ (50 − z)MnO ⭈ 50Fe2O3 with z having values of 0, 5, 10, 15, and 20 respectively. Weighed amounts of the precursor powders (AR grade) were mixed in a mortar under acetone for 2 h. The mixture was then calcined at 1073 K for 2 h. The calcined powder was cold pressed at a pressure of 5 bars. The resulting pellets were J. Mater. Res., Vol. 14, No. 10, Oct 1999
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sintered at 1273 K for 2 h. The crystalline phases present were identified by diffractograms taken in a Rich Seifert 3000P x-ray diffractometer. The microstructure of the materials was investigated with a JEM 200 CX transmission electron microscope. Mag
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