Magnetic properties of Mg-ferrite nanoparticles

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Abstract

MgFe2O4 nanoparticles encapsulated in amorphous SiO2 were prepared by means of a wet chemical method, and the magnetic properties were studied. The diameters of these particles were estimated from X-ray diffraction patterns as ranging from 3 to 8 nm. Magnetization measurements were carried out for each sample under a ±50 kOe field. The blocking temperature, Tb, was determined to be around 30 K from the temperature dependence of both the field-cooled (FC) and the zero-field-cooled (ZFC) magnetization. The MH curve indicated the presence of ferromagnetic behavior with hysteresis below the blocking temperature, Tb. A large coercive force of about 1000 Oe was found at 5 K. Heat treatment was carried out for the prepared samples using an electric furnace in air. A clear difference in magnetization was found between the quenched samples and annealed samples. This phenomenon suggests that the Mg2+ ions in the spinel structure would be randomly distributed into both the A- and the B-site.

Introduction

Magnetic nanoparticles have been drawing much attention because they show unique features such as quantum size effects and magnetic tunneling. They are also important for technological applications, not only in high-density magnetic recording systems but also as a material in the medical field. We studied the magnetic properties of Ni–Zn ferrite nanoparticles and found remarkable characteristics [1]. In this paper, we present the synthesis of Mg–ferrite (MgFe2O4) nanoparticles ranging from 2.2 to 6.1 nm in size, and their magnetic characteristics. Mg–ferrite bulk crystal has an inverse spinel structure, where the A sublattice contains half of the Fe3+ ions and the B sublattice contains the other half together with all of the Mg2+ ions. Here, the A sites are tetrahedral sites and the B sites are octahedral sites. It is very interesting that MgFe2O4 shows magnetism even though Mg2+ ions are non-magnetic. This may be due to the incomplete inverse spinel structure of MgFe2O4. There is also a remarkable phenomenon where the ion distribution of Mg2+ ions varies depending on the quenching temperature. This phenomenon suggests that the saturation magnetization and the transition temperature vary with the heat treatment conditions. So, in this article, the effect of heat treatment on magnetizations depending on the annealing time will also be reported.

Section snippets

Experiment

Mg–ferrite nanoparticles were prepared by means of wet chemical method using a mixture of MgCl2·6H2O, FeCl2·4H2O and Na2SiO4·9H2O in aqueous solutions. The resulting precipitates were washed several times with distilled water and dried in a water bath at about 350 K. The obtained solids were annealed at temperatures between 873 and 1073 K in air. All samples were also quenched in air at room temperature after annealing.

The prepared samples were examined by means of Cu Kα X-ray (λ=0.154 nm) powder

Results and discussion

The X-ray powder diffraction patterns showed amorphos SiO2 around 2θ=23 and peaks of the spinel structure with a lattice constant of about 0.843 nm for the 3.1-nm particles. This value is slightly larger than that for the 0.836-nm bulk crystal. No other phase was found in the patterns, even though the patterns for the samples annealed at lower temperatures were fairly broad. So the prepared samples were found to be MgFe2O4 in amorphous SiO2. The size of the nanoparticles was determined from the

Acknowledgement

This work was partially supported by a Grand-in Aid for Science Research (No. 15510089) from the Japan Society for the Promotion of Science.

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