Synthesis of high-coercivity non-stoichiometric cobalt ferrite nanocrystals: Structural and magnetic characterization

doi:10.1016/j.matchemphys.2011.12.049

Materials Chemistry and Physics

Volume 132, Issues 2–3, 15 February 2012, Pages 999-1006

https://doi.org/10.1016/j.matchemphys.2011.12.049 Get rights and content

Abstract

The magnetic properties in nanoscale ferrite materials are strongly dependent on the crystal size, morphology, and cation distribution in the lattice. The present work addressed the synthesis of Co-substituted ferrite nanocrystals were attempted at various staring Fe:Co mole ratios (3:1, 2:1, 1.7:1, and 1.4:1) and the corresponding structural and magnetic properties determined. The synthesis of the ferrite powders was carried out by the conventional and modified coprecipitation method. The later consists of contacting the metal ions solution with hydroxide ions at controlled flow-rates to promote the heterogeneous nucleation, where earlier produced ferrite nuclei will act as seeds, and hence crystal growth. The actual Fe:Co mole ratios in the as-synthesized samples were determined by energy dispersive X-ray spectroscopy (EDS). Obtained nanocrystals were also characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), and Mössbauer spectroscopy techniques. Cobalt ferrite nanocrystals ranging between 11 and 19 nm exhibited coercivity values between 114 and 4412 Oe. The variation in coercivity values of cobalt ferrite nanocrystals with different compositions was mainly attributed to the remarkably enlargement of crystal size under flow-rate controlled synthesis conditions, and the particular distribution of cations between A- and B-sites in addition to surface anisotropy contribution.

Highlights

► Non-stoichiometric cobalt ferrite and the composition-dependence of their magnetic properties. ► 78% of the total Fe present in starting solutions was incorporated into the ferrite. ► Flow-rate controlled synthesis promoted crystal growth and changes in cation distribution. ► High coercivity was attained due to surface anisotropy, cation distribution, and crystal size.

Introduction

The magnetic properties of cobalt ferrite arise as consequence of diverse factors such as crystal size [1], [2], morphology [3], chemical composition [4], [5], and/or cation distribution [6]. Pure and bulk cobalt ferrite (CoFe₂O₄) has inverse spinel structure (Co_xFe_1−x)[Co_1−xFe_1+x]O₄, where Co²⁺ ions have preference for the octahedral site (B) while Fe³⁺ ions distribute equally between tetrahedral [A] and octahedral sites. Substitution of metal ions by foreign cations (transition metals [7] or rare earth [8]) is a common practice when a change in A–B interaction is desired. Because the Fe_A³⁺–Fe_B³⁺ super-exchange interaction is different from the case of Co_A²⁺–Fe_B³⁺, any deviation from the stoichiometric Fe:Co mole ratio (2:1) will lead to an atomic rearrangement between A- and B-sites and/or creation of vacancies [9], with the possibility of other zero-order structural defect types. Consequently, a change in magnetic properties will take place with the variation of the listed factors.

Among the preparation methods available for the production of magnetic nanoparticles (i.e. coprecipitation [10], sol–gel, hydrothermal [11], mechanical milling [12], organic precursor), the coprecipitation method was selected to produce these cobalt ferrite nanocrystals because the synthesis parameters can be easily modified in order to tune magnetic properties. On this basis, cobalt ferrite nanocrystals with different initial Fe:Co mole ratios (3:1, 2:1, 1.7:1, and 1.4:1) were synthesized and characterized in order to determine their structural, morphological, compositional features, and magnetic properties. In addition to chemical composition, the effect of flow-rate of addition of reactants, which leads to crystal growth, was studied [13]. In addition to crystal growth, cation distribution, internal magnetic field, morphology, and chemical composition, the presence of a secondary phase (α-Fe₂O₃), was identified as responsible of the observed wide range of magnetic properties.

Accordingly, cobalt ferrite nanocrystals with initial Fe:Co mole ratios in reacting aqueous solutions of 3:1, 2:1, 1.7:1, and 1.4:1 were produced by the conventional and modified coprecipitation methods. For this later case, the flow-rate at which the reactants were contacted with the boiling alkaline solution was set to 0.67 mL min⁻¹. The reaction time was 1 h in all our experiments.

Section snippets

Materials

All reagents were of analytical grade and used without further purification. CoCl₂·6H₂O (ACS, 98–102%, Alfa Aesar) and FeCl₃·6H₂O (ACS, 97–102%, Alfa Aesar) are the cobalt and iron precursors while NaOH (pellets, 98%, Alfa Aesar) acts as precipitant agent.

Synthesis of cobalt ferrite nanocrystals

The ferrite nanocrystals were produced by contacting an aqueous solution of Fe and Co ions (Fe:Co mole ratios of 3:1, 2:1, 1.7:1, and 1.4:1) with a 0.34 M NaOH solution. The co-precipitation reaction took place under conventional and flow-rate

XRD analyses

Fig. 1, Fig. 2 show the XRD patterns for the ferrite powders synthesized at various Fe:Co mole ratios by the conventional and flow-rate controlled coprecipitation route, respectively. All peaks correspond to a cubic ferrite structure. The average crystallite size and lattice parameter of cobalt ferrite nanocrystals synthesized under the above-described conditions are summarized in Table 1. In the conventional coprecipitation method, the average crystallite size of the ferrite particles ranged

Conclusions

Cobalt ferrite nanocrystals ranging between 11 nm and 19 nm were successfully synthesized by the conventional and modified (flow-rate controlled) coprecipitation methods. TEM analyses evidenced the crystal enlargement for the powders synthesized under flow-controlled conditions; the crystals exhibited a nearly squared habit. EDS analyses on the ferrite nanocrystals revealed that approximately 78% of the total Fe present in starting solutions was incorporated into the ferrite structure. The

Acknowledgements

This material is based upon work supported by the NSF-EPSCoR Institute for Functional Nanomaterials (IFN). TEM analyses were carried out at National High Magnetic Field Laboratory supported by NSF Cooperative Agreement No. DMR-0084173 by the State of Florida.

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Synthesis of high-coercivity non-stoichiometric cobalt ferrite nanocrystals: Structural and magnetic characterization

Abstract

Highlights

Introduction

Section snippets

Materials

Synthesis of cobalt ferrite nanocrystals

XRD analyses

Conclusions

Acknowledgements

Chem. Eng. Sci.

J. Magn. Magn. Mater.

J. Eur. Ceram. Soc.

J. Magn. Magn. Mater.

J. Solid State Chem.

Microelectron. J.

J. Magn. Magn. Mater.

Mater. Lett.

Nano Today

J. Magn. Magn. Mater.

J. Appl. Phys.