Tuning of magnetic properties in cobalt ferrite by varying Fe+2 and Co+2 molar ratios

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Abstract

Different grades of magnetic cobalt ferrite (CoFe2O4) nanoparticles were synthesized with various molar ratios of Fe+2 to Co+2 ions in the initial salt solutions by the co-precipitation method. The crystal structure and morphology of the nanoparticles are obtained from X-ray diffraction and transmission electron microscopy studies. Fourier transform infrared spectroscopy analysis exhibited the Fe–O stretching vibration ~540 cm−1, confirming the formation of metal oxide. The magnetic studies demonstrate that all of the nanoparticles are superparamagnetic at 300 K. The saturation magnetization and coercivity of the CoFe2O4 nanoparticles are affected by the molar ratios of Fe+2 to Co+2 ions. Among all the synthesized nanoparticles, the system with 75:25 molar ratio of Fe+2 to Co+2 ions with a particle size of 13 nm showed a high magnetization of 90 emu/g.

Introduction

Magnetic nanoparticles (MNPs) have shown enormous popularity in the field of biomedical science due to their wide applications. MNPs such as ferrous and ferric as well as cobalt ferrite oxides are commonly used as a means for magnetically induced hyperthermia, particularly in regards to the treatment of tumors [1], [2], [3], [4], [5], [6], [7], [8]. While mixed in hydrogels, MNPs are used for stimuli-induced drug release, cell separation, and as contrast agents for magnetic resonance imaging and even tissue engineering [1], [5]. All of these require that these nanoparticles should have high magnetization behavior and overall narrow particle size distribution, typically in the range of ~100 nm, with uniform physical and chemical properties. Oxide based spinal ferrites are very promising for hypothermic treatment, and among all, iron oxides are widely used for biomedical application due to its well known biocompatibility [9]. However, in order to improve the magnetic efficiency of these oxides material, high magnetic anisotropy and moment are desirable.

The bi-metallic magnetic cobalt ferrites have already been proposed for biomedical applications, which has large magnetic anisotropy compared to other oxide ferrites [10], [11], [12]. The magnetocrystalline anisotropy of Fe3O4 nanoparticles is 14 kJ/m3 while this is as high as 380 kJ/m3 for the CoFe2O4. This large difference allows the possibility of controlling the magnetic properties of iron oxide nanoparticles by doping small amounts of Co+2 ions in Fe3O4. CoFe2O4 has a spinel crystal structure, which crystallizes to cubic closed pack structure of oxygen ions in which tetrahedral (A) and octahedral (B) sites are being occupied by cations. The general formula of CoFe2O4 with cations distribution is represented as (Co+21−cFe+3c)A[Co+2cFe+32−c]BO4, where c is the inversion parameter. Additionally the quantities such as magnetization, coercivity, permittivity, and conductivity are greatly influenced by porosity, grain size and structure of the nanoparticles. Also the properties of these magnetic particles are critically dependent upon the synthesis procedure that makes interest to explore the cause and effect relationship between synthetic strategies and properties of the materials. Magnetic cobalt ferrite has been synthesized by various methods, such as water-in-oil microemulsion, mechanical milling, co-precipitation, hydrothermal methods, and sol–gel like techniques [13], [14], [15], [16], [17], [18], [19], [20]. Among these, the co-precipitation synthesis route is the most convenient one for controlling the morphology and properties of the magnetic nanoparticle. The particle size and magnetic properties can be greatly varied with pH, salt concentration, temperature, and counter ion nature. In past a lot of studies on change of particle size with different reaction parameters have been performed [21], [22], [23]. Recently, Pereira et al. have synthesized magnetic MFe2O4 nanoparticles by a novel one step coprecipitation method using alkaline agents [24]. In spite of the development of various solution based synthesis routes, the synthesis of cobalt ferrite with the desirable size and magnetic properties is still a challenge. To our knowledge, no detailed study has been performed establishing the correlation between the particle sizes and magnetic properties of nanoparticles with respect to the change in molar concentration of Fe+2 and Co+2 ions, keeping Fe+3 ion concentration fixed in the initial salt solutions.

The aim of the present work is to provide a systematic study of the synthesis of well-defined nanocrystalline cobalt ferrite powders that have excellent magnetic properties. The magnetic properties of the complex oxides are modified by the compositional variations by synthesizing different grades of magnetic cobalt ferrite by varying the Fe+2 and Co+2 ion concentrations in the initial salt solutions. We compared the physical and magnetic properties of the nanopowders of different grades and compositions. The crystallinity and the morphology of the nanoparticles were analyzed with an X-ray diffractometer (XRD) and transmission electron microscopy (TEM). The saturation of magnetization of the magnetic particles was determined with Superconducting Quantum Interference Device (SQUID) magnetometer.

Section snippets

Materials

Iron (III) chloride hexahydrate (FeCl3∙6H2O), iron (II) chloride tetrahydrate (FeCl2∙4H2O), 3-aminopropyl triethoxy silane (APS) and citric acid (CA) were obtained from Sigma-Aldrich. CoCl2∙6H2O and ammonium hydroxide (NH4OH) was obtained from Alfa Aesar. All the chemicals and materials used in this work are used as purchased without further purification. The salt solutions were prepared by dissolving desired molar concentration of FeCl3∙6H2O, FeCl2∙4H2O and CoCl2∙6H2O in DI water.

Synthesis of magnetic nanoparticles

Magnetic

Results and discussion

By changing the molar ratio of FeCl2∙4H2O to CoCl2∙6H2O different grades of cobalt ferrite nanoparticles were synthesized by the co-precipitation method. In all the cases the concentration of Fe+3 ions was kept constant. The total molar concentrations of the (II) ions in all the cases are half of the (III) ion concentration. As mentioned above, the ratio of the molar concentration of the Fe+3 and (Fe+2 and Co+2) are fixed to 2:1, by the method of 0.2 M of Fe(III) and 0.1 M of Fe(II) and Co(II)

Conclusions

We have successfully prepared a series of cobalt ferrite nanoparticles, which is based on mixed types of metal-ion precursors, with varying the molar ratio of Fe+2 to Co+2 ions. XRD and TEM studies revealed the presence of high crystalline spinel structure. The experimental calculated molar concentrations of Fe/Co ions of different nanoparticles systems from EDS are well matched with the theoretical values. FTIR analysis revealed the formation of metal oxide and the reduction of maghemite phase

Acknowledgment

This work is supported by the DoD (CEAND) Grant no. W911NF-11-1-0209 (US Army Research Office), NSF-CREST (CNBMD) Grant no. 1036494 (HRD) and NSF-RISE Grant no. HRD-0931373.

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    Present address: Virginia State University, Department of Chemistry and Physics, VA 23806, USA.

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