Finite size effects on the structural and magnetic properties of sol–gel synthesized NiFe2O4 powders

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Abstract

Nanoparticles of nickel ferrite have been synthesized by the sol–gel method and the effect of grain size on its structural and magnetic properties have been studied in detail. X-ray diffraction (XRD) studies revealed that all the samples are single phasic possessing the inverse spinel structure. Grain size of the sol–gel synthesized powders has been determined from the XRD data and the strain graph. A grain size of 9 nm was observed for the as prepared powders of NiFe2O4 obtained through the sol–gel method. It was also observed that strain was induced during the firing process. Magnetization measurements have been carried out on all the samples prepared in the present series. It was found that the specific magnetization of the nanosized NiFe2O4 powders was lower than that of the corresponding coarse-grained counterparts and decreased with a decrease in grain size. The coercivity of the sol–gel synthesized NiFe2O4 nanoparticles attained a maximum value when the grain size was 15 nm and then decreased as the grain size was increased further.

Introduction

Synthesis of advanced ceramics as nanoparticles is currently gaining widespread interest in material processing technology [1], [2], [3], [4], [5]. Owing to the extremely small dimensions of nanostructured materials, a major portion of the atoms lie at the grain boundaries, which in turn is responsible for superior magnetic, dielectric and mechanical properties in these materials compared to their conventional coarse grained counterparts [6], [7], [8]. The different chemical processes currently in vogue for the synthesis of nanoparticles include co-precipitation [9], combustion method [10], sol–gel process [11], spray pyrolysis [12], micro emulsion technique [13] and hydrothermal process [14]. Among these methods, sol–gel process allows good control of the structural and magnetic properties of ceramic materials. The advantages of this method include processing at low temperatures, mixing at the molecular level and fabrication of novel materials.

Nanosized magnetic particles exhibit unique properties and have promising technological applications in high-density recording, colour imaging, ferrofluids, high-frequency devices and magnetic refrigerators [15], [16]. Nanoparticles of magnetic ceramic materials are also widely used as contrasting agents in magnetic resonance imaging (MRI), replacement of radioactive materials used as tracers and delivery of drugs to specific areas of the body. From the application point of view, the most significant properties of magnetic ceramic materials, namely magnetic saturation, coercivity, magnetization and loss, change drastically as the size of the particles move down into the nanometric range [17], [18], [19]. Among the different ferrites, which form a major constituent of magnetic ceramic materials, nanosized nickel ferrite possess attractive properties for application as soft magnets and low loss materials at high frequencies [20]. Moreover, there are numerous reports wherein anomaly has been reported in the structural and magnetic properties of spinel ferrites in the nanoscale [21], [22], [23]. A typical example is zinc ferrite, which is a normal spinel exhibiting antiferromagnetism with a Neel temperature of ∼10.5 K in the micron regime [24]. However, nanoparticles of zinc ferrite prepared by different techniques show ferrimagnetic characteristics with ordering temperatures above room temperature. Nickel ferrite in the ultrafine form, is an inverse spinel exhibiting noncollinear spin structure and the magnetic moment at low temperatures is appreciably lower than the value for the bulk material [25]. Chinnasamy et al. found a mixed spinel structure for nickel ferrite when the grain size is reduced to a few nanometers [26]. They deducted this with the help of Mössbauer spectroscopy, magnetization and EXAFS. They observed that in addition to the canting of surface spins because of the broken exchange bonds, the core spins could also have canted spin structure, resulting from the occupation of the tetrahedral sites by Ni2+ ions. Kodama et al. [27], [28] observed anomalous magnetic properties for their organic coated nickel ferrite nanoparticles. Hence, the synthesis of nickel ferrite as nanoparticles and the investigation of their various physical properties depending on the grain size continue to be an active area of research in the field of material science.

In the present investigation, nickel ferrite was prepared by sol–gel technique and they were heat treated at different temperatures to synthesize magnetic particles with varying grain size and study the finite size effects of their magnetic properties. To the best of our knowledge, such a study has seldom been reported in sol–gel derived samples in which there is a higher degree of crystallinity than in the co-precipitated samples.

Section snippets

Synthesis techniques

Phase pure NiFe2O4 has been prepared by sol–gel technique taking Fe(NO3)3·9H2O and Ni(NO3)2·6H2O in a 2:1 ratio and dissolving them in ethylene glycol at about 40 °C. After heating the sol of the metal compounds to around 60 °C a wet gel is obtained. The obtained gel when dried at about 100 °C self ignites to give the desired product. The samples were also fired at 300, 600 and 900 °C for 12 h.

X-ray diffraction studies

The structural characterization of all the four samples were carried out by the X-ray diffraction (XRD)

Results and discussions

The XRD pattern of all the four samples of NiFe2O4 powders synthesized by the sol–gel technique is depicted in Fig. 1. All the characteristic peaks of NiFe2O4 are present in the diffraction pattern. The XRD data agrees well with the standard JCPDS values [29]. However, a sharp increase in the crystalline nature of the nickel ferrite powders is observed as the firing temperature was increased which is recorded as a decrease in the broadening of the peaks in the diffraction pattern. This clearly

Conclusion

The XRD studies of the sol–gel synthesized NiFe2O4 showed that it has the inverse spinel structure and the X-ray data agreed well with the reported data. It was observed that in addition to a change in the grain size, strain is also induced during the firing process. The magnetic studies carried out using the Vibrating Sample Magnetometer showed that the specific saturation magnetization (σs) of the nanosized NiFe2O4 decreased as the grain size decreased. As the grain size was increased by the

Acknowledgments

MG thanks the University Grants Commission, Government of India for the fellowship. MRA and SSN acknowledge financial assistance provided by DST (File No SP-S2-M- 64/96). AMJ is grateful for the financial assistance offered in the form of a post doctoral fellowship in Nanoscience and Nanotechnology by DST, Govt of India.

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