STRUCTURAL, OPTICAL AND MAGNETIC PROPERTIES OF SPINEL OXIDE NANOPARTICLES

Abstract

The AB2O4 type magnetic transition metal oxides with cubic spinel structure are important class of materials due to their wide potential applications, such as sensors, catalysts, Li-ion batteries, high density magnetic recordings, data storage, magnetic resonance imaging and drug delivery. These materials have some important properties such as superparamgnetism, exchange bias effect, spin canting/spin glass, anisotropy. These properties could be tuned by varying shape and size of the materials which could be tuned with the help of synthesis method. The cationic distribution of the spinel ferrites also plays a very important role to determine the structural optical and magnetic properties of the material. In these spinel oxides, oxygen atoms form an fcc close-packed structure whereas A2+ and B3+ ions occupy either the tetrahedral (A-site) or the octahedral (B-site) interstitial sites. There are 8 A-sites at which the metal cations are tetrahedrally coordinated with oxygen, and 16 B-sites which possess octahedral coordination. There are mainly two types of spinel structures, normal and inverse spinel structures. In normal spinel structure, divalent atoms (A2+) occupy tetrahedral site and trivalent atoms (B3+) occupy octahedral sites, i.e. cationic distribution corresponds to (A2+)[B2 3+]O4. On the other hand, in the inverse spinel structured compounds the divalent atoms occupy octahedral site and trivalent atoms are equally distributed at both tetrahedral and octahedral sites and therefore cationic distribution is represented as (B3+)[A2+B3+]O4. However, when both trivalent and divalent atoms occupy both the tetrahedral as well as octahedral sites, it is known as mixed spinel or partially inverse spinel structure. In most of the spinels, the cationic distributions possess an intermediate degree of inversion where both sites contain a fraction of the A2+ and B3+ cations. The cationic distribution depends on the inversion parameter which is the deciding factor of crystal structure of the spinel metal oxides. Magnetically, spinel ferrites display ferrimagnetic ordering. The magnetic moments of cations in the A and B-sites are aligned parallel with respect to one another. Between the A and B-sites the arrangement is antiparallel and as there are twice as many B-sites as Asites, there is a net moment of spins yielding ferrimagnetic ordering for the crystal. The choice of metal cation and the distribution of ions between the A and B-sites, therefore offers a tunable magnetic system. For example, Fe3O4 has inverse spinel structure with cationic distribution (Fe3+)tetra[Fe2+Fe3+]octO4 . On the other hand, CoFe2O4 and CuFe2O4 iv with the cationic distribution (Co1-δ 2+Feδ 3+)tetra[Coδ 2+Fe2-δ 3+]octO4 and (Cuδ 2+Fe1-δ 3+)tetra[Cu1- δ 2+Fe1+δ 3+]octO4, respectively, are partially inverse spinel structured compounds. Moreover, Co3O4 and NiCo2O4 with cationic distributions (Co2+)tetra[Co3+]octaO4 and (Ni2+Co2+)tetra [Co3+]octaO4, respectively, exhibit normal spinel structure. Compared to the bulk materials, spinel ferrites nanoparticles have much unique properties such as superparamagnetism, spin disorders: spin canting/spin glass, high coercivity, lower Curie temperature and high magnetic susceptibility etc. Recently, superparamagnetism in spinel ferrite nanoparticles have attracted much interest due to their possible application in drug delivery and magnetic resonance imaging. In the present thesis work, we have studied the structural, optical and magnetic properties of some spinels nanoparticles synthesized by co-precipitation method. We have observed some novel and unique magnetic behavior, such as metamagnetic transition, superparamgnetism and spin canting in the grown nanoparticles. The present thesis is divided into six chapters. The chapters are as follows: The chapter 1 contains an introductory aspect and survey of the field. This chapter covers the fundamentals of magnetism and exceptional feature of magnetism in nanodimensions. How the characterstics of the material changes in nano-field. The unique and exceptional features such as spin frustration, finite size effect, surface disorder and spin canting are explained in this chapter. The crystal structure, optical and magnetic properties of spinel ferrites and corresponding phenomena to understand the novel properties observed in our synthesized materials, i.e. Metamagnetic transition, spin canting and surface spin disorder. The systematic literature review and motivation of the present work is also summarized in this chapter. The chapter 2 describes the prominent experimental techniques and methodology employed in the present investigations for synthesis and study of structural, optical and magnetic properties of synthesized nanoparticles of ferrites. In this chapter, we have described the models and methods with help of them we have used to analysis our data. The chapter 3 has two parts. In the first part we have described in first part the work related to the structural, optical and magnetic properties of Fe3O4 nanoparticles and in the second part the v structural, optical and magnetic properties of Co3O4 nanoparticles have been presented. The Fe3O4 nanoparticles have been synthesized at different pH (~ 7, 11 and 12) via co-precipitation method. The Fe3O4 nanoparticles synthesized at pH ~ 12 has been further annealed at 230 °C with the aim to study the effect of particle size variation on the structural, optical and magnetic properties. The x-ray diffraction (XRD) results of as synthesized Fe3O4 nanoparticles reveal formation of spinel structure with space group Fd- 3m. Further, XRD, FESEM and TEM results confirm the nanocrystalline nature of the as synthesized samples and variation in particle size with pH and after annealing. The optical measurements show two band gaps in Fe3O4 nanocrystalline samples. Field dependent magnetization measurements (M-H) reveal superparamagnetic nature at room temperature and ferromagnetic behavior at low temperature (~5 K). Furthermore, M-H plots measured at 5 K show presence of metamagnetic transition in all Fe3O4 samples. The metamagnetic transition along with ferromagnetic behavior at low temperature in Fe3O4 nanoparticles are observed first time in the present work to the best of our knowledge. Further the value of magnetization decreases with decreasing particle size at both temperatures. The fitting of the field cooled (FC) temperature dependent magnetization (M-T) data with modified Bloch-spin wave model with additional surface disorder term and mixed magnetic phases indicates surface spin disorder and mixed magnetic phases in the as synthesized samples, which may be the possible reason for the existence of metamagnetic transition in the samples. The correlation between the observed magnetic properties and structural characteristics of the samples with the synthesis parameters (pH value and annealing effect) has been described and discussed in this chapter. In the second part of Chapter 3, we have synthesized the Co3O4 nanoparticles with the help of low temperature co-precipitation method and studied the structural, optical and magnetic properties. The X ray diffraction analysis of the synthesized samples reveals the formation of single phase cubic spinel structure with the space group Fd-3m. The FESEM and TEM results indicate the formation of nano-sized particles. The optical measurement reveals the two band gaps ~2.77 and 1.67 eV in the sample. The magnetic measurement shows weak ferromagnetic interaction in Co3O4 along with usual paramagnetic nature at room temperature. However, at low temperatures the sample shows antiferromagnetic vi interaction. The correlation between the structural and observed magnetic and optical properties of Co3O4 nanoparticles has been described and discussed in this chapter. The chapter 4 contains four sub sections. In the first section we have described structural, optical and magnetic properties of CoFe2O4 nanoparticles synthesized by annealing at 400 ºC, 600 ºC and 800 ºC via a co-precipitation method. The second section describe the effect of variation of pH (~ 7, 10 and 12) on the structural, optical and magnetic properties of CoFe2O4 nanoparticles. The third and fourth sections are related to the study of the effect of Cr and Mn doping, respectively, on the structural, optical and magnetic properties of CoFe2O4 nanoparticles The structural analysis of the CoFe2O4 samples, synthesized via co-precipitation method at different annealing temperature and at different pH, indicates the structural distortion at the octahedral site which has been found correlated with the observed room temperature magnetic moment of the CoFe2O4 nanoparticles. Magnetic studies reveal multiple magnetic phases and surface spin disorder in the samples. Spin canting/glass behaviour is also observed in the as synthesized CoFe2O4 nanoparticles. In addition to these, the as synthesized samples also show a novel metamagnetic transition due to the coexistence of antiferromagnetic and ferromagnetic ordering, simultaneously. The coexistence of antiferromagnetic and ferromagnetic phases is attributed to spin canting/surface spin disorder in the nanocrystalline samples. Mossbauer data also supports the presence of spin canting in the samples. The Cr and Mn doped samples have been synthesized with compositions MxCo1- xFe2O4 (M= Mn/Cr, x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) via co-precipitation method. The Xray diffraction patterns of the synthesized samples reveal the formation of single phase cubic spinel structure with the space group Fd-3m for all compositions. However, an impurity peak of α-Fe2O3 has been observed in the XRD patterns of samples with Cr and Mn composition, x ≥ 0.4. However, this impurity phase of α-Fe2O3 is not observed in the MnFe2O4 sample (x=1) synthesized by annealing at 500 oC. The magnetic measurements show ferrimagnetic interaction in MxCo1-xFe2O4 nanoparticles at room temperature. However, MnFe2O4 sample synthesized by annealing at 500 °C shows superparamagnetic vii behavior. We have fitted the M-H curve of MnFe2O4 synthesized by annealing at 500 °C with modified Langvenin function to confirm superparamagnetism. However, Cr doped samples indicate the metamagnetic transition at low temperature. The M-T data also indicates the surface disorder in the synthesized samples. The correlation between the structural and observed magnetic properties of synthesized nanoparticles has been described and discussed in this chapter. The chapter 5 contains three sections. The first section is related to the study of the Structural, Optical and magnetic properties of FeCo2O4 nanoparticles, wherever the second and third sections contain the work related to the Effect of Mn and Zn doping, respectively, on structural and magnetic properties of FeCo2O4 nanoparticles. The nanocrystalline samples of FeCo2O4 have been synthesized via co-precipitation method. The X-ray diffraction, FTIR, Raman and XPS results confirm the formation of mixed spinel structure of the synthesized sample. The FESEM and TEM results exhibit the nanocrystalline nature of the synthesized FeCo2O4 samples. The optical measurement shows two band gaps ~ 2.48 eV and 1.51 eV, in the sample. The magnetic measurement shows ferrimagnetic interaction in the sample at room temperature. Furthermore, M-H measurements of the sample show interesting metamagnetic transition below 200 K. The Mn and Zn doped samples have been prepared with the compositions MxFe1- xCo2O4 (M=Mn/Zn, x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) with help of co-precipitation method. The X-ray diffraction, Raman and FTIR results reveal the mixed spinel phase in the as synthesized samples. The FESEM and TEM results confirm that the samples are nanocrystalline. The FESEM and TEM results indicate that the microstructures of the FeCo2O4 sample changes with doping level (x) of Mn. XPS results show that each metal ion in the as synthesized samples are present in two possible oxidation states, such as Co2+- Co3+, Fe2+-Fe3+ and M3+-M4+. From Optical measurements, it has been observed that the band gap of as synthesized samples decreases with increase in Mn and Zn doping concentration. The M-H measurements at room temperature show that magnetization decreases with the increment of Mn and Zn content in samples. The metamagnetic transition observed in FeCo2O4 samples at low temperature decreases with increase in Mn viii and Zn content in the samples. The magnetic versus temperature measurements indicate that the TC shifts towards lower temperature with increasing Mn and Zn concentration. Furthermore, a sharp ferromagnetic to paramagnetic transition is observed at 183K and 150K in MnCo2O4 and ZnCo2O4, respectively. The chapter 6 contains the brief summary of the work presented in the thesis through chapter’s three to five. The overall comments and recommendations have also been given in this chapter.