This thesis lays importance in the preparation and characterization of a few selected representatives of the ferrite family in the nanoregime.

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1 Preface Nanotechnology is an all inclusive one and is increasingly playing a lead role not only in material science, but also in areas like biology, information technology, photonics and biotechnology. Nanoscience is growing at a faster pace than anticipated and rapid strides are made in this regime. With the advent of nanoscience and nanotechnology, new devices are emerging and with the help of nanotechnology, the obsolete are replaced. Quantum Mechanics played a very seminal role in understanding the fundamental properties at the nanolevel and so physicists naturally take the lead in exploiting this brand new area of science for the benefit of human kind. From time immemorial, magnetism and magnetic materials has been playing a significant role in making life more humane and helping scientists to device new gadgets for various applications. The domain of magnetism or more precisely nanomagnetic materials is bound to play a profound role in helping devise newer applications envisaged using nanotechnology. Its common knowledge that together with magnetism and nanotechnology, devices based on giant magneto resistance will soon see the light of the day. With world wide concern for environment, scientists have been scouting for alternatives for gas based refrigeration and in this journey, they have found an answer in nanocrystalline magnetic materials as potential magnetic refrigerants which are eco-friendly and viable. Ferrimagnetic materials based on ferrites have been contributing their might in various applications like radiofrequency circuits, high quality filters, rod antennas, transformer cores, read/write heads for high speed digital tapes and other devices. The research on ferrites and materials based on ferrites can be traced to the preliminary research carried out by Snoek, Smith and Wijin, Cullity,

2 Chikazumi and others. These materials are revisited again by physicists and chemists because quantum size effects of these materials are pronounced at nanodimensions. At the nanosize, these materials exhibit superlative magnetic and electrical properties and are now a subject of intense research especially to delve into the fundamental aspects like quantum mechanical effects at the nanosize. With the emergence of nanotechnology there is renewed interest in ferrites since many of the useful properties of these materials can be modified suitably for applications in magnetic storage, as precursors for ferrofluids, as contrast enhancing agents in Magnetic Resonance Imaging (MRI), as magnetic refrigerant materials in magnetic refrigeration technology and also as magnetically guided drug delivery agents. The ease with which the structural and magnetic properties of these materials can be tailored has made ferrites an ideal candidate for studying the size effects at the nanoregime from a fundamental perspective. As far as applications are concerned, the properties can be tailored by a judicious choice of cations present in the ferrite materials. Ferrites are also important for magnetic refrigeration applications because at nanodimensions, they exhibit phenomena like superparamagnetism, spin glass behaviour and blocking. Though most of the electrical and magnetic properties of these materials are well understood at the micron regime, there are grey areas where lot of research needs to be carried out. For example, the electrical properties of nanoferrites are not well understood and very little literature exists on the mechanism of conduction. Manganese zinc ferrites belong to the class of mixed spinel ferrites. The equilibrium distribution of cations in the bulk structure is influenced by a number of factors namely ionic radii, ionic charge, lattice energy, octahedral site preference energy and crystal field stabilization energy. In the coarser regime, Zn 2 ' has a strong preference for tetrahedral sites while Ne+ exhibits a strong octahedral preference in spine! ferrites. Cations like Mn2+ IMn 3 + are found to be influencing the magnetic,

3 structural and electrical properties considerably. In the nanoregime, in the case of spinel ferrites, there are distinct deviations in magnetic properties with respect to their bulk counterparts. However the role of cations vis a vis their occupancy of octahedral sites instead of tetrahedral sites is still not clear as regard their influence in deciding the overall magnetic properties of these materials in the nanoregime. It is in this context that a study of nanosized MnFe204 with varying concentration of zinc assumes significance. Ferrites belonging to the series, MnJ_xZnxFe204 (for x=o, 0.1, ) provide an ideal platform to check various hypotheses. Cobalt ferrite distinguishes itself among ferrites on account of its wide ranging applications in magnetic recording devices, magneto-optical devices, sensors based on magnetostrictive property etc. They exhibit interesting magnetic properties with their major contribution coming from the magnetocrystalline anisotropy of cobalt. Magnetic metal nanoparticles are also a subject of intense research because of their potential applications. In this context, iron and nickel needs special mention. However, these materials in the nanoregime possesses large surface area and prone to oxidation. Hence passivation of these nanopartic\es is a prerequisite and nanocomposites based on these nanosized metal nanopartic\es are sought after for various applications in catalysis, bio sensors, spin polarized devices, carriers for drug delivery etc. Template assisted preparation of metal nanoparticles are a viable alternative. Inexpensive methods which can be carried out in ordinary laboratories are often adapted to. Thus the synthesis of nanocrystalline spinej ferrites and metal polymer nanocomposites and the evaluation of their multifunctional properties are significant both from a fundamental as well as from an applied perspective. This thesis lays importance in the preparation and characterization of a few selected representatives of the ferrite family in the nanoregime. The

4 candidates being manganese zinc ferrite and cobalt ferrite prepared by coprecipitation and sol-gel combustion techniques respectively. The thesis not only stresses importance on the preparation techniques and optimization of the reaction conditions, but emphasizes in investigating the various properties namely structural, magnetic and electrical. Passivated nickel nanocomposites are synthesized using polystyrene beads and adopting a novel route of ion exchange reduction. The structural and magnetic properties of these magnetic nanocomposites are correlated. The magnetocaloric effect (MCE) exhibited by these materials are also investigated with a view to finding out the potential of these materials as magnetic refrigerants. Calculations using numerical methods are employed to evaluate the entropy change on selected samples. The results are reported in the thesis. This proposed thesis 1S entitled "On the Synthesis and M ultifunctional Properties of some Nanocrystalline Spinel Ferrites and Magnetic Nanocomposites" and consists of eight chapters. The significance of nanomagnetic materials and their applications in nanotechnology are briefly introduced in Chapter 1. The general structural, magnetic and electrical properties of the ferrites in the bulk and nanoregime are briefly discussed. A comprehensive picture of different types of nanocomposite, their synthesis procedure, properties and applications are also included in this chapter. Finally the motivation and objectives of the work are outlined. Chapter 2 deals with analytical techniques like X-ray diffraction and transmission electron rnicroscopy (TEM), used for the structural characterization of the magnetic nanoparticles. Energy dispersive spectrum (EDS) and inductively coupled plasma analysis (ICP), used for verifying the stochiometry are also provided in the chapter. The details of magnetic measurements and iv

5 magnetocaloric measurements are also mentioned. Theoretical and experimental aspects of dielectric and electrical measurements are also explained in this chapter. Chapter 3 essentially discusses the synthesis, structural and magnetic characteristics of the mixed ferrite series Mnl_xZnxFe204 (for x=o, 0.1, ). Emphasis is given to the impact of zinc substitution on these characteristics. A detailed analysis of the electrical properties of manganese zinc mixed ferrites as a function of zinc substitution is given is Chapter 4. The various conduction mechanisms for the different compositions are also suggested. Chapter 5 presents the synthesis and characterization of cobalt ferrite nanoparticles. The particulars of structural magnetic and electrical characteristics are analysed and correlated. The novel technique of synthesis of metal polystyrene nanocomposites (Nickel-polystyrene and iron-polystyrene nanocomposites) forms the main theme of Chapter 6. The effect of cycling (the loading reduction cycle involved in the synthesis) in enhancing the structural and magnetic characteristics is also depicted in this part of the thesis. In Chapter 7, the indirect technique of measuring magnetocaloric effect (MCE) is employed to measure the magnetocaloric properties of cobalt ferrite nanoparticles and Nickel polystyrene nanocomposites. The importance of the measurements is highlighted. Chapter 8 is the concluding chapter of the thesis and in this chapter the salient observations and the inferences drawn out of these investigations are presented in a nutshell. The scope offurther work is also proposed here. v