Role of Rare-earth Ions on Structural, Dielectric and Magnetic Properties of Nanosized CoFe2O4 and its Scope for Application

Abstract

This thesis work preferentially aspires to tune the structural, dielectric, and magnetic properties of undoped and rare-earth (RE) (Ho, Gd, Nd, La) doped CoFe2O4 nanoparticles synthesized by glycine nitrate method auto-combustion technique. RE ions possess unpaired 4f electrons that influence the microstructure, electrical and magnetic properties of the substituted ferrite. When the partial replacement of Fe3+ in spinel structure by RE ions is done, they modify the cell symmetry and induce internal stress. As a result, not only the structural properties such as cell parameters, the lattice constant, the crystallite size of the material changes but also modifies the electric, dielectric and magnetic properties. It is observed that substitution of RE has improved the properties of ferrites but a detailed investigation has not been examined much. So in the present study influence of RE ions Ho, Gd, Nd, La in cobalt ferrite nanoparticles is investigated. Structural, electrical, dielectric, and magnetic properties are investigated in detail. An investigation on the electrical transport characteristics of a series of RE doped cobalt ferrite is carried out to achieve an insight into the different mechanism that operates at nano dimensions. The enhanced properties of the as-synthesized nanoparticles have been explained in terms of cation distribution, structural modifications, and synthesis techniques. Among the different RE elements, lanthanum ion substituted cobalt ferrites are generally of great interest as La3+ possesses a decisive influence on the structural dielectric and magnetic properties when compared to other RE ions. Furthermore, replacing of La3+ for Fe3+ in CFO shows a significant saturation magnetization value, which is suitable for magnetic recording applications. The aforementioned work has been described systematically in eight different chapters as follows:
Chapter I consists of a brief introduction to nanotechnology, ferrites, types and classification, and magnetism and their classifications, properties of nanospinel ferrites, literature, and motivation of the present work. The role of RE substitution in cobalt ferrite and possible modification has been discussed. Further, in the ensuing section, the vi orientation of the research problem proposed and its scope for the future is discussed in detail.
Chapter II provides a detailed description of the synthesis techniques used to synthesize ferrite nanoparticles. Though various synthesis techniques are used for preparing ferrites, in this present chapter we have briefly discussed glycine nitrate auto-combustion, co-precipitation, and solid-state techniques and green synthesis using Aloe-vera plant extract. Among the aforementioned synthesis technique, we have preferred glycine nitrate auto-combustion for the preparation of rare-earth substituting cobalt ferrite. The detail information of the experimental methods and physical characterization techniques used to characterize the synthesized ferrite such as structural properties i.e. X-ray diffraction and Fourier transform Infrared (FTIR) spectroscopy, FESEM and TEM for morphological analysis, impedance and modulus analysis, dielectric properties, Magnetic properties and Mössbauer study has been provided.
Chapter III begins with the structural characterization of undoped CoFe2O4. All the nanoparticles have been synthesized by glycine nitrate method using analytical reagent grade nitrates. The electrical, dielectric, and magnetic studies of CoFe2O4 nanoparticles are discussed in detail. The analysis of complex impedance study using theoretical modeling and the electrical transport properties (ac conductivity and dc resistivity), dielectric properties, fitting of dielectric dispersion behaviour by using a modified Debye model is provided. The dielectric constant, dielectric loss and dielectric loss tangent are found to decrease as a function of frequency and temperature.
Chapter IV introduces the incorporation of Holmium (Ho3+) in CoFe2O4 having the chemical formula CoFe2-xHox04 (x= 0.0, 0.1, 0.3, 0.5, 0.7). The particle size obtained from the XRD data well-matched with calculated particle size from the micrographic images obtained. A non-Debye type relaxation was obtained for the samples and is explained based on Maxwell-Wagner polarization following the Koop's theory. Magnetic measurements such as saturation magnetization, coercivity, and remnant magnetization were found to be decreasing with increasing Ho3+ substitution which was due to anisotropic A-B site interactions. Mössbauer data elucidate the existence of a negligible superparamagnetic component due to nanosized particles. High-frequency behavior, low losses, and high DC electrical resistivity suggest the Ho doped CoFe204 to be used in novel electromagnetic devices.
Chapter V deals with the structural, electrical, dielectric and magnetic properties of Gadolinium (Gd3+) substituted CoFe2O4 having the chemical formula CoFe2-xGdx04 (x= 0.0, 0.1, 0.3, 0.5, 0.7). The obtained XRD diffraction patterns obtained is a single phase with the mark of Gd(OH)3 at higher Gd content. Surface elucidation reveals that the grain size decreases with increasing the Gd dopant content. The temperature-dependent dielectric constant, dielectric loss, electrical resistivity, and conductivity studies signify the presence of a net correlation and microstructural effect. Impedance analysis indicates a decrease in resistance with the presence of grain boundary contribution and a minimum grain contribution. The variation in magnetic properties of Gd doped CFO is correlated to the variation of grain size on Gd3+ doping, corroborating the incorporation of Gd3+ into the CFO matrix. The synthesized ferrites are found to be suitable in electromagnetic devices operating at high frequencies.
Chapter VI covers the structural, electrical, dielectric and magnetic properties of Neodymium (Nd3+) substituted CoFe2O4 having the chemical formula CoFe2-xNdx04 (x= 0.0, 0.1, 0.3, 0.5, 0.7). The investigation of crystal parameters using the XRD technique of the synthesized samples indicates that all the materials crystallize in the spinel structure which is also confirmed by the FTIR and Mössbauer spectra. DC electrical resistivity suggests that the as-synthesized samples exhibited semiconducting behavior. Incorporation of Nd increases the dielectric constant of the samples massively and a very low loss is obtained. This favours the synthesized samples to have applications at high frequencies. Magnetic studies signify that the inclusion of Nd3+ ions weakens the hysteresis loop, which also reduces the coercive field (Hc), favouring its application in magneto-recording devices.
Chapter VII covers the structural, electrical, dielectric and magnetic properties of Lanthanum (La3+) substituted CoFe2O4 having the chemical formula CoFe2-xLax04 (x= 0.0, 0.1, 0.3, 0.5, 0.7). The XRD data reveals the existence of an ortho-ferrite phase at higher La3+ doping. frequency-dependent complex impedance analysis suggests the existence of grain, grain boundary co-contribution, and temperature-dependent relaxation. An enhancement in the dielectric values was obtained at lower frequency making the material suitable for high-frequency devices. Magnetic properties such as saturation magnetization and remnant magnetization were found to decrease with an increase in La concentration.
Chapter VIII outlines the work carried out in the thesis with the future scope. The obtained results in this thesis will provide the usefulness of the data for further research on RE doped ferrites with improved quality using other methods or the same under better experimental conditions. The use of other characterizations like high- temperature XRD, viii time-resolved spectroscopy, optical, and UV spectroscopy is believed to illustrate a broader view of our samples. The results of these nanoferrites may be useful for application in various fields of science, industry, and technological fields.