Magnetic Properties Of CoFe₂O₄-based Composites Prepared By Hydrothermal Synthesis

Spinel cobalt ferrite (CoFe2O4) is a kind of ferrimagnetic oxide. As one of typical magnetic materials, it is of great significance in the fundamental science and technology application. CoFe2O4has its specific physical, chemical, catalytic and magnetic properties. For examples, cobalt ferrite possesses large magnetocrystalline anisotropy constant K1, high Curie temperature, low power loss at high frequencies, good hardness, high coercivity Hc, temperate saturation magnetization, and steady chemical property. These characteristics make CoFe2O4have broad applications in fields of the catalyst, electromagnetic wave absorption, high-density information storage field, drug targeting, ferrofluids, gas sensors, magnetic resonance imaging and so on.Recently, CoFe2O4-based nanocomposite magnetic materials have attracted extensive interets because they exhibit great magnetic performance improvement and physical phenomena such as exchange bias and exchange spring effects. Therefore, it is well worth conducting a series of experiments to investigate the CoFe2O4-based nanocomposite materials.In this dissertation, we have prepared samples via a hydrothermal method and studied the effects of stirring speed, reacting temperature and post-treatment in different ambients on the ingredient, structure and magentic properties of final products.We use various kinds of instruments to study the crystal structure, morphology, lattice fringe, and magnetic properties of samples, such as X-ray diffractometer (XRD), scanning electron microscope (SEM),(High resolution) transmission electron microscopy ((HR)TEM), and quantum design superconducting quantum interference device (SQUID). All measurements were carried out at room temperature. The obtained results have theoretical and practical significance for improving the properties of CoFe2O4-based nanocomposites. This dissertation is divided into six chapters, and the main content of each chapter is as follows:In the first chapter, we introduce the application background, crystal structure and magnetic properties of CoFe2O and overview the research progress of CoFe2O4-based nanocomposites. Finally we point out the research motivation and significance of this dissertation.In the second chapter, we mainly introduce the synthesis and characterization methods of CoFe2O4-based nanocomposites, as well as the starting materials and instruments for experiment.In chapter three, by changing stiring speeds Vs (0,100,200,300,400r/min), we prepared a series of CoFe2O4-based nanocomposites. With increasing Vs, the ingredient of sample changed from CoFe2O4Co0.7Fe0.3(CFO/CF) to single-phase CoFe2O4(CFO). The maximum magnetization and coercivity of obtained samples were88.9emu/g and3010Oe, respectively. With increasing Vs, the amount of soft magnetic CF phase decreases, resulting in the decrease of saturation magnetization Ms; the remanence magnetization (Mr) increases and the maximum remanance to saturation magentization ratio (M,/Ms) reaches0.67; The coercivity (Hc) achieves a maximum at Vs=200r/min. These phenomena can be well explained by magnetocrystalline anisotropy, dipolar interaction and shape anisotropy. The impedance of the materials was also influenced by stirring speed and the related mechanism was also discussed.In chapter four, by changing reaction temperature (80,120,140,160and180℃), we prepared the CoFe2O4/Coo.7Feo.3nanocomposites with different relative content. We used XRD, SQUID, SEM,(HR)TEM and SAED to characterize samples. All samples are composed of octahedral CoFe2O4nanoparticles and spherical-like Coo.7Feo.3nanoparticles. The maximum magnetization and coercivity were191emu/g and1311Oe, respectively, which have not been observed for the CoFe204/Coo.7Feo.3system before. The large saturation magnetization can be attributable to the larger mass ratio of Co0.7Fe0.3to CoFe2O4, which contacted intimately. Magnetic dipolar interaction, which leads to the decrease of Ms and Mr/MS, plays an important role in magnetic properties. The coercivity changed with reaction temperature and exhibited complex variation, and its mechanism deserves further investigation.In chapter five, by changing the ambient conditions (in air and in H2), we annealed the precursor powder obtained from hydrothermal reaction. The precursor powder contain CoFe2O4and Co(OH)2. After annealed in air and in H2ambients, the precursor powders become ferrimagnetic CoFe204/antiferromagnetic CO3O4and Co/Fe alloy, respectively. For the samples annealed in air, the giant exchange bias and coercivity enhancement were observed below the Neel temperature (TN) of CO3O4which can be attributed to the exchange coupling between ferrimagnetic and antiferromagnetic phases. For the samples annealed in H2ambient, a solid solution of Fe and Co (Co3Fe7) was obtained; the average atomic moment for Co3Fe7is2.4μB, larger than that of separated Fe or Co. We also show that the magnetic parameters can be sensitively tuned by varying processing conditions, which may be useful for obtaining the desired magnetic performance.In chapter six, we make a summary for the whole dissertation and make a prospect for the further investigation.