Magnetic Exchange Interactions: The Synthesis and Characterization of Superparamagnetic NiFe2O4 Nanocrystals
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Magnetic nanoparticles have been pursued for various applications that include catalytic systems, data storage, biomedical imaging, drug delivery, sensors, and nanoelectronics. For many of these applications it is essential to tune the magnetic properties through careful synthetic manipulation to control chemical composition, morphology, and crystallite size. Further understanding of the fundamental relationships between magnetic properties and microstructure is necessary to meet the emerging technological needs. Spinel ferrite nanoparticles provide a flexible system to study this relationship due to their unique ferrimagnetism, variable chemical composition, and chemical stability. The purpose of this thesis is to study the unique magnetic properties through the synthesis of spinel ferrite based nanoparticles of variable composition, exchange-coupled hard/soft core@shell nanoparticles, and exchange biased ferrimagnetic (FiM)@ antiferromagnetic (AFM) nanoparticles. Chapter 2 studies synthesis of redox-sensitive NiFe2O4 nanoparticles through modification of the aminolytic synthetic method (a method developed in the Zhang group). Through seed-mediate modified-aminolytic synthesis a study of the size-dependent magnetic properties of NiFe2O4 nanoparticles ranging from ~4-10 nm in diameter was performed as well an investigation into their ferromagnetic resonance properties. Chapter 3 utilizes the versatile nature of the modified-aminolytic synthesis to study how the substitution of different divalent metal cations (A = Co2+, Zn2+, Mn2+, and etc.) alters the magnetic properties of mixed composition Ni1-xAxFe2O4 nanoparticles. This series is to demonstrate a strategy to tune ferromagnetic resonance of spinel ferrites through understanding the relationship between the effective magnetic anisotropy and ferromagnetic resonance absorption. This endeavor should allow for the design of tunable microwave materials via control of chemical composition to meet the requirements of communication and antenna technological. Chapter 4 expands on the versatility of the seed-mediated modified-aminolytic method to synthesize bimagnetic exchange-coupled core@shell ferrimagnetic nanoparticles in which the core and shell materials have varied magnetic anisotropy energies (EA). The design of exchange-coupled nanomaterials has been pursued as a method to achieve rare-earth free permanent magnets through the creation of exchange spring magnets that is a hybrid of beneficial magnetic properties of the hard/soft ferrimagnetic phases such as the large effective magnetocrystalline anisotropy (coercivity HC) of the hard phase and large saturation magnetization (MS) to achieve a magnetic squareness factor (Mr/Ms) closer to 1. Ferromagnetic resonance spectroscopy demonstrates that microwave absorption is largely dictated by the surface state of the nanomaterial and careful design of a core@shell heterostructure can lead to strategies to tune material properties to meet technological needs. Chapter 5 studies the synthesis of nanoparticle analogs of bulk antiferromagnetic materials such as NiO and CoO. These antiferromagnetic nanoparticles demonstrate unique superparamagentic behavior in sub-10 nm single domain crystals. Chapter 6 investigates the magnetic phenomena of exchange bias (EB) in bimagnetic core@shell (FiM@AFM) nanoparticles. Again utilizing the versatility of the seed-mediated modified-aminolytic method the synthesis of core@shell (FiM@AFM) nanoparticles can be studied with control over core/shell dimensions and chemical composition. Herein we will report the largest exchange field shift (HE) in nanoparticle system, increased thermal stability of superparamagnetic nanoparticles, observe vertical shift along the magnetization axis, and report strategies to tune exchange bias derived properties. Through synthetic design we can tune the anisotropy energy (EA) of the ferrimagnetic and antiferromagnetic phases and additionally we can tune the interface coupling energy (Eex) allowing for the fine control of EB magnetic properties. Chapter 7 in the pursuit of understanding ferromagnetic resonance properties of spinel ferrite nanoparticles we also demonstrated the practical application of fabricated nanomagnetic films for use in communication/antennae technologies. This chapter will summarize the fabrication of these nanomagnetic films along with some results from our collaborative research pursuits with the Dr. Papapolymerou research group at Michigan State University. The novel synthesis and study of magnetic metal oxide nanocrystals has expanded our fundamental understanding of superparamagnetism, ferromagnetic resonance, spin-order in antiferromagnet nanoparticles, and interfacial exchange coupling interactions in core@shell nanostructures. Spinel ferrite nanocrystals are promising ultra-high frequency absorption materials with tunable properties capable of meeting engineering requirements. The observation of massive exchange bias shifts in core@shell (NiFe2O4@CoO) nanostructures challenges superparamagnetic limitations of magnetic nanoparticles and expands the potential of solid-state magnetic recording media.