Tunable Magnetic Properties of Spinel Ferrite Nanoparticles: Ferromagnetic Resonance and Physical Model of Magnetic Permeability

Spinel ferrite nanoparticles have been widely applied in high frequency applications such as microwave absorption, signal filter, etc. The design of spinel ferrites with various properties has been a hot topic to satisfy different application requirements. The goal of this dissertation is to explore the tunability of magnetic properties especially at high frequencies with the manipulation of size, chemical composition, magnetic couplings, and super exchange interactions. Chapter 2 studies the magnetic coupling, cation distribution, and magnetic properties (susceptibility, hysteresis, FMR spectrum, etc.) of hard and soft spinel ferrite nanoparticles. The hard (CoFe2O4) and soft (MnFe2O4, MgFe2O4, and ZnFe2O4) nanoparticles show discrepancy on magnetic properties due to spin-orbit coupling strength difference. The magnetic property difference among soft nanoparticles has been demonstrated to be attributed to the variation of cation distribution. Chapter 3 investigates the effect of Al3+ substitution in MnAlxFe2-xO4 and CoAlxFe2-xO4 nanoparticles. It has been demonstrated that the magnetic properties under DC and AC applied field are size dependent. Composition dependent study reveals the preference of Al3+ on octahedral sites, which consequently affect the number of super exchange interactions and magnetic behavior. Chapter 4 explores the tunability of superparamagnetic and ferromagnetic resonance properties of CoxMg1-xFe2O4 and MnxMg1-xFe2O4 nanoparticles. The incorporation of Co2+ is proved to have significant influence on the magnetic properties of CoxMg1-xFe2O4 nanoparticles, which is mainly caused by magnetic coupling and magnetic anisotropy change. Broad tunability of magnetic properties has been achieved in CoxMg1-xFe2O4 series of nanoparticles. Chapter 5 reveals the effect of annealing process on the cation distribution and FMR spectrum of ZnFe2O4 nanoparticles. As-prepared ZnFe2O4 nanoparticles are demonstrated to have high inversion degree of 0.46, which decreases with higher annealing temperature. The number of super exchange interaction diminishes with smaller inversion degree. Chapter 6 develops a physical model for complex magnetic permeability of spinel ferrite nanoparticles using DC SQUID magnetometer and FMR data. The model reveals the intrinsic permeability information over broad range of frequency and applied field. In Chapter 7, the complex permeability of spinel ferrite nanoparticles with various chemical compositions is simulated and compared. The correlation of saturation magnetization, magnetic anisotropy, and FMR behavior with magnetic permeability has been found.Ph.D