Magnetic Investigation of Nano-sized Materials Incorporating 3d Ions

Abstract

Nanomaterials have a wide range of applications that are attributed to their unique physicochemical properties. Among them, magnetic nanoparticles are of high importance because they have useful applications in areas as diverse as catalysis, magnetic data storage, photovoltaics, energy storage, and spintronics. This thesis has explored in detail the magnetic properties of three major classes of synthesized magnetic nanomaterials under the variation of different experimental conditions such as temperature, field or frequency, and using two different magnetic techniques, SQUID magnetometry and EPR spectroscopy. The study has provided key information on the behaviours of all these samples. SQUID magnetometry has been used to examine the magnetic behaviour of the different nanomaterials over a wide temperature range, probing the structural transformations that accompany the magnetic changes. EPR spectroscopy at different microwave frequencies has been used in the thesis to examine the electronic configuration of the novel nanomaterials, the structural features, and the nature of magnetic exchange occurring within the particles. This thesis includes an introductory Chapter 1 that reviews the key theory used to interpret the magnetic behaviours of the studied nanoparticles. In Chapter 2, the SQUID and EPR techniques are reviewed, from theory and instrumentation to experimental procedures used in the thesis. Three other chapters summarize the results of the experimental studies conducted on the different classes of nanomaterials doped with transition metals. Chapter 3 explores the properties of manganese-doped lead sulphide nanomaterials with different paramagnetic concentrations, formulated as Pb1-xMnxS4 (x = 0.02, 0.04, 0.06, 0.08 and 1.0). In all cases, the investigation has revealed the occurrence of a magnetic phase transition from a superparamagnetic (SPM) phase at high temperatures to a ferromagnetic (FM) state at low temperatures, with a transition temperature that increases from 15K (x = 0.02) to 25-30K (x = 0.06 and 0.8). Interestingly, the data show that antiferromagnetic (AFM) ordering occurs in all nanoparticles, irrespective of their doping level, with FM ordering dominating the low temperature behaviours due to ineffective antiferromagnetism associated with these small superconducting nanoparticles. Surface interactions between plays a role in this behaviour. Chapter 4 discusses the magnetic behaviours of antimony sulphide nanocrystals doped with different concentrations of 3d transition metal ions, of formula Sb2(1-x)M2xS3 (0.2 ≤ x ≤1.0; M = Ni, Co or Fe). The analyses of magnetic data show of thermal-dependent systems related to the doping of the magnetic ion and a magnetic ordering of paramagnetic, superparamagnetic and ferromagnetisim. In Chapter 5, the spinal ferrite nanoparticles as AFe2O4 (A = Co, Zn and Mn) investigation shows apparent typical spin glasses behaviour at low temperatures due to freezing the magnetic moments near the blocking temperature. The measurements extend to study the effect of the particle size in Fe3O4 on the size between 4.3 to 6.0 nm indicating that the nanomaterial anisotropy is the result of the large surface-to-volume ratio of the particle size anisotropies and the exchange interaction between the inter-particles.