Chemically synthesized Iron-Platinum binary alloy nanoparticles

In this dissertation, we explored the fabrication of FePt nanoparticles prepared by a solution-phase synthesis route and characterized their structural/ microstructural and magnetic properties both to gain a fundamental understanding and to check their compatibility for technological applications in ultra high density magnetic storage media.;Monodispersed Fe-Pt alloy NPs (nanoparticles) have been prepared by thermal decomposition of iron pentacarbonyl [Fe(CO)5] and reduction of platinum acetylacetonate [Pt(acac)2] with dibenzyl ether in the presence of oleic acid (OA) and oleyl amine (OAm) as surfactants. The composition of the nanoparticles was adjusted by changing the Fe(CO)5/Pt(acac) 2 molar ratio while fixing the Pt(acac)2 amount. Two phases of Fe-Pt binary alloy, FePt3 and FePt, were obtained successfully with the molar ratios of 1.5 and 2.1, respectively. The size of FePt NPs was tuned in the range of 3-6 nm by controlling the injection temperature of the iron precursor. It was found that, low injection temperature of precursors and the usage of surfactants as a reaction solvent, together with a slow heating to a low refluxing temperature were the key parameters for the formation of cubic nanoparticles. Spherical, cubic (with rounded edges) and octapod shapes were successfully produced by changing the OAm/OA molar ratio. Nanorods were formed by simply adjusting the injection time of the surfactants.;Although it was reported in the literature that the dominant mechanism of formation of NPs involves the initial formation of platinum rich clusters followed by the gradual diffusion of iron atoms into these clusters during the synthesis, in this work it is clearly shown that Fe rich seeds do form in the early stages of the reaction. And it was these competitive nucleation sites that cause a compositional distribution between individual FePt particles in the final sample, although a narrow distribution is measured for the overall composition.;As-synthesized NPs exhibit the chemically disordered A1 (fcc) structure. We have shown that in order to obtain the L10 phase with high magnetocrystalline anisotropy, one should heat treat the as-made nanoparticles at high temperatures (800 °C) and with long periods of annealing time (>120 min). However, aggregation and sintering were inevitable with these parameters. Therefore, we have employed several different approaches for the sintering prevention of chemically synthesized FePt NPs such as formation of FePt NPs/carbon multilayered structures by sputtering of carbon onto FePt NPs previously deposited on a silicon substrate, impregnation of NPs into ordered mesoporous silica (SBA-15) and coating the NPs with a SiO2 shell. In addition to these approaches, we also propose the use of a revolutionary technique which is the construction of hollow mesoporous zirconia shells with exactly one FePt NP in each shell. Among these methods, FePt@SBA-15 mesoporous structures, FePt@SiO2 coating and FePt@hm-ZrO2 encapsulation showed very promising results. A high coercivity value of 8.6 kOe was obtained without a significant size change by annealing at 700 °C for 2h with the FePt@SBA-15 structures. However, minor aggregation on the surface of SBA-15 or within the pores was observed due to diffusion of the NPs.;The sintering problem was solved completely by the silica coating technique. We have found that higher temperatures and longer annealing times (900 °C for 24 and 48h) are necessary to develop the coercivity (∼ 13 kOe) for these NPs due to the restricted geometry. By investigating the inter-particle interactions via Henkel plots and the relaxation measurements, it is concluded that interactions (either exchange of dipolar) are absent or negligibly weak and magnetization reversal is governed by coherent rotation. When samples with multi-particle occupancy per silica shell were annealed at 900 °C for 12 h, a high coercivity value of ∼ 8 kOe was obtained at room temperature. Hence, it is concluded that the sintering of the NPs works in favor of the phase transformation from low anisotropic A1 to highly anisotropic L10 phase.;Size dependence of the coercivity was investigated on the silica coated samples and it was found that the data follows a similar trend with theoretical prediction for the coherent rotation model where non-interacting single domain particles with uniaxial anisotropy are assumed.;The preliminary data for the annealed FePt@hm-ZrO2 structure showed very promising results with room temperature coercivities reaching ∼ 16 kOe without a change in the size (5.8 nm) of the FePt NPs.