Synthesis, assembly, and integration of magnetic nanoparticles for nanoparticle-based spintronic devices

Abstract

Recently, the emergence of nanoparticle-based spintronic devices is believed to promote the future development of magnetics and spintronics due to their unique advantages and intriguing properties. To build these devices, self-assembly strategies can be employed as bottom-up fabrication methods to orderly organize and effectively integrate the magnetic nanoparticles either inside the active layers or on the surfaces of spintronic devices. The arrangement and composition of the assembled nanoparticles can be properly tuned through external stimuli and chemical reactions respectively. From the application viewpoint, the nanoparticle-based spintronic devices possess intrinsic inter-particle barriers and additional intra-particle surface/interface, which provide favorable sites for spin-dependent electron tunneling and scattering as realized in conventional spintronic devices with multilayer thin-film structures. Furthermore, due to the interplay between the magnetic nanoparticles and other magnetic nanostructures in the nanoparticle-based devices, novel phenomena and enhanced performance can be anticipated resulting from spin transport and spin accumulation. This thesis presented the chemical synthesis, controlled assembly, and device integration of magnetic nanoparticles, and further demonstrated the fabrication of several types of nanoparticle-based spintronic devices, followed by the investigations of their magnetization switching process and magnetotransport behaviors. At first, spherical Fe2O3 and polyhedral Fe3O4 nanoparticles were synthesized and a magnetic-field guided assembly method was developed to build nanoparticle superstructures with high crystallinity and controlled alignment. Characterization and calculation results evidence that the nanoparticle superstructures exhibit enhanced coercivity and increased magnetization along the assembly direction, which are associated with their magnetic anisotropy and dipole interaction. These nanoparticle superstructures can be potentially used as functional materials for the fabrication of magnetic storage devices. Following this, composite Fe3O4-Ag nanoparticles were synthesized and a controlled interfacial assembly method was developed to construct the nanoparticles as continuous films on patterned substrates, producing nanoparticle-based magnetoresistive (MR) devices. Magneto-transport and electron-transport studies reveal that this MR device exhibits enhanced MR ratio and electron tunneling behavior, which are attributed to the intra-particle material interfaces and inter-particle organic barriers. The fabrication procedure can be further extended to other composite magnetic nanoparticles, enabling the construction of magnetic nanodevices from multi-functional nanomaterials. Subsequently, monodisperse Fe2O3 nanoparticles were synthesized and integrated as compact nanoparticle coatings onto pseudo-spin-valve (PSV) devices by solution processing and magnetically-guided assembly. The nanoparticle coating shows both structural order and crystallographic alignment, leading to its enhanced magnetism along easy-axis directions. The nanoparticle-integrated PSV device exhibits enhanced device performance in terms of MR ratio and switching field. The hybrid device structure and the particle-thin film interaction mechanism pave a novel way to modulate the magnetic and magnetotransport properties of spintronic devices. Finally, spin-polarized CoFe2O4 magnetic nanoparticles were synthesized and integrated as uniform nanoparticle films onto spin-valve (SV) devices through a modified interfacial assembled technique. The feasibility of nanoparticle assembly at ambient environment enables the convenient transfer of nanoparticle films to various substrates. Magnetic properties and magnetoresistance behaviors of the nanoparticle-integrated SV device were systematically investigated. The measurement results and the systematic analysis provide insights into designing spintronic devices with novel device structures and improved performances.