Nanoscale Structures in Near-Equiatomic Fe-Ni Films and Particles

The Fe-Ni binary system is ferromagnetic, electrically conductive, and displays a high dimensional stability in the composition range between 30 and 45 at.%Ni. The material is consisting of earth-abundant elements and continues to find applications in digital memory devices, precision instruments, and micro-electromechanical systems (MEMS). The Fe-Ni components being used in these applications are predominantly of the A1-type crystal structure, a chemically disordered crystal structure that belongs to the cubic system. As a soft ferromagnet, A1 Fe-Ni are used for the linear control and sensing of magnetic fields, but they are not suitable for any of the permanent magnet applications, such as the traction motors in electric or hybrid vehicles and the generators in wind turbines. However, the discovery of L10 Fe-Ni, a chemically ordered crystal structure that belongs to the tetragonal system, first in high-flux neutron targets and later in meteorites, revealed a high first-order magnetocrystalline anisotropy constant on the order of 0.84 MJ/m^3 or higher, giving rise to the possibility of producing a permanent magnet based on a material system free of rare-earths and precious metals.

The synthesis of L10 Fe-Ni has been a longstanding challenge. So far, the human-made synthesis of L10 Fe-Ni were only realized in the laboratories in the form of thin films, nanoprecipitates, or nanoparticles by a few methods that are far from viable for the production of bulk materials. In order to synthesize L10 Fe-Ni, one has to circumvent the sluggish ordering kinetics and the limited thermodynamic driving force imposed by the low order-disorder temperature of the system. A fundamental understanding of the relationship between the nanoscale structure and the synthesis condition may provide critical insights for one to implement a phase transformation landscape, at which the ordered phase can form within a timescale viable for bulk production.

The A1-to-L10 Fe-Ni chemical ordering phase transformation is a first-order phase transformation. Based on the thermodynamic and the kinetic parameters of the order-to-disorder phase transformation, the synthetic challenge of L10 Fe-Ni can be tackled from at least two directions: (1) increase the thermodynamic driving force of the ordering transformation; (2) decrease the kinetic barrier of the ordering transformation.

The thermodynamic driving force for the ordering transformation can be increased by replacing the A1 Fe-Ni phase with an initial phase that has a higher molar Gibbs free energy than that of the A1 Fe-Ni phase. A search for alternative metastable Fe-Ni phases with a near-equiatomic composition (i.e., 40 ~ 60 at.%Ni) is therefore of a significant importance for the synthesis of L10 Fe-Ni.

On the other hand, the ordering kinetics can be accelerated by lowering the activation barrier of lattice diffusion, which is a sum of both the vacancy formation energy and the vacancy migration energy. In the neighborhood of a non-equilibrium crystal defect, such as a surface, a grain boundary, or a dislocation, the activation energy required for lattice diffusion is expected to be reduced, because the vacancy concentrations in the neighborhoods of these crystal defects are supposed to be higher than the equilibrium level. A nanoscale structure indicative of a high density of any of these non-equilibrium defects is therefore also of a significant importance for the synthesis of L10 Fe-Ni.

In this dissertation, the nanoscale structures of near-equiatomic Fe-Ni synthesized by different methods are investigated with an aim to provide insights for the synthesis of L10 Fe-Ni. Chapter 1 is an introduction. Chapter 2 - 4 are dedicated to the electrodeposition of Fe-Ni films. Chapter 2 investigated the conditions under the activation limit, during the deposition of which a metastable BCC phase was observed to form. Chapter 3 investigated the anomalous codeposition mechanism encountered under the activation limit, in order to understand the origin of the through-thickness composition gradient. Chapter 4 investigated the conditions close to the mass transfer limit, during the deposition of which a high surface area growth front was observed to form. Chapter 5 and 6 are dedicated to two methods based on the use of pulsed laser. Chapter 5 investigated the structures arise from the pulsed laser irradiation of electrodeposited Fe-Ni films. It was demonstrated that the pulsed laser irradiation affects the crystallographic texture of the Fe-Ni films. Chapter 6 investigated the structures in the Fe-Ni nanoparticles synthesized by pulsed laser ablation in liquids. A nano-size metastable non-cubic phase and a high density of planar defects were observed. The significances of these observations upon the synthesis of L10 Fe-Ni are discussed in the respective chapter.