Item Details

Strain-Induced Microstructural and Ordering Behaviors of Epitaxial Fe38.5Pd61.5 Films Grown by Pulsed Laser Deposition

Steiner, Matthew
Thesis/Dissertation; Online
Steiner, Matthew
Fitz-Gerald, James
Magnetic thin films of 3d-4d/5d transition metal alloys such as Fe-Pt, Co-Pt, and Fe-Pd are of technological interest due to their ordered L10 tetragonal intermetallic phase, exhibiting large magnetocrystalline anisotropies of K ~ 10^7 to 10^8 ergs/cm3. Anisotropies of this magnitude are comparable to lanthanide based 3d-4f rare earth permanent magnets, which have become ubiquitous since their development in the 1970s. Despite their prevalence, rare earth magnets are limited by a vulnerability to corrosion, as well as brittleness due to a lack of available slips systems in their complex crystal structures; both causing intractable problems for nanoscale applications. In juxtaposition, the strong hard-magnet properties of 3d-4d/5d magnetic alloys, combined with the ductility and chemical inertness of their ennobled metallic nature, allow these material systems to remain above the thermally induced KV/kBT superparamagnetic limit at the nanometer scale. This combination of properties is ideal for applications in ultra-high-density magnetic storage or micro-electro-mechanical systems. Within this class of materials, Fe-Pd alloys possess comparatively moderate magnetocrystalline anisotropies relative to Co-Pt and Fe-Pt. The Fe-Pd phase diagram, however, exhibits a considerably lower range of order-disorder transition temperatures, rendering the material well-suited for nanostructured magnetic applications by enabling lower processing temperatures. In addition, the higher economic demand for Pt makes Pd-based alternatives of considerable technological interest. Experimental work to date near the Fe38.5Pd61.5 eutectoid between the chemically ordered L10 and L12 phases of the Fe-Pd system, bounding one side of the technologically relevant L10 phase region, is limited and has left large uncertainties in the experimental phase diagram. The related Co-Pt system has been shown to decompose under bulk conditions into a novel, strain-induced chessboard microstructure at the eutectoid composition between its ordered L10 and L12 intermetallic phases due to coherency strain, making it likely that other 3d-4d/5d material systems may also produce unique strain-induced microstructural behavior. Strain-induced effects are indeed observed for the Fe38.5Pd61.5 thin films presented, but they are of a considerably different nature than the microstructural behavior produced at the Co-Pt eutectoid. Epitaxial films of Fe38.5Pd61.5 at the L10-L12 eutectoid composition have been grown on MgO (001) oriented substrates by pulsed laser deposition, probing this unexplored region of the binary diagram and solidifying a gap in the experimental record. This thesis advances the scientific understanding of ordered magnetic films by introducing two new strain-induced ordering phenomena. Thin films of Fe38.5Pd61.5 deposited above 600°C are found in a single ordered phase, initially surmised to be L12 due to magnetic properties and the location of the X-Ray Diffraction (XRD) peaks. Careful analysis of peak intensities results in the prediction of anomalous long-range ordering parameters. Quantitative XRD analysis of the films confirms that this is due to a perturbation in the Pd-site occupancy of the non-stoichiometric Fe atoms in the films; resulting in a hybridization of the L10 and L12 ordered structures. This L1' hybridized ordered structure, first postulated by thermodynamic principles to exist for the Au-Cu system†, is believed to be induced by the accommodation of epitaxial strain from the substrate. Classical notations are expanded in discussion of the L1' hybrid phase, defining two independent long-range ordering parameters. Along with its verification, the thermodynamic behavior of this new strain-induced phase is addressed in relation to the equilibrium phase diagram. In addition to this new hybrid phase, Fe38.5Pd61.5 films grown at 550°C have been found to possess a unique two-phase microstructure of prismatic 50 to 100 nm disordered A1 secondary phases with <110> oriented facets, embedded within an ordered L12 matrix. Large strain energies during the early stages of 550°C epitaxial film growth are hypothesized to lead to this two-phase decomposition and the abnormal precipitation of a disordered phase from an ordered state. A L10-L12 two-phase region, as predicted by the equilibrium diagram, remains unobserved. † W. Shockley, J. Chem. Phys. 6, 130 (1938)
Date Received
University of Virginia, Department of Engineering Physics, PHD (Doctor of Philosophy), 2013
Published Date
PHD (Doctor of Philosophy)
Libra ETD Repository
Logo for In CopyrightIn Copyright


Read Online