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Author ORCID Identifier

https://orcid.org/0009-0008-2835-2661

Date Available

6-7-2027

Year of Publication

2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Chemical and Materials Engineering

Faculty

Paul F. Rottmann

Faculty

Matthew Beck

Abstract

This dissertation investigates two functional applications of high-entropy alloys (HEAs) in thin-film form, unified by the question of how their multi-element character can be turned to engineering advantage at the nanoscale. The two parts address distinct application domains but share a common framework of magnetron-sputter deposition, multi-modal nanoscale characterization, and quantitative comparison against experimental standards or theoretical predictions.

Part I examines the intermediate-temperature oxidation behavior of a refractory multi-principal-element alloy (RMPEA) thin-film system. Magnetron-sputtered Cr₁₀(MoNbTaW)₉₀ films, with and without a 35 nm aluminum capping layer, were annealed in air at 300, 400, and 500 °C for 10 and 20 hours and characterized by XRD, FIB-SEM, TEM with SAED and STEM-EDS, XPS, and nanoindentation. Uncoated films developed a thick amorphous mixed-oxide layer containing all five metals in non-segregated form, with substantial mechanical degradation (hardness 18 to 12.5 GPa, modulus 330 to 240 GPa at 500 °C). Al-capped films instead formed a compact, nanocrystalline γ-Al₂O₃ layer ~20 nm thick that inhibited oxygen ingress and preserved mechanical integrity throughout the test matrix. The XPS-derived oxidation hierarchy was supported by the FactSage-predicted Ellingham diagram. The work establishes that thin metallic-aluminum capping layers can kinetically lock in a protective scale on RMPEAs, with direct implications for next-generation high-temperature structural applications.

Part II addresses the suitability of soft-magnetic HEA thin films as a permalloy alternative for artificial-spin-ice (ASI)-based neuromorphic and reservoir computing. Magnetron-sputtered Al₀.₂₅CrFeCoNi films (10–30 nm), capped in situ with ≈5 nm Al, were characterized by SQUID magnetometry (5–380 K) and broadband FMR spectroscopy (15–20 GHz). The films were ferromagnetic at room temperature with saturation magnetization ~200 emu/cc and coercivity ~30–250 Oe- a marked deviation from the paramagnetic bulk alloy, indicating that sputter deposition produces an ordering temperature substantially elevated above the bulk value. ZFC/FC M–T data showed thickness-independent magnetization at 5 K crossing over to thickness-dependent behavior above 150 K, consistent with surface-anisotropy-dominated behavior and yielding a surface anisotropy Ks of order 0.1 mJ m⁻². Field-swept FMR on the Al-seeded 15 nm film returned g = 1.97 ± 0.004, μ₀Meff = 0.40 ± 0.01 T, and Gilbert damping α = 0.019 ± 0.001 with μ₀ΔH₀ = 51 ± 16 Oe- roughly three times the zero-temperature first-principles prediction of Kudrnovský et al. and 2.4 times the permalloy benchmark. The magnetic transition temperature of 250–275 K provides operating margin near room temperature. To the author's knowledge, this is the first thickness-dependent SQUID study of AlₓCrFeCoNi thin films below 50 nm, the first FMR-derived Gilbert damping value for any AlₓCrFeCoNi composition, and the first direct experimental test of the Kudrnovský prediction for this system. A custom broadband VNA-FMR spectrometer was developed in parallel and validated against a 25 nm permalloy reference film.

Together, the two projects demonstrate that process-compositional-structural complexity is instrumental in tuning passivation potential and magnetic behavior in high-entropy alloys, overcoming limitations of conventional alloys.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2026.316

Archival?

Archival

Funding Information

This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC-0024346

This work was performed in part at the Electron Microscopy Center at the University of Kentucky, members of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-1542164).

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Available for download on Monday, June 07, 2027

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