Optimization of Co₂MnSi Heusler Alloy Thin Films for Spintronic ApplicationsdissertationsAbudushalamu, YilidaercreatorXiao, GangAdvisorBrown University. Department of Physicssponsor2026, None p.born digitalThesis (Sc. M.)--Brown University, 2026thesesCo₂MnSi (CMS) is a Co-based full-Heusler alloy predicted to exhibit half-metallic ferromagnetism and high spin polarization, making it a promising electrode material for magnetic tunnel junctions (MTJs). Translating these ideal properties into sputtered thin films requires careful control of composition, crystalline ordering, surface roughness, and the CMS/MgO interface. This thesis investigates the growth and early device integration of sputtered CMS thin films, progressing from composition control and baseline film growth to MgO-based interface optimization and MTJ stack design. An exploratory composite Co/Mn/Si target was first evaluated before the arrival of a commercial CMS alloy target. XPS analysis showed a strongly Mn-deficient and Si-rich composition relative to the ideal Co:Mn:Si = 2:1:1 ratio, indicating that simple geometric control of target area was insufficient to achieve stoichiometric CMS under the present conditions. CMS films subsequently deposited on Si/SiO₂ substrates from the bulk target established a reproducible baseline: the films showed CMS-related crystalline reflections in XRD, soft in-plane ferromagnetism over a deposition temperature range of 480–540°C, and metallic resistivities of 58–77 μΩ·cm. The project then transitioned to MgO substrates to move toward device-relevant heterostructures. AFM measurements showed that a high-temperature in-situ substrate anneal followed by MgO buffer deposition reduced local template roughness to 0.057 nm. CMS deposited at room temperature on this template (MNC072) preserved a smooth surface of 0.178 nm with Ms = 741 emu/cm³ but no resolvable in-plane anisotropy. CMS deposited at 550°C with the in-situ buffer (MNC074) showed higher Ms = 791 emu/cm³ and clear fourfold in-plane magnetic anisotropy consistent with epitaxial CMS/MgO alignment, at the cost of increased surface roughness. These results identify the central optimization challenge: achieving both smooth morphology and magnetocrystalline anisotropy simultaneously. Preliminary MTJ device work included MgO barrier-rate calibration by X-ray reflectivity and the design of photolithographic masks for future transport measurements. The proposed stack architecture, switching strategy based on coercive field asymmetry, and fabrication sequence are defined, establishing a process foundation for future CMS/MgO/CMS magnetotransport measurements.spintronicsHeusler alloysMagnetic tunnel junctionsTunneling magnetoresistanceFerromagnetismMagnetic anisotropyEnglishRPB20260516