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Influence of Structural and Compositional Heterogeneities on Nanoscale Thermal Transport and Magnetization Dynamics

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The ability to predict and control thermal/magnetic properties is crucial for numerous applications. Incorporating materials with distinct structure-property relationships could offer tantalizing new possibilities in the design of denser electronic components with efficient thermal management. Hence, it is important to understand how variations in elemental composition and structural inhomogeneities influence thermal transport and/or magnetization. The primary goal for my dissertation research is to use Time-Domain Thermo-Reflectance (TDTR) to generate 2-dimensional thermal conductivity maps of two distinct material systems: (a) ferromagnetic Co-Fe alloys, and (b) Al-PVDF nanocomposites. I also use Time-Resolved Magneto-Optic Kerr Effect (TR-MOKE) to investigate magnetization dynamics in Co-Fe alloys. First, I summarize the experimental results of the magnetization dynamics in Co-Fe alloys. Through TR-MOKE experiments, I show that Co-Fe compositions that exhibit low Gilbert damping parameters (at the nanosecond timescale) also feature prolonged ultrafast demagnetization responses (at the femtosecond timescale) upon photoexcitation. Thus, I report a strong correlation between the dynamics at both timescales, indicating that the same physical mechanisms likely govern both phenomena. Next, I interrogate the thermal conductivity in Co-Fe alloys. I conduct TDTR measurements on Co-Fe thin films and arc-melted alloys, and spatially map the thermal conductivity on a Co-Fe diffusion multiple. I report two main results: (i) the thermal conductivity does not appear to be strongly influenced by crystalline disorder in Co-Fe alloys, and (ii) Co-Fe compositions that feature ultralow magnetic damping also exhibit significantly high non-electronic contribution to thermal transport. I hypothesize that the magnon thermal transport is likely to be very high at these compositions. Finally, I present high-resolution thermal conductivity maps of Al-PVDF nanocomposite films with varied Al volume fractions (0 – 50%). My thermo-reflectance mapping technique has sub-micron resolution and demonstrates how thermal transport properties vary spatially across the polymer, metal, and metal-polymer interfaces. I show that increasing the Al volume fraction to 50% enhances the bulk thermal conductivity of the polymer film by a factor of 2. In certain areas with coalesced Al particles, the local thermal conductivity dramatically increases by a factor of 250. A careful understanding of the spatial variation in the thermal conductivity will aid in the prediction of flame propagation and combustion characteristics in Al-PVDF films.

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