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Microstructure, strain, and magnetostructural coupling in intermetallics

  • Author(s): Levin, Emily Elizabeth
  • Advisor(s): Seshadri, Ram;
  • Pollock, Tresa
  • et al.

There exists a wealth of unexplored functionality in the relationship between strain and properties, specifically magnetism. While the flexible electronics of today attempt to preserve functionality during bending, we anticipate technology where bending or stretching adds utility. To do this, we must examine the coupling between strain/strain gradients and magnetic properties. We have studied these effects in two-phase materials, where interfaces between precipitate and matrix phases lead to large, built-in strain gradients in bulk materials. We show that complex microstructures in biphasic Heuslers may be manipulated through careful processing. Tuning knobs such as composition and heat treatments allow for systematic variation of the strain gradient at interfaces. Nanoscale precipitates evolve from fully coherent, and therefore highly strained, nanoplatelets to semicoherent spheres with misfit dislocations to relieve the misfit strain, leading to a modulated strain field along the interface. These insights into phase separation and interface characterization in the Heuslers TiNi1+xSn and NbCo1+xSn provide the tools to engineering microstructure in (Ti,Zr)CoSb–MnCoSb, a model system to study the effects of interfacial strain on magnetic structure. This thesis will discuss microstructural engineering of advanced thermoelectric, magnetic, and magnetocaloric materials, which have applications in efficient refrigeration, waste heat recovery, and eco-friendly power generation, as well as next-generation technology.

Magneto-plastic coupling is examined in the Heusler MnAu2Al, where we observe a drastic reduction in net magnetization as a result of ambient temperature mechanical processing such as hand grinding. Using a combination of magnetic measurements, X-ray diffraction, electron microscopy, and DFT, we elucidate the mechanism for this magnetization reduction. This remarkable ordered intermetallic has a particularly low antiphase boundary formation energy. The change in chemical order present at these boundaries results in antiferromagnetic exchange interactions across the planar fault. Thus, the magnetic domains are pinned anti-aligned to each other, and cancel out to reduce the contribution to the net moment. We expect that this effect is not limited to MnAu2Al, but present in several other ordered intermetallics.

Finally, a computational parameter termed the magnetic deformation ΣM, a proxy for intrinsic magnetostructural coupling, was used to screen magnetic compounds to identify viable magnetocaloric candidates. Several promising material systems were found to have a significant magnetocaloric effect and highly tunable temperature ranges of operation. These materials have potential in magnetic refrigeration and thermomagnetic power generation. We observe through temperature dependent synchrotron X-ray diffraction that there is anisotropic spontaneous magnetostriction in orthorhombic (Pnma) Mn2–xCoxP, where the thermal contraction trends from the paramagnetic state cease to describe the structure below the Curie temperature. This indicates strong coupling between magnetism and structure. A combination of experimental and computational techniques reveal the individual site contributions and ferrimagnetism present in these materials, depending on composition. The trends in ΔSM, the figure of merit for magnetocaloric materials, with respect to composition, are reproduced by the computationally derived ΣM.

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