Pathways for tailoring the magnetostructural response of FeRh-based compounds

Materials systems that undergo magnetostructural phase transitions (simultaneous magnetic and structural phase changes) have the capability of providing exceptional functional effects (example: colossal magnetoresistance effect (CMR), giant magnetocaloric (GMCE) and giant volume magnetostriction effects) in response to small physical inputs such as magnetic field, temperature and pressure. It is envisioned that magnetostructural materials may have significant potential for environmental and economic impact as they can be incorporated into a wide array of devices ranging from sensors for energy applications to actuators for tissue engineering constructs. From the standpoint of fundamental scientific research, these materials are interesting as they serve as model systems for understanding basic spin-lattice interactions.;In this work, the near-equiatomic phase of FeRh serves as a test bed for understanding the magnetostructural phenomena in intermetallic alloys due to its relatively simple crystal structure (cubic with B2 (CsCl)-type ordering) and its reported ability to undergo a first-order magnetic phase change from antiferromagnetic (AF) to ferromagnetic (FM) ordering, with an accompanying 1 % volume expansion in the unit cell near room temperature ( Tt ~ 350 K). Overall, three interrelated but largely unexplored aspects concerning the FeRh system have been examined here: (1) influence of nanostructuring on the magnetostructural response; (2) influence of simultaneous application of pressure and magnetic field on the magnetostructural response; (3) correlations between chemical modification of the lattice and the magnetostructural response. Bulk FeRh-based samples in this study were synthesized using the arc-melting technique and nanostructuring of the system was achieved via rapid solidification processing (melt-spinning) of the arc-melted precursor. Structure-property correlations between the parent equiatomic FeRh compound and its nanostructured/chemically-modified counterparts were examined using a variety of structural and magnetic probes including x-ray diffraction (synchrotron and laboratory based), transmission electron microscopy (TEM) and magnetometry. Overall, the results achieved in this work provide predictive capability and pathways for tailoring the magnetostructural behavior and the associated functional response of FeRh systems for potential technological applications such as magnetic refrigeration and heat-assisted magnetic recording media. Further, insight is gained into the mechanism of magnetostructural phenomena at the fundamental atomic level. In particular, the experimental evidence obtained in this work suggests that the magnetostructural response of FeRh-based compounds depends upon both the electronic state of the system and the magnetovolume effect.;Despite the success achieved in this Dissertation, many open questions regarding the first-order magnetostructural transition in FeRh systems still persist. The concluding chapter of this Dissertation provides recommendations for future experiments that may be conducted to develop a more advanced understanding of the fundamental thermodynamic and kinetic factors influencing the magnetostructural phase transformation process in FeRh and related intermetallic compounds. Further, it is anticipated that computational studies aimed at modeling the magnetostructural behavior of FeRh-based ternary alloys using ab initio calculations and density functional theory will be useful for providing a theoretical framework to the results obtained in this study. Despite the success achieved in this Dissertation, many open questions regarding the first-order magnetostructural transition in FeRh systems still persist. The concluding chapter of this Dissertation provides recommendations for future experiments that may be conducted to develop a more advanced understanding of the fundamental thermodynamic and kinetic factors influencing the magnetostructural phase transformation process in FeRh and related intermetallic compounds. Further, it is anticipated that computational studies aimed at modeling the magnetostructural behavior of FeRh-based ternary alloys using ab initio calculations and density functional theory will be useful for providing a theoretical framework to the results obtained in this study.