Degree

Doctor of Philosophy (PhD)

Department

Department of Physics and Astronomy

Document Type

Dissertation

Abstract

Solid-state caloric effects, such as the magnetocaloric (MCE) and barocaloric (BCE) effects, may be utilized in future cooling technologies that are more efficient and environment-friendly. Large caloric effects often occur near phase transitions, especially near coupled first-order magnetostructural transitions (MST), and are initiated by external parameters, such as magnetic field or hydrostatic pressure. In this dissertation, the effects of pressure, temperature, and magnetic field on the phase transitions in three material systems are studied in order to elucidate how the respective caloric effects are affected.

In the first study, the realization of a coupled MST in a MnNiSi-based system through isostructural alloying is explored, which resulted in a giant conventional MCE. The MST shifts towards lower temperature with increasing applied hydrostatic pressure, whereas it shifts towards higher temperature with an increase in magnetic field. The strong pressure dependence along with a large volume change during the MST suggested the possibility of pressure-induced BCE in this material. In a subsequent study, we observed a giant hydrostatic pressure induced inverse BCE through pressure-dependent calorimetric measurements. The multiple caloric effects in the same material for the same phase transition qualify this material as a multicaloric material.

In second study, we investigated the hydrostatic pressure dependence of the metamagnetic transitions in DyRu2Si2, which shows multiple metamagnetic transitions at atmospheric pressure. With the application of moderate hydrostatic pressure, the metamagnetic transitions disappeared, but then reappeared with increasing pressure. We discuss the pressure-induced magnetostrictive behavior, the variation of the entropy changes with pressure, and a possible origin of the pressure-dependent behavior in light of the variation of the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interactions.

For x = 0.25 in the Ni2Mn1-xCuxGa Heusler alloy series, the structural and magnetic transitions coincide to create a coupled first-order MST. Since giant MCE was reported for this system, it is useful to understand the underlying physics driving the coupling of transitions. Although first-order transitions cannot be investigated through the critical behavior analysis, the structural and magnetic transitions in Ni2MnGa (parent alloy) and Ni2Mn0.85Cu0.15Ga are not coupled. In this case, investigating the critical behaviors of the two alloys near their second-order phase transitions will provide insight as to how the magnetism in these materials evolve with increasing copper doping. In this study, through the calculated critical exponent values, we identified the universality classes which best described the parent and Cu-doped (x = 0.15) alloys. The exponent values shed light into the range of the magnetic interactions, and the evolution of the interactions due to the non-magnetic Cu substitution scheme. This type of analysis can be performed in other material systems to get a picture of the systematic trends, through doping or other processes, such as applied pressure, that lead to first-order phase transitions.

Date

11-10-2018

Committee Chair

Stadler, Shane

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