Phase Transitions and Magnetocaloric Effects in Metastable Phases of Intermetallic Compounds

Author

Tej Bahadur Poudel Chhetri, Louisiana State University and Agricultural and Mechanical CollegeFollow

Degree

Doctor of Philosophy (PhD)

Department

Department of Physics and Astronomy

Document Type

Dissertation

Abstract

The interplay between spin and lattice degrees of freedom in magnetic materials often results in interesting magnetoresponsive effects. The magnetocaloric effect is one such magnetothermal phenomenon that may be utilized in energy-efficient magnetic cooling systems. This effect becomes more prominent near a coupling of magnetic and structural transitions, i.e., magnetostructural transitions (MSTs). This dissertation investigates phase transitions (magnetic and structural) in metastable phases of MnNiGe- and MnCoGe-based compounds generated through high-pressure synthesis/annealing and thermal quenching techniques and assesses their magnetocaloric effects.

First, the effects of doping, hydrostatic pressure, and thermal quenching on phase transitions and related magnetocaloric effects in Mn_1−xCo_xNiGe system are systematically explored. The substitution of Co at the Mn site reduces the structural transition temperature, and changes the magnetic interaction from an antiferromagnetic state (AFM) to a ferromagnetic state (FM) in the low-temperature phase, resulting in a magnetostructural transition in a wide temperature range. Similarly, the application of hydrostatic pressure and/or thermal quenching in Mn_1−xCo_xNiGe (x = 0.03 and 0.05) leads to the reduction of their structural transition temperatures, thereby forming magnetostructural transitions in the periphery of room temperature, yielding large magnetic entropy changes (up to -80 Jkg⁻¹K^-1 ).

Next, metastable phases are generated in Mn_1−xCo_xNiGe (x = 0.05 and 0.08) by annealing at 800 °C followed by rapid cooling at ambient pressure and under high pressure. The pressure-thermal processing establishes magnetostructural transitions in Co-doped MnNiGe samples and improves their magnetocaloric effects. Following these results, phase transitions in metastable phases generated through high-pressure annealing in stoichiometric MnNiGe are investigated. An increase in annealing pressure continuously shifts the structural transition toward lower temperature and stabilizes the high-temperature hexagonal phase at room temperature in stoichiometric MnNiGe without any compositional modifications.

Finally, phase transitions and associated magnetocaloric effects in the metastable phases of stoichiometric MnCoGe are investigated. The results show that the structural transition in MnCoGe can be driven effectively by increasing the annealing pressure, resulting in a first-order MST without elemental substitutions. Our findings establish the approach of applying high pressure during thermal processing/synthesis as an effective tool to tailor the functional properties of magnetocaloric material systems, without modifying their compositions.

Date

6-28-2023

Recommended Citation

Poudel Chhetri, Tej Bahadur, "Phase Transitions and Magnetocaloric Effects in Metastable Phases of Intermetallic Compounds" (2023). LSU Doctoral Dissertations. 6204.
https://repository.lsu.edu/gradschool_dissertations/6204

Committee Chair

Stadler, Shane