Electronic Theses and Dissertations

Date

2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

Committee Chair

Sanjay R Mishra

Committee Member

Gladius Lewis

Committee Member

M. Shah Jahan

Committee Member

Shawn Pollard

Committee Member

Xiao Shen

Abstract

Magnetocaloric oxides materials have garnered increasing attention as environmentally friendly materials that may be used in systems that are alternatives to conventional gas-based refrigeration, a notable example being low-temperature magnetic cooling applications. In this study, the structural, magnetic, and magnetocaloric properties of Al3+-doped rare-earth orthochromites (RECrO3, where RE = Ho, Er, Gd, Tm, Tb, and La) were systematically investigated. A gradual decrease in lattice parameters and unit cell volume was observed from LaCrO3 to TmCrO3, consistent with the expected lanthanide contraction across the rare-earth series. The substitution of Al3+ at the Cr3+ site induced lattice contraction, reduced octahedral distortion, and modified Cr–O–Cr bond angles, thereby influencing magnetic superexchange interactions. Structural refinement confirmed a decrease in orthorhombic strain and an approach toward ideal perovskite geometry, enhancing structural stability. Magnetic measurements revealed that Al3+ doping weakened Cr3+-O2--Cr3+ exchange pathways, leading to suppressed Néel temperatures and reduced coercivity, though enhanced magnetization was observed in HoCr0.5Al0.5O3 due to strengthened Ho–Cr coupling. Density functional theory (DFT) calculations supported these trends, showing reduced energy differences between magnetic configurations and increased Cr–O hybridization. Magnetocaloric measurements revealed second-order phase transitions, accompanied by a notable enhancement in entropy change (−ΔSₘ), particularly for HoCr0.5Al0.5O3, which exhibited a 36% increase (8.83 J/kg·K) over its undoped counterpart. These results establish Al3+ doping as a viable strategy for tuning the magnetic and magnetocaloric performance of perovskite oxides, with promising implications for low-temperature magnetic refrigeration applications. Building on the effects of single-site doping, a comprehensive investigation of rare-earth (RE = Gd3+, Er3+, and Tm3+) and Al3+ co-doping in RECr0.5Al0.5O3 demonstrates a synergistic approach to fine-tuning structural, microstructural, and magnetic properties for optimized magnetocaloric performance. Co-doping induces a targeted lattice contraction, reduced octahedral tilting, and altered Cr–O–Cr bond angles, resulting in improved crystal symmetry and enhanced superexchange interactions. These structural modifications are accompanied by reduced crystallite and particle sizes, as confirmed by FESEM and spectroscopic analyses, indicating suppressed grain growth. DFT simulations reveal that co-doping alters the magnetic ground states, reduces the magnetic anisotropy energy, and enhances Cr–O hybridization, thereby facilitating soft magnetic behavior. Magnetic measurements confirm reduced TN, second-order transitions, and the emergence of weak ferromagnetism via Dzyaloshinskii–Moriya interactions. Among the singly doped samples, Ho0.67Gd0.33CrO3 showed the best performance, followed by Er- and Tm-doped samples. For co-doped systems containing both rare-earth and Al3+ ions, Ho0.67Tm0.33Cr0.5Al0.5O3 exhibited the highest magnetocaloric effect, with an enhancement of 45%. This was followed by Er (20%) and Gd (5%) co-doped samples, making them excellent candidates for low-temperature magnetic refrigeration. In conclusion, it was observed that approximately a 76% increase in MCE for Ho0.67Gd0.33Cr0.5Al0.5O3, a 50% improvement for Ho0.67Er0.33Cr0.5Al0.5O3, and a 20% enhancement for Ho0.67Tm0.33Cr0.5Al0.5O3, all compared to the parent compound HoCrO3. These results highlight the effectiveness of dual-site doping as a powerful strategy for synthesizing magnetically soft, and structurally robust perovskites that have enhanced maximum entropy change and, as such, may find use in low-temperature magnetic refrigeration systems. This work offers a significant contribution to the development of energy-efficient and environmentally benign refrigeration technologies. By combining experimental and theoretical insights, it establishes a clear structure–property relationship in Al3+ and rare-earth co-doped RECrO3 systems. The observed enhancements in magnetocaloric response, achieved through precise control of lattice and magnetic parameters, open pathways for rational material design. These findings not only deepen our understanding of complex oxide magnetism but also provide a blueprint for optimizing perovskite materials for solid-state cooling. Ultimately, the study advances the broader goal of sustainable and scalable low-temperature magnetic refrigeration solutions. Keywords: Orthochromites, autocombustion, x-ray diffraction, crystal structure, Rietveld refinement, density functional theory (DFT), magnetization, magnetic isotherms, Arrott plots, magnetic entropy change

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest.

Notes

Open Access

Download

Share

COinS