Electronic Theses and Dissertations



Document Type


Degree Name

Doctor of Philosophy



Committee Chair

Sanjay R Mishra

Committee Member

Gladius Lewis

Committee Member

Jeffrey G Marchetta

Committee Member

Shawn D Pollard

Committee Member

Xiao Shen

Committee Member

Muhammad S Jahan


The magnetocaloric effect (MCE) is the heating or cooling of a magnetic material due to the application of a magnetic field. Magnetic refrigeration (MR) based on the MCE is a promising alternative to conventional gas compression-based cooling techniques. Because of the increased interest in these materials over the last two decades, less expensive materials, such as garnet and perovskite oxides, have been investigated as possible MC materials by altering their microstructure. In this view, the motivation for this work is to explore the effect of impurity atoms in these oxides from the technological and scientific point of view and highlight the general consequences on the magnetic and magnetocaloric properties of these compounds. The work presents a detailed characterization of the structural, magnetic, and magnetocaloric properties of three oxide systems, namely (1) lanthanum strontium magnetite perovskite composites, (2) aluminum-doped gadolinium garnets, and (3) rare-earth-doped gadolinium garnets. All compounds were prepared a facile one-step autocombustion process. Structure-property relation in the studied compounds was established via various analytical techniques, including x-ray diffraction, scanning electron microscopy, Fourier transformed infrared spectroscopy, Mossbauer spectroscopy, physical property measurement system, and density functional theory (DFT). The presence of antiferromagnetic MO phase in (1-x) La0.45Nd0.25Sr0.30MnO3 (LSMO ) – x Wt.% MO (M = Cu, Co, and Ni) nanocomposites brings in a second-order magnetic phase transition in the LSMO phase. Consequently, La0.45Nd0.25Sr0.3MnO3 - 2.5 Wt.%CuO composite displayed a maximum entropy change (ΔSM) ~ -4.0 J/kg/K, highest among studied composites, and is 110% higher than that obtained with La0.45Nd0.25Sr0.3MnO3. La0.45Nd0.25Sr0.3MnO3–5.0 Wt.%NiO displayed the highest relative cooling power (RCP) value ~ 299 J/kg among all studied composites. The magnetic changes in the composite are ascribed to alteration of Mn3+-O2—Mn4+ superexchange interaction strength due to M ion migration within and the surface of LSMO grain. Magnetization of aluminum-doped Gd3Fe5-xAlxO12 (0.0 ≤ x ≤ 1.0) compound enhanced with non-magnetic aluminum ion replacing magnetic iron ions with the spin-down moment. Consequently, the maximum magnetic entropy change, derived from magnetic isotherms in field up to 3 T, in Gd3Fe5-xAlxO12, was observed to vary from ~1.94 to ~3.07 J. K-1 kg-1 for x = 0.0 and x = 1.0, respectively. While an RCP value of 226 J.kg-1 was measured for x = 0.25 sample. Furthermore, the study was extended to understanding the influence of rare-earth ion doping in Gd3-xRExFe5O12 (RE = Y, Nd, Sm, and Dy, x = 0.00, 0.25, 0.50, and 0.75). At low temperature, the Gd3+ (L = 0) spin moment couples positively with the Fe3+ site moment, increasing the magnetization value of the compound. The presence of rare-earth ions with a finite orbital moment, such as Nd3+, Sm3+, and Dy3+, affects the RE3+(Nd, Sm, Dy)- O2- - Fe3+(Tetra.) bond-angle and bond-length in favor of enhancing the superexchange interaction with Fe3+ (tetrahedral site, spin-down). Consequently, RE3+ spins align opposite to the net spin moments of Fe3+ sites, leading to a decrease in magnetization value at low temperatures. Maximum entropy change, ΔSMmax, increased with Dy3+ and Sm3+ doping, whereas Nd3+ and Y3+ decreased ΔSM with x content. Moreover, the maximum magnetic entropy change value, ∆S_M^Max, increased ~ 7% for Dy3+ x = 0.75. These values are relatively high and comparable to some noticeable magnetocaloric materials. The findings in these studies show that a careful choice of dopants can be used to tune the structure-property relation toward enhancing the MCE in the studied compounds. The study will help the community working in the field of MC materials with a fundamental understanding of the underlying structure-property of materials to futuristic engineering of novel MC materials.


Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest.


Open access