Ph.D. (Doctor of Philosophy)
Department of Physics
Multiferroic materials having unusual properties of simultaneous presence of ferroelectricity (FE) and magnetism, such that FE can be controlled by magnetic field and magnetization by electric field have attracted interest due to their promising applications in electrically controllable microwave elements, magnetic field sensors, and possibly in spintronics. In this dissertation, the single phase and single ion multiferroic properties of the ceramic oxide antiferromagnets (AFM) Sr1-xBax Mn1-yTiyO3 with perovskite structure are discussed. These compounds were designed and developed in our laboratory by understanding critical parameters controlling FE and magnetic properties, advancing the synthesis processes, studying the structural behavior and intricate transitions, and characterizing dielectric and magnetic properties over the complex phase diagram of the manganites system. Multiferroelectricity was achieved by forcing bonds between magnetic transition metals and oxygen under internal tension through the proper selection of the sizes and charges of the A-site and B-site cations as predicted based on the Goldschmidt tolerance factor.
Synchrotron X-ray diffraction (XRD) and Neutron powder diffraction (NPD) experiments were used to study the structural, FE and magnetic properties. In the conventional FE titanates BaMn1-yTiyO3, the solubility limit of manganese was extended to 1-y ~ 0.16, and the XRD data taken at Argonne’s National Laboratory Advanced Photon Source showed increased temperatures of the coupled structural-ferroelectric transitions above 400 K. A complete solid solution range of manganese ion on the Ti-site was obtained in SrMn1-yTiyO3 (0 ≤ y ≤ 1), which showed cubic structure at room temperature. I was not able, however to induce the expected FE properties. Samples with 0 ≤ y ≤ 0.3 showed antiferromagnetic (AFM) order below 233 K.
For the double-substituted compounds near x = 0.45–0.65 and y = 0–0.1, a transition from cubic to the non-centrosymmetric FE tetragonal P4mm structure was increased to ~ 430 K on heating, and observed at ~ 60 K lower temperatures on cooling showing unusually large thermal hysteresis characteristic of the first order transition. The FE distortions c/a that are larger than the distortions of the nonmagnetic BaTiO3 have been observed with a maximum estimated polarization Ps of ~ 29 μC/cm2 at ~ 220 K. At lower temperatures, Ps calculated by atomic positions of the refined NPD data was suppressed by less than 50% due to development of AFM phase, indicating very strong coupling between FE and AFM. Similar Ps were found for x = 0.43–0.45 and y = 0 compounds as determined by the measured c/a ratios from the lattice parameters and by the shift of the Mn/Ti and oxygen atomic positions from the NPD, but with larger suppressions of Ps, ~75% observed below the AFM transitions. Experiments using hydrostatic pressure showed rapid suppression of the FE transition and an increase of the AFM transition as confirmed also by magnetic measurements. In search of new multiferroic materials I found the new solid solution system BaMnxFe1-xO3 with the perovskite structure present up to 66.7% and, a recent XRD indicated multiferroic behavior for x = 0.30.
Chapagain, Kamal, "Discovery and Study of Single-Phase and Single-Ion Manganese Perovskite Multiferroics" (2018). Graduate Research Theses & Dissertations. 6911.
Northern Illinois University
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