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Title:Inelastic light scattering studies of novel magnetodielectric mechanisms in transition metal CoCr2O4 and rare earth Ce2O3
Author(s):Sethi, Astha
Director of Research:Cooper, Lance
Doctoral Committee Chair(s):Shoemaker, Daniel P
Doctoral Committee Member(s):Abbamonte, Peter; Wagner, Lucas K
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
spin fluctuations
crystal field excitation
Abstract:In this thesis, I present our Raman scattering studies to elucidate the distinct microscopic mechanisms that contribute to the magnetodielectric phenomena in diverse d- and f-electron systems. In particular, I study the low energy excitations including magnons, phonons and crystal field excitations - in two representative systems - the d-electron transition metal oxide CoCr2O4 and the f -electron rare earth oxide Ce2O3. In CoCr2O4, we observe a T1g symmetry magnon in the magnetodielectric phase below TC = 94 K.This magnon has an anomalously large Raman intensity, suggesting the presence of large spin fluctuations that strongly modulate the dielectric response in CoCr2O4. We show that an increasing magnetic field decreases the magnon Raman intensity, suggesting that the magnetodielectric behavior in CoCr2O4 is associated with a field-induced suppression of the spin fluctuations. We verify this further by showing that a similar decrease of the magnon intensity is observed under pressure, which serves to increase the magnetic anisotropy and consequently suppress the spin fluctuations. Our studies clarify that the mechanism behind magnetodielectricity in the spinel CoCr2O4 is different from the field-induced domain reorientation mechanism that is applicable to several magnetodielectric spinels. Additionally, our simultaneous field- and pressure-dependent measurements demonstrate that applied pressure or strain can serve as effective methods to control the magnetodielectric behavior in CoCr2O4. In Ce2O3, we observe energetically proximate vibrational and electronic crystal field excitations, which hybridize and lead to the emergence of coupled vibronic excitations in the magnetodielectric phase below TN = 6.2 K. The Raman intensities of these vibronic excitations indicate a strong modulation of the dielectric response due to these excitations in Ce2O3. We demonstrate that an external magnetic field effectively tunes the energies and intensities of these vibronic excitations, suggesting a novel contribution to the magnetodielectric behavior in Ce2O3 that is associated with a field-tunable vibronic coupling between the vibrational and electronic crystal field excitations. We also observe evidence for magnetostructural changes below TN that likely explain the abrupt enhancement of vibronic coupling in the magnetodielectric phase of Ce2O3. Such a vibronic coupling related microscopic contribution to the magnetodielectric phenomena in the rare-earth compounds such as Ce2O3 is distinct from those in the transition metal compounds, which have the crystal field excitations far removed in energy from the vibrational excitations. The emergence of vibronic excitations in the magnetodielectric phase of rare-earth compound Ce2O3 is akin to the emergence of electromagnon excitations in the multiferroic phase of transition metal compounds.
Issue Date:2020-08-27
Rights Information:Copyright 2020 Astha Sethi
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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