Inelastic light scattering studies of novel magnetodielectric mechanisms in transition metal CoCr2O4 and rare earth Ce2O3

Sethi, Astha

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https://hdl.handle.net/2142/109328 Copy

Description

Title

Inelastic light scattering studies of novel magnetodielectric mechanisms in transition metal CoCr2O4 and rare earth Ce2O3

Author(s)

Sethi, Astha

Issue Date

2020-08-27

Director of Research (if dissertation) or Advisor (if thesis)

Cooper, Lance

Doctoral Committee Chair(s)

Shoemaker, Daniel P

Committee Member(s)

• Abbamonte, Peter
• Wagner, Lucas K

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• magnetodielectric
• magnetostructural
• vibronic
• magnon
• 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.

Graduation Semester

2020-12

Type of Resource

Thesis

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Copyright 2020 Astha Sethi

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