Quantum magnetism in cerium-based binary compounds

Cote, Alexandra

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https://hdl.handle.net/2142/121312 Copy

Description

Title

Quantum magnetism in cerium-based binary compounds

Author(s)

Cote, Alexandra

Issue Date

2023-06-26

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

MacDougall, Gregory J

Doctoral Committee Chair(s)

Cooper, S Lance

Committee Member(s)

• Beck, Douglas H
• Clark, Bryan 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)

• cerium
• magnetism
• cerium sesquioxide
• cerium telluride
• cerium tritelluride
• neutron diffraction
• muon spin rotation

Abstract

Cerium-based compounds have afforded an experimental landscape teeming with unusual phenomena. The magnetic properties of these materials have been especially interesting, as 4f electrons may exhibit either localized or itinerant behavior. Studying the magnetic order of these materials is therefore a crucial step toward a comprehensive understanding of the behavior of the 4f electrons in these systems. Investigation of the magnetic order in cerium-based materials often takes a multi-faceted approach as the behavior is complex, with instances of hidden order, small magnetic moments, and multipolar order yielding results that are typically at odds with those expected by a traditional dipole picture. In this work we use a combination of neutron scattering and muon spin relaxation to probe the magnetic properties of four cerium-based systems, the metastable Ce2O3 and the layered chalcogenides, CeTe2, CeTe3, and Ce2Te5. Ce2O3 has established itself as a hidden order system, as several investigations using neutron diffraction have previously failed to uncover signatures of magnetic order despite a clear magnetic transition in heat capacity and magnetic susceptibility data. In addition, a giant magnetodielectric effect and vibronic excitations have been reported in this material and are associated with this purported magnetic transition. To investigate the claims of magnetic order and the emergent vibronic excitations, we use a combination of neutron diffraction, triple-axis and time-of-flight inelastic neutron scattering, and muon spin relaxation techniques. Our inelastic neutron scattering and muon spin relaxation results provide the first evidence for long-range magnetic order in this material. Analysis of the collective results suggest that the magnetic order is of a k=0 antiferromagnetic order with spins aligned preferentially within the planes. Assuming dipolar moments in our analysis gives rise to large inconsistencies between neutron diffraction and muon data sets in a way that provides strong evidence for the ordering of higher-order moments. The cerium tellurides, CeTe2, CeTe3, and Ce2Te5, consist of distorted salt-like Ce-Te layers separated by square nets of Te atoms. Each of these materials possesses a different number of square Te nets, influencing the effective crystal field environment and correlation effects which, in turn, influences the magnetic properties. An investigation of the magnetic properties of the cerium telluride materials is therefore a great way to correlate the local environment and interactions to the magnetic properties. Neutron diffraction and muon spin relaxation are used to examine the magnetic order in this series and comment on the trends with varying layer composition. Magnetic Bragg peaks appeared below the ordering transition in neutron diffraction on CeTe2. Refinement of this diffraction data confirmed the emergence of a ferrimagnetic structure. Muon spin rotation on the same material showed a strong relaxation of the asymmetry with a temperature-dependence of the relaxation mirroring that seen in Ce2O3, though the absence of clear oscillations implies the presence of disorder or multiple muon sites. In neutron diffraction measurements on both CeTe3 and Ce2Te5 magnetic Bragg peaks were not observed below the ordering transitions, revealing a hidden order problem in these two materials. Muon spin rotation measurements on CeTe3 showed a strong relaxation similar to that seen in CeTe2, but with a changing sample volume fraction, suggesting a first-order transition. While no magnetic Bragg peaks were seen in neutron diffraction on Ce2Te5, existence of long-range magnetic order below the ordering transition was confirmed with the emergence of oscillations in the muon spin rotation data, similar to the case of Ce2O3.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Alexandra Cote

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