Cryogenic scanning transmission electron microscopy of quantum materials

Ni, Haoyang

Permalink

https://hdl.handle.net/2142/129506 Copy

Description

Title

Cryogenic scanning transmission electron microscopy of quantum materials

Author(s)

Ni, Haoyang

Issue Date

2025-03-27

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

Zuo, Jian-Min

Doctoral Committee Chair(s)

Zuo, Jian-Min

Committee Member(s)

• Chi, Miaofang
• Huang, Pinshane
• Hoffman, Axel
• Chiang, Tai-Chang

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Electron microscopy
• 4D-STEM, Cryogenic scanning transmission electron microscopy
• Quantum materials
• Charge density wave
• Ferromagnetism

Abstract

Low temperature phase transitions in quantum materials often lead to emergent and exotic properties, which are primarily determined by correlations between the four fundamental degrees of freedom – lattice, charge, orbital and spin. Understanding the microscopic origins of these phase transitions is critical to enable the theory-guided design of quantum materials and devices that are capable of retaining quantum phenomena at the targeted temperatures and beyond. To achieve this goal, a microscopic probe sensitive to crystal structure, charge distribution and magnetic ordering is a prerequisite. In this study, we advance cryogenic (4-dimensional or 4D) scanning transmission electron microscopy (STEM) based imaging techniques to investigate the low-temperature phase transitions in two prototypical quantum materials EuAl4 and Fe5-xGeTe2. They are selected for study because the strong spin-lattice coupling in these materials related to several quantum mechanical phenomena, including coexisting charge density wave and chiral spin density waves, topological spin states like skyrmions and thermal treatment dependent magnetization curves. First, using cryogenic 4D-STEM we visualize long wavelength incommensurate charge density wave (ICDW) in EuAl4 below the transition temperature of 140 K. The periodic lattice distortion (PLD) associated with the ICDW formation is mapped in real space, which we show to have a temperature dependent wavelength and coherence. The PLD introduces periodic local symmetry fluctuation with alternating in-plane polarity determined by differential phase contrast imaging. Using the newly developed method of electron exit-wave power spectra (EWPS) analysis, we then show that the symmetry breaking originates from the periodic, transverse atomic displacements. Importantly, the displacements on Al and Eu sublattices have different phases determined for the first time. This critical information enables us to construct a PLD model that can account for the periodically alternating in-plane polarity in EuAl4. Second, we investigate the in-plane magnetic domain structure in Fe5-xGeTe2 in the ab-plane. The magnetic phase diagram is established by performing Lorentz 4D-STEM and mapping magnetic domain structure as a function of temperature and external magnetic field μ0Hext. The result shows a stripe-to-bubble transition at temperatures below 270 K driven by μ0Hext. Different magnetic states can be stabilized in Fe5-xGeTe2 by having different magnetic history of zero-field cooling and field cooling. The transition from vortex-like type-I magnetic bubbles to type-II magnetic bubbles with parallel domain wall can be driven by in-plane external field, and the type-II bubble orientation is determined by external field direction. Combining micromagnetic simulations and our experimental Lorentz 4D-STEM observations, we conclude that Fe5-xGeTe2 is a centrosymmetric ferromagnet with weak out-of-plane anisotropy. Last but not least, we determine the correlation between the crystal structure, local composition and magnetic domain structure in Fe5-xGeTe2 by comparing two samples, one is quenched and the other is furnace cooled. Our results show that thermal history greatly influences the spatial variation of Fe composition in Fe5-xGeTe2, which leads to different crystallographic domain structures in the quenched and the furnace cooled Fe5-xGeTe2. In the quenched Fe5-xGeTe2, phase separation results in the coexistence of Fe-poor secondary and Fe-rich primary phases, while the furnace cooled Fe5-xGeTe2 exhibits greater structural and compositional homogeneity. Specifically, Fe(1)-Ge ordered and disordered layers coexist in the primary phase, whereas the secondary phase consists solely of disordered layers, where intralayer Fe(1)-Ge ordering is strongly influenced by local Fe composition. The phase separation in the quenched Fe5-xGeTe2 greatly impacts the magnetic domain structure. Cryogenic Lorentz 4D-STEM imaging and micromagnetic simulations reveal that the Fe-poor secondary phase undergoes a spontaneous in-plane to out-of-plane anisotropy transition below 150 K, while Fe-rich primary phase persists to be an out-of-plane anisotropic ferromagnet. Our results provide direct evidence of the coupling between local crystal structure and magnetic ordering in Fe5-xGeTe2. Our findings on prototypical systems of EuAl4 and Fe5-xGeTe2 provide insights into each material system and highlight the potential of cryogenic (4D) STEM as a transformative and versatile tool for understanding the microscopic mechanisms driving phase transitions in quantum materials. The ability of cryogenic (4D) STEM to visualize structural, charge and spin phase transitions at low temperatures lays the methodological foundation for future studies, targeted at understanding the correlation between lattice, charge, orbital and spin at atomic and nanometer scales.

Graduation Semester

2025-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129506

Copyright and License Information

Copyright 2025 Haoyang Ni

Owning Collections