Author ORCID Identifier
Semester
Fall
Date of Graduation
2024
Document Type
Dissertation
Degree Type
PhD
College
Eberly College of Arts and Sciences
Department
Physics and Astronomy
Committee Chair
Mikel B. Holcomb
Committee Member
Udaya B. Halabe
Committee Member
Wathiq Abdul-Razzaq
Committee Member
Joonhee Lee
Abstract
Superconducting and magnetic thin films are commonly used in sensors for analytical detectors, especially those that operate at very low temperatures, often below 1 Kelvin. These highly sensitive detectors, capable of functioning across a broad range of wavelengths, have garnered significant interest in scientific research. The focus of this thesis is to address a material challenge associated with an emerging technology known as microwave kinetic inductance detectors (MKIDs). The research involves the study of Al/Si and Al-Mn/Si-based thin films, as well as an investigation into the magnetic properties of LaMnO₃ thin films. LaMnO₃, a perovskite thin film that plays a crucial role in certain heterostructures, is particularly intriguing due to its complex magnetic phase diagram.
Using x-ray absorption spectroscopy (XAS), we examined the residues and adsorbates created during various standard lithography and etch steps typically employed to pattern thin aluminum films into device structures. This study revealed the formation of aluminum oxide (α-Al₂O₃) and aluminum fluoride (β-AlF₃). A correlation between low two-level system (TLS) microwave loss and XAS spectra was also observed, suggesting lower relative quantities of α-Al₂O₃ and β-AlF₃, as well as a more pronounced O K-edge pre-edge peak. Additionally, this research explored the chemical impact of different process steps, including standard silicon substrate wafer cleaning, SF₆ plasma etching, fluorocarbon passivation, and exposure to photoresist adhesion promoters during lithography, using control samples for comparison.
Al has a superconducting transition temperature Tc of about 1.175 K, however, NASA needs to operate some devices at lower temperatures to reduce thermal noise. The introduction of magnetic impurities, such as manganese, systematically reduces Tc. For instance, in Al-Mn thin films with 3000 ppm of Mn, the Tc of as-deposited films decreases to 60 mK. The development of Al-Mn thin films is followed by a post-annealing process. Detectors annealed at higher temperatures exhibit an increase in transition temperature (Tc) depending on the annealing temperature and duration. Using extended x-ray absorption fine spectroscopy (EXAFS), we analyzed the local structure around Mn in Al and discovered that Mn clustering increases with higher annealing temperatures. The longer diffusion length at a higher annealing temperature indicates a higher probability of Mn clustering. Clustering of Mn atoms lessens the scattering center for Cooper pairs due to the superconducting aluminum host. Thus, with prolonged annealing time and rising temperature, Mn atoms come closer to each other near grain boundaries, which affects the Tc of our transition edge sensors.
The magnetic properties of LaMnO₃ thin films grown on SrTiO₃ substrates using pulsed laser deposition were also investigated. These thin films are pseudomorphically strained by SrTiO₃, but their out-of-plane lattice parameter varies between 3.89 Å - 3.93 Å. This variation relates to changes in oxygen content during film growth. XPS and XAS studies revealed that the Mn cations in the films exhibit a mixed valence state of Mn³⁺/Mn⁴⁺. The observed ferromagnetic magnetization is attributed to the double exchange interaction between Mn³⁺-O-Mn⁴⁺, even though bulk stoichiometric LMO typically shows A-type antiferromagnetic order. Depth-dependent magnetization measurements from polarized neutron reflectivity were compared to those obtained from a vibrating sample magnetometer, and the strength of magnetization correlated with the Mn⁴⁺ content. Additionally, spontaneous magnetization reversal was observed in zero-field-cooled (ZFC) magnetization cycles. The study also examined field-dependent variations in blocking temperature (TB), freezing temperature (Tf), and compensation temperature (Tcomp) associated with the ZFC cycle. The blocking temperature followed Kneller's relation, TB(H)=TB(0)(1−H/HK)2, while the freezing and compensation temperatures exhibited exponential decay with increasing field. Finally, the temperature dependence of coercivity was analyzed, showing a mixture of weakly interacting superparamagnetic and antiferromagnetic phases. Below the blocking temperature, coercivity followed the relationship HC(T)=HC(0)[1−(T/TB)1/2].
The study of impurities and absorbates in Al thin films during fabrication, along with the clustering of dopants during annealing and their impact on the superconducting transition temperature, will be crucial for understanding the performance and consistency of Al thin film-based superconducting devices. Additionally, this thorough investigation into the intriguing magnetic properties and magnetization reversal in LaMnO3 will be valuable for advancing research on its application in magnetic heterostructure devices.
Recommended Citation
Bhandari, Ghadendra Bahadur, "X-ray Absorption and Related Studies in Superconducting Devices and Magnetic Thin Films" (2024). Graduate Theses, Dissertations, and Problem Reports. 12640.
https://researchrepository.wvu.edu/etd/12640
Comments
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