Thesis Defense
Student: Ualisson Donardelli Bellon
Program: Geofísica
Title: “Structure and magnetic stability of natural iron oxides in the vortex-state”
Advisor: Prof. Dr. Ricardo Ivan Ferreira da Trindade
Judging Committee
- Prof. Dr. Ricardo Ivan Ferreira da Trindade – Orientador - IAG/USP
- Dr. Lesley Nagy - University of Liverpool, United Kingdom
- Prof. Dra. Lisa Tauxe - University of California San Diego, US
- Prof. Dr. Richard J. Harrison - University of Cambridge, United Kingdom
- Prof. Dr. Liao Chang - Peking University, China
Abstract
Magnetic minerals hold crucial records of ancient magnetic fields, providing insights into the geological history of planets. Various physical and chemical processes generate magnetic minerals with differing grain sizes and morphologies, directly affecting their physical properties and ability to record magnetic fields. While uniformly magnetized single domain (SD) grains are well understood, the properties of pseudo-single domain (PSD) grains, which are slightly larger and non-uniformly magnetized, remain poorly studied. This research combined numerical modeling, classical rock magnetic analysis, and synchrotron- based chemical and tomographic techniques to explore how the characteristics of nanoscale grains influence their macroscale magnetic properties. The focus was on Neoproterozoic cap carbonates from South America to study magnetic mineral growth in sedimentary environments. These rocks exhibit secondary natural magnetization with unique magnetic properties, such as distorted hysteresis loops and anomalous hysteresis ratios. To address distorted hysteresis, a modified Gamma-Cauchy function was proposed to unmix coercivity components, correlating them with different magnetic populations in the samples. Magnetic measurements and in-situ synchrotron-based chemical analysis revealed sub-micron magnetite associated with clay minerals, indicating the formation of PSD grains due to thermally triggered processes that occurred during the Gondwana supercontinent assembly, and a subsequent cooling that blocked their remanence. Once samples with targeted PSD grains were identified and properly characterized, the study transitioned from macroscale to nanoscale analysis. The potential of Ptychographic X-ray Computed Nano-tomography (PXCT) and micromagnetic modeling was then investigated by analyzing the magnetic properties of micrometric maghemite grains, modeled after giant Paleoproterozoic magnetofossils aligned in a magnetosome-like structure. This model demonstrated that chains of multivortex grains can produce efficient magnetotaxis even at low magnetic field intensities. Applying these methods to the carbonate rocks, PXCT identified hundreds of iron oxide grains with varied sizes and shapes. Some grains, particularly those near the SD to single-vortex (SV) transition, exhibited multiple domain states that were unstable for long-term field recording. Additionally, certain grains showed temperature-dependent domain state occupancy probabilities, complicating the recovery of past magnetic field intensities. In summary, we show that advanced chemical and imaging techniques, combined with numerical models, can unravel a wide range of macroscopic magnetic properties. This versatile methodology has the potential to be applied to various terrestrial and extraterrestrial materials, offering new perspectives on the magnetic histories of planets and other celestial bodies.
Keywords: Magnetic mineralogy, Nano- tomography, Magnetic vortex-state, Micromagnetic models, Paleomagnetism