High Temperature Magnetic Imaging of Titanomagnetites

天然存在的磁性氧化铁以微小矿物颗粒的形式存在于岩石、沉积物、土壤以及考古和外星材料中。通过各种地质过程和不同程度的可靠性，氧化铁颗粒可以记录和存储有关古代行星磁场方向和强度的古磁信息。其中一个过程是热磁记录，即在地磁场存在的情况下，火成岩中的磁性矿物通过从高温冷却到环境表面温度而被磁化。了解热磁记录的物理机制是准确恢复自然材料古磁信息的必要条件。然而，我们的理论理解是基于最简单的内部微磁结构（单畴），只适用于最小的晶粒尺寸，而不包括更丰富的大晶粒，表现出复杂的结构（多畴）。本研究将通过直接观察晶粒的微磁结构及其随实验施加的磁场和控制温度的变化来解决多畴晶粒的热磁记录问题。这项研究活动将提高我们研究高温下磁性行为的基本物理的能力，并将提高我们对常见磁性矿物的磁性行为的理解。这项研究将有利于地球科学家调查地球和太阳系其他地方行星磁场的起源和演变。自然记录介质，地球科学家努力从中检索地磁场的古代历史信息，最常见的不是保存他们的数据在良好的，高保真的磁性矿物颗粒，而是更复杂的微磁结构颗粒。随着高温磁力显微镜（MFM）的发展，对普通磁性矿物中与温度相关的微磁结构的直接高分辨率观察最近成为可能，该显微镜能够成像高达约400°C的磁模式。该项目将重点研究天然氧化铁和合成氧化铁晶粒中磁化结构随温度、外加弱场、晶粒尺寸和先前磁性处理的变化过程。这将使研究小组能够研究自然磁场记录的机制，了解它们在地质时期对热或化学套印的敏感性，并设计出优化的实验室方法，从自然记录介质中恢复古磁场数据。这里支持的研究将提供对负责热磁化的微磁机制的深入了解，以及允许在实验室中通过实验确定古代地磁场古强度的基本过程。

Naturally occurring magnetic iron oxides in the form of microscopic mineral grains are common in trace amounts in rocks, sediments, and soils as well as in archeological and extraterrestrial materials. Through various geological processes and to varying degrees of reliability, iron-oxide particles can record and store paleomagnetic information about the direction and strength of ancient planetary fields. One such process is thermomagnetic recording, whereby magnetic minerals in igneous rocks are magnetized by cooling from high temperatures to ambient surface temperatures in the presence of the geomagnetic field. Understanding the physical mechanisms for thermomagnetic recording is essential for accurate recovery of paleomagnetic information from natural materials. Yet our theoretical understanding is predicated on the simplest internal micromagnetic configurations (single-domain), appropriate for just the smallest grain sizes, and does not cover the more abundant larger grains exhibiting complex configurations (multi-domain). This research will address thermomagnetic recording in multi-domain grains by direct observations of their micromagnetic configurations, and their changes with experimentally applied fields and controlled temperatures. The research activity will improve our ability to study the fundamental physics of magnetic behavior at elevated temperatures and will improve our understanding of the magnetic behavior of common magnetic minerals. This research will benefit geoscientists investigating the origins and evolution of planetary magnetic fields on Earth and elsewhere in the solar system.Natural recording media, from which geoscientists strive to retrieve information on the ancient history of geomagnetic field, most commonly do not hold their data in well-behaved, high-fidelity magnetic mineral grains, but rather in grains with more complex micromagnetic structures. Direct high-resolution observation of the temperature-dependent micromagnetic structures in common magnetic minerals has recently become possible with the development of high-temperature magnetic force microscopes (MFM) capable of imaging magnetic patterns up to approximately 400°C. This project will focus on a detailed MFM study of the evolution of magnetization structures in grains of natural and synthetic iron oxides as functions of temperature, applied weak fields, grain size, and prior magnetic treatments. It will allow the team to study the mechanisms of natural magnetic recording, to understand their sensitivity to thermal or chemical overprinting over geologic time, and to devise optimized laboratory methods for the recovery of paleomagnetic field data from natural recording media. The research supported here will provide insight into the micromagnetic mechanisms responsible for thermoremanent magnetization and the fundamental processes that allow estimates of the paleointensity of the ancient geomagnetic field to be determined experimentally in the laboratory.