High Temperature Magnetic Imaging of Titanomagnetites

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.