A combined experimental and computational study of nanopaleomagnetic recorders in meteoritic metal
A nanoscale intergrowth of fine tetrataenite particles in an iron rich matrix, known as the `cloudy zone' has recently been recognised as a stable paleomagnetic recorder. It is found in meteorites containing iron-nickel alloy that have developed the characteristic Widmanst"atten pattern. However, the close particle proximity and high magnetocrystalline anisotropy of the cloudy zone make it a highly unconventional material to use in paleomagnetic studies. Open questions about the formation of the cloudy zone, its exact composition and structure, and how it acquires a remanence remain unanswered. To use the cloudy zone as a reliable and accurate paleomagnetic recorder, its properties have to be well understood.
The particle size in the cloudy zone has been measured to be between $\sim$500 nm and $\sim$10 nm, however, it is rarely higher than $\sim$150 nm. This fine lengthscale makes it difficult to study the cloudy zone with conventional methods. No known single method can provided the solution to all current problems concerning the use of the cloudy zone as a paleomagnetic recorder. Therefore, to explore the properties of the cloudy zone a multi-method approach using advanced nanoscale investigation techniques and theoretical calculations was adopted. The formation of the cloudy zone within the context of the Fe-Ni phase diagram was studied using Monte Carlo simulations. These simulations were supplemented by Density Functional Theory (DFT) calculations. DFT was also used to explore the preferred chemical ordering schemes of the matrix as well as its ground magnetic state. The 3D structure of the cloudy zone was imaged with sub-nanometer resolution using Atom Probe Tomography (APT). This was one of the first applications of APT to image meteoritic metal. Accurate composition measurements of the matrix and tetrataenite as well as kamacite were made. Synchrotron M"ossbauer spectroscopy was used to provide high spatial resolution information of the magnetic state of the matrix and tetrataenite in the cloudy zone as well as the surrounding metal. The matrix was conclusively demonstrated to be paramagnetic at room temperature as a bulk material.
X-Ray holography was used for the first time to directly image magnetisation of individual paleomagnetic remanence carriers under high applied fields. In-field hysteresis behaviour of individual particles in the cloudy zone was measured by directly imaging the sample magnetisation with a resolution of $\sim$25 nm. The cloudy zone was found to consist of strongly interacting single domain particles. The experimental observations were supported by modelling.
The combined approach of multiple methods was capable of providing answers to some of the important questions about the cloudy zone. The cloudy zone was found to be highly stable against remagnetisation by applied external fields. If the particle size is below $\sim$80-50 nm the cloudy zone was found to consist of isolated single-domain tetrataenite particles sitting in a paramagnetic matrix. The matrix might become ferrimagnetic at very low temperatures. Depending on how the magnetisation is measured, the measurement might be affected by the matrix changing its magnetic state to ferromagnetic at surfaces. This phenomena should not affect the overall stability of the cloudy zone as a paleomagnetic recorder. Due to the close proximity of the tetrataenite particles, there are strong magnetostatic interactions between them. This finding demands that new methods be developed for correct interpretation of the remanence recorded in the cloudy zone.