Variations in the intensity of the geomagnetic field are connected to the geodynamo in the Earth's core, and the history of these variations can provide information on the evolution of the core and the core-mantle boundary. However, to acquire reliable data on paleointensity one must avoid many pitfalls. Efforts to develop more reliable paleointensity methods are hampered by our poor understanding of how the field is recorded in all but the smallest crystals. In particular, we know almost nothing about how the recording process is affected by dislocations.
Dislocations are imperfections that impose internal stresses on a crystal. These stresses couple with the magnetization. There is good reason to expect that dislocations are common in the most important minerals for paleomagnetism, submicron ferromagnetic crystals, but there is almost no hard evidence for their presence. Nor is anything known about the effect they would have on the magnetic properties in such crystals. We propose to make a major advance in this knowledge using both experiment and theory.
Samples from a variety of geologic settings, as well as synthetic magnetite samples, will be examined using transmission electron microscopy (TEM) to characterize the dislocations. We will calculate the stress fields around dislocations, taking advantage of a theoretical breakthrough at Lawrence Livermore Laboratories. The stress fields will be incorporated in a numerical micromagnetic model to calculate the coupling between dislocations and magnetization.
The micromagnetic model will be used to model the acquisition of thermoremanent magnetization (TRM) as minerals cool. It will incorporate realistic grain sizes and shapes, as well as crystallographic dislocation geometries, determined from the TEM measurements. The model will be used to calculate the effect of dislocations on (1) magnetic hysteresis as a function of temperature, (2) low temperature demagnetization and (3) TRM acquisition. We will attempt to reproduce a variety of anomalous results that have been reported for TRM experiments in large magnetite crystals, and we will investigate various protocols for measuring paleointensity experiments. This work should lead to a better understanding of the protocols and better paleointensity methods.
This work will include the first systematic search for dislocations in small magnetite crystals, the first calculations of stress fields around dislocations in finite crystals, and the first calculations of the fine-scale coupling between dislocations and magnetization in any material. It also involves the first unconstrained physical model of TRM for crystals with nonuniform magnetization. It may lead to better paleointensity methods and should also be important to applications of environmental magnetism.
These advances are made possible by a new partnership between two university scientists (one of whom is a Beginning Investigator) and a researcher at a government laboratory. This partnership has great potential for future studies of the interaction between stress and magnetization.
