Collaborative Proposal: The Effect of Dislocations on Magnetic Properties of Small Titanomagnetite Minerals

地磁场强度的变化与地核中的地球发电机有关，这些变化的历史可以提供有关地核和核幔边界演化的信息。然而，要获得可靠的古烈度数据，必须避免许多陷阱。由于我们对除了最小的晶体外，磁场是如何被记录下来的了解不多，因此开发更可靠的古强度方法的努力受到了阻碍。特别是，我们几乎不知道记录过程如何受到位错的影响。位错是在晶体上施加内应力的缺陷。这些应力与磁化耦合。我们有充分的理由认为，位错在古地磁学中最重要的矿物亚微米铁磁晶体中很常见，但几乎没有确凿的证据证明它们的存在。也不知道它们对这种晶体的磁性有什么影响。我们建议使用实验和理论在这方面取得重大进展。将使用透射电子显微镜（TEM）检查来自各种地质环境的样品以及合成磁铁矿样品，以表征位错。我们将利用劳伦斯利弗莫尔实验室的理论突破，计算位错周围的应力场。应力场将被纳入一个数值微磁模型，以计算位错和磁化之间的耦合。微磁模型将用于模拟矿物冷却时热释电磁化（TRM）的采集。它将结合现实的晶粒尺寸和形状，以及晶体位错的几何形状，从TEM测量确定。该模型将用于计算位错对（1）作为温度函数的磁滞，（2）低温退磁和（3）TRM采集的影响。我们将试图重现各种异常的结果，已报告的TRM实验在大磁铁矿晶体，我们将调查各种协议测量古强度实验。这项工作应导致更好地理解协议和更好的古强度方法。这项工作将包括第一次系统地搜索小磁铁矿晶体中的位错，第一次计算有限晶体中位错周围的应力场，以及第一次计算任何材料中位错和磁化之间的精细尺度耦合。它还涉及第一个无约束的物理模型TRM的晶体与非均匀磁化。它可能会导致更好的古强度方法，也应该是重要的环境磁学的应用。这些进展是由两名大学科学家（其中一人是初级研究员）和一名政府实验室研究员之间的新伙伴关系实现的。这种伙伴关系有很大的潜力，为未来的研究应力和磁化之间的相互作用。

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.