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

地磁场强度的变化与地核的地球动力有关，这些变化的历史可以提供地核演化和地核-地幔边界的信息。然而，要获得可靠的古强度数据，必须避免许多陷阱。开发更可靠的古强度方法的努力受到了阻碍，因为我们对除了最小的晶体之外的所有晶体中如何记录磁场的理解不足。特别是，我们几乎不知道记录过程是如何受到错位的影响的。位错是在晶体上施加内应力的缺陷。这些应力与磁化耦合。有充分的理由认为，位错在古地磁、亚微米铁磁晶体等最重要的矿物中是常见的，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。我们打算利用实验和理论在这方面取得重大进展。来自各种地质环境的样品，以及合成磁铁矿样品，将使用透射电子显微镜（TEM）来表征位错。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来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磁化（TRM）的获取。它将结合现实的晶粒尺寸和形状，以及晶体位错几何形状，由透射电镜测量确定。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。我们将尝试重现在大型磁铁矿晶体中报道的TRM实验的各种异常结果，我们将研究测量古强度实验的各种方案。这项工作将有助于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。它还涉及非均匀磁化晶体的TRM的第一个无约束物理模型。这可能会导致更好的古强度方法，对环境磁学的应用也有重要意义。这些进步是由两位大学科学家（其中一位是初级研究员）和政府实验室的一名研究员之间的新伙伴关系实现的。这种伙伴关系对未来研究应力和磁化之间的相互作用具有很大的潜力。

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.