Cortés-Ortuño David, Fabian Karl, de Groot Lennart V
Paleomagnetic Laboratory Fort Hoofddijk Department of Earth Sciences Utrecht University Utrecht The Netherlands.
Norwegian University of Science and Technology (NTNU) Trondheim Norway.
J Geophys Res Solid Earth. 2022 May;127(5):e2022JB024234. doi: 10.1029/2022JB024234. Epub 2022 May 23.
Micromagnetic tomography (MMT) is a technique that combines X-ray micro computed tomography and scanning magnetometry data to obtain information about the magnetic potential of individual grains embedded in a sample. Recovering magnetic signals of individual grains in natural and synthetic samples provides a new pathway to study the remanent magnetization that carries information about the ancient geomagnetic field and is the basis of all paleomagnetic studies. MMT infers the magnetic potential of individual grains by numerical inversion of surface magnetic measurements using spherical harmonic expansions. The magnetic potential of individual particles in principle is uniquely determined by MMT, not only by the dipole approximation, but also more complex, higher order, multipole moments. Here, we show that such complex magnetic information together with both particle shape and mineral properties severely constrains the internal magnetization structure of an individual grain. To this end, we apply a three dimensional micromagnetic model to predict the multipole signal from magnetization states of different local energy minima. We show that for certain grains it is even possible to uniquely infer the magnetic configuration from the inverted magnetic multipole moments. This result is crucial to discriminate single-domain particles from grains in more complex configurations such as multi-domain or vortex states. As a consequence, our investigation proves that by MMT it is feasible to select statistical ensembles of magnetic grains based on their magnetization states, which opens new possibilities to identify and characterize stable paleomagnetic recorders in natural samples.
微磁层析成像（MMT）是一种将X射线显微计算机断层扫描和扫描磁强计数据相结合的技术，用于获取有关嵌入样品中的单个颗粒磁势的信息。恢复天然和合成样品中单个颗粒的磁信号为研究携带古代地磁场信息的剩余磁化强度提供了一条新途径，而剩余磁化强度是所有古地磁研究的基础。MMT通过使用球谐展开对表面磁测量进行数值反演来推断单个颗粒的磁势。原则上，单个颗粒的磁势不仅由偶极近似，而且由更复杂的高阶多极矩唯一确定。在此，我们表明，这种复杂的磁信息以及颗粒形状和矿物特性会严重限制单个颗粒的内部磁化结构。为此，我们应用三维微磁模型来预测不同局部能量最小值的磁化状态产生的多极信号。我们表明，对于某些颗粒，甚至可以从反演的磁多极矩中唯一推断出磁结构。这一结果对于区分单畴颗粒与处于更复杂结构（如多畴或涡旋态）的颗粒至关重要。因此，我们的研究证明，通过MMT根据磁颗粒的磁化状态选择统计集合是可行的，这为识别和表征天然样品中稳定的古地磁记录器开辟了新的可能性。
