How do magnetic interactions in nanoscale intergrowths affect palaeomagnetic interpretations?

我们都熟悉这个实验，我们把一张纸放在一块条形磁铁上，通过在纸上撒铁屑来成像磁场。铁屑与磁铁的磁场对齐，描绘出众所周知的偶极场模式。在这种类型的实验中，磁性粒子可以自由地向高场区域移动。如果粒子被粘在纸上，使它们不能移动，那么粒子内的磁化强度就会移动。事实上，粒子内的磁化会旋转以与磁体的方向场对准，但当然这种旋转对眼睛来说是不可见的。粒子内的磁化与磁场对准并且即使我们移除原始磁化场也保持对准的能力意味着粒子能够产生场方向的永久记录。小磁性颗粒的这种记录能力已被人类利用了50多年，并用于磁带录音机和硬盘计算机存储等领域。在磁带和驱动器中，记录介质都是由涂有磁性微粒的薄膜组成的。这种技术允许计算机驱动器存储大量数据，这些数据可以一次又一次地删除和记录，但这种高可靠性只能通过确保所有磁性颗粒具有一致的粒度和化学纯度来实现。磁性颗粒不仅是人造的，而且在大多数岩石和土壤中自然存在。然而，与人造记录介质不同，这些天然存在的磁性矿物可以具有各种各样的化学成分，粒度和颗粒浓度。在某些情况下，这些磁性矿物将在单个颗粒内包含区域，每个区域具有略微不同的化学结构。这些独特的磁性矿物将在岩石或土壤形成时响应环境条件而发展。例如，从同一座火山喷出的熔岩可以产生不同的磁性矿物，这取决于熔岩冷却的速度，土壤中含有的磁性矿物取决于有机物质的含量。因此，能够正确识别磁性矿物的类型，可以告诉我们一些关于样品形成环境的信息。磁性矿物的准确识别是重要的另一个原因。天然磁性矿物在形成时会记录地球磁场的方向。然而，记录的可靠性在很大程度上取决于存在的磁性矿物的类型。某些矿物的磁化方向可能更多地受到矿物本身的晶体结构的影响，而不是地磁场。这被认为是可能的情况下，当磁性颗粒是化学分区，如前所述。因此，重要的是要了解不同的多带颗粒如何能够进行磁记录，并开发简单的测试来区分哪些矿物系统是可靠的地磁场记录器，哪些不是。

We are all familiar with the experiment where we place a piece of paper on top of a bar magnet and image the magnetic field by sprinkling iron filings on to the paper. The iron filings align with the magnetic field of the magnet, tracing out the well-known dipole field pattern. In this type of experiment the magnetic particles are free to move toward the high field regions. If the particles had been glued to the paper so that they could not move, then the magnetisation within the particles would have moved instead. In fact the magnetisation within the particles would rotate to align with the direction field of the magnet, but of course this rotation would not have been visible to the eye. The ability of the magnetisation within a particle to align with a magnetic field and remain aligned even if we remove the original magnetising field, means the particles are able to produce a permanent record of the field direction. This recording capability of small magnetic particles has been exploited by man for over 50 years, and used, for example, in magnetic tape recorders and for hard drive computer storage. In both tapes and drives the recording media consists of a thin film coated with a fine power of magnetic particles. Such technology allows computer drives to store huge amounts of data which can be deleted and recorded time and time again, but this high reliability can only be achieved by ensuring that all the magnetic particles are of a consistent grain size and chemical purity. Magnetic particles are not just man-made, but also occur naturally in most rocks and soils. Unlike man-made recording media, however, these naturally occurring magnetic minerals can have a wide variety of chemical compositions, grain sizes, and particle concentrations. In some cases these magnetic minerals will contain zones within a single grain with each zone having a slightly different chemical structure. These distinct magnetic minerals will develop in response to environmental conditions at the time when the rocks or soils are formed. For example, lavas thrown out from the same volcano can produce different magnetic minerals depending on how slowly the lavas cool, and soils will contain different magnetic minerals depending on the amount of organic material present. Being able to correctly identify the type of magnetic mineralogy, therefore, can tell us something about the environment in which that sample was formed. Accurate identification of the magnetic minerals is important for another reason. Natural magnetic minerals will record the direction of the Earth's magnetic field as the minerals are formed. However, the reliability of the recording will greatly depend upon the types of magnetic minerals that are present. It is possible that the direction of magnetisation in some minerals is more influenced by the crystalline structure of the mineral itself rather than the geomagnetic field. This is thought to be likely in the case when a magnetic grain is chemically zoned, as mentioned earlier. It is therefore important to understand how different multi-zoned grains are able to make a magnetic recording, and develop simple tests to distinguish which mineral systems are reliable recorders of the geomagnetic field, and which are not.