Ferromagnetic Resonance of Interacting Particles: A Theoretical Study (Renewal)

在几乎任何一种淡水或咸水中，都能找到制造磁铁链的细菌。它们被称为趋磁细菌，它们利用磁铁链在地球磁场中定位自己，这有助于它们找到能够茁壮成长的深度。当它们死亡时，磁铁会以“磁化石”的形式存在；它们是少数能留下永久记录的微生物群之一。这些记录不仅提供了细菌的信息，还提供了它们所经历的环境条件；在发现磁铁的地层中，它们提供了细菌死亡前后地球磁场的记录。古地磁学家已经尝试开发出可靠地识别沉积物中磁性化石的磁性方法；但是这些方法不能总是区分细菌产生的磁性矿物和无机磁性矿物。加州理工学院的一个小组开发的一项测试显示出了希望。他们使用了铁磁共振（FMR），将样品暴露在微波频率的电磁场中并测量其磁响应。这可能是唯一的测试，可以区分一链磁铁和一个单一的延长磁铁。然而，现有的FMR理论不能模拟多个磁体，如果它们通过各自的磁场相互作用。在这个项目中，PI将开发软件，实现FMR的一般理论，用于相互作用的单畴磁铁（像偶极子一样的磁铁）。它将足够通用，允许大小、形状和方向变化。该软件不仅能够模拟商业电子自旋共振光谱仪（只能测量单个样品方向上的功率吸收）的输出，还能够模拟可以旋转样品并获得相位信息的仪器。该软件将用于计算磁铁链的FMR光谱，因为它们在活的细菌中，以及在细菌死亡后，随着链被破坏和磁铁氧化，光谱的变化。它将用于确定FMR何时可以用于可靠地识别磁化石。

In nearly any body of fresh or salt water, there is a depth at which bacteria can be found that manufacture chains of magnets. Called magnetotactic bacteria, they use the chains of magnets to orient themselves in the Earth's field, and this helps them find the depth at which they can thrive. When they die, the magnets remain as "magnetofossils"; they are one of the few groups of microorganism to leave a permanent record. This record provides not only information on the bacteria, but also the environmental conditions they experienced; and in layers where the magnets are found, they provide a record of the Earth's magnetic field around the time the bacteria died. Paleomagnetists have attempted to develop magnetic methods that reliably identify magnetofossils in sediments; but these methods cannot always discriminate between magnetic minerals produced by bacteria and inorganic magnetic minerals.A test developed by a group at Caltech shows promise. They use ferromagnetic resonance (FMR), which involves exposing a sample to microwave-frequency electromagnetic fields and measuring the magnetic response. This may be the only test that can distinguish a chain of magnets from a single elongated magnet. However, the existing theory for FMR cannot model more than one magnet if they interact with each other through their magnetic fields. In this project, the PI will develop software that implements a general theory of FMR for interacting single-domain magnets (ones that act like dipoles). It will be general enough to allow the sizes, shapes and orientations to vary. The software will be capable not just of modeling the output of commercial electron spin resonance spectrometers, which only measure power absorption in a single sample orientation, but also instruments that can rotate the sample and obtain phase information. The software will be used to calculate FMR spectra for chains of magnets, as they are in live bacteria, and changes in the spectra as the chains become disrupted and the magnets oxidized after the bacteria die. It will be used to determine when FMR can be used to reliably identify magnetofossils.