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.
