In the mud at the bottom of a pond live bacteria that carry tiny magnetic compass needles inside them. These needles are chains of magnetite crystals that help the bacteria swim along Earth's magnetic field, like a compass needle, in search of better conditions. When the bacteria die, the chains remain in the sediment and can be preserved for millions of years.
Geologists find such chains in ancient rocks and use them to learn when life appeared on Earth. But how do you tell a chain assembled by a living bacterium from a random clump of magnetic particles? It turns out that biogenic chains have distinctive magnetic properties. Scientists measure how such particles magnetize and demagnetize, and from the shape of the curves they determine whether life was involved.
Until now, calculations were done for spherical particles, but in many bacteria the crystals are elongated, like grains of rice. In the new work, simple formulas were derived for the interaction of such elongated magnets, and the behavior of whole chains of them was calculated. This will allow a more accurate reading of Earth's magnetic record.
as written by the authors arXiv:2601.16861
Magnetotactic bacteria synthesize linear chains of magnetite nanoparticles within their bodies, which allow the bacteria to navigate the Earth's magnetic field in search of the best habitat. Biogenic magnetite particles, called magnetosomes, are very promising for use in biomedicine. Magnetosome chains have also been found in ancient fossils and sediments. The study of magnetofossils provides valuable information about the Earth's biological past. The presence of biogenic magnetite in ancient rock samples can be detected by measuring ferromagnetic resonance spectra, first-order magnetization reversal curves, or quasi-static hysteresis loops. Theoretical analyses of these experiments generally assume that magnetosomes are spherical nanoparticles, although the shape of some types of magnetosomes is close to spheroidal one. In this work, simple formulas for describing the magneto-dipole interaction of oriented spheroids are obtained and quasi-static hysteresis loops of randomly oriented magnetosome chain assembly consisting of elongated spheroids are calculated.
Magnetosomes are so stable that they are found in rocks over 2 billion years old, and they still retain their magnetization.