Constraining the magnetic properties of ultrafine- and fine-grained biogenic magnetite

Four samples containing ultrafine- and fine-grained magnetite of magnetoferritins and magnetotactic bacteria cells were magnetically characterized at both room and low temperatures. Transmission electron microscopy analysis showed that the biometrically synthesized magnetoferritins (M-HFn) have magnetite cores with a mean size of 5.3 ± 1.2 nm inside protein shells, while Magnetospirillum gryphiswaldense MSR-1 cell produced intracellular magnetosome magnetites have a mean size of 29.6 ± 7.6 nm, arranged in a single chain. A pure M-HFn sample (M 1 ), MSR-1 whole cell sample (M 4 ) and two samples (M 2 , M 3 ) mixing M-HFn with MSR-1 whole cells in different weight percentages were measured, including hysteresis, temperature dependency of magnetization and remanence and frequency dependence of AC susceptibility at low temperature. At room temperature, the ultrafine-grained magnetite core of M-HFn of M 1 sample has a typical superparamagnetic (SP) behavior. The chain-arranged magnetosome magnetite of MSR-1 cells of M 4 sample shows a stable single-domain (SD) state. At low temperature, the M 2 sample with ~ 16 wt% SD magnetosome magnetite and the M 3 sample with ~ 43 wt% SD magnetosome magnetite behave somewhat similar to the M 1 (pure M-HFn), due to the SP component from M-HFn magnetite. With the dominance of SP magnetite in samples M 1 , M 2 , and M 3 , the coercivity and saturation remanence decrease significantly as temperature increasing from 5 to 20 K. Of note, the magnetization and frequency dependence of AC susceptibility at low temperature are sensitive to SP magnetites in measured samples. The magnetosome magnetite produced by MSR-1 has a Verwey transition temperature at around 100 K, which is consistent with previous observations on magnetotactic bacteria. This study provides useful clues for identification of SP and SD magnetite in sediments, as well as related potential biomedical and biomagnetic applications.