Abstract
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 (M1), MSR-1 whole cell sample (M4) and two samples (M2, M3) 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 M1 sample has a typical superparamagnetic (SP) behavior. The chain-arranged magnetosome magnetite of MSR-1 cells of M4 sample shows a stable single-domain (SD) state. At low temperature, the M2 sample with ~ 16 wt% SD magnetosome magnetite and the M3 sample with ~ 43 wt% SD magnetosome magnetite behave somewhat similar to the M1 (pure M-HFn), due to the SP component from M-HFn magnetite. With the dominance of SP magnetite in samples M1, M2, and M3, 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.
Highlights
Superparamagnetic (SP) magnetite has been found in numerous geological samples, e.g., soils, pelagic sediments, tuffs and ice sheets, usually with some distinct magnetic properties; they are of great interests in rock magnetism, environmental magnetism and paleomagnetism
Each MSR-1 cell contains a single chain that consists of 10–15 octahedral magnetite magnetosomes, which have a mean size of 29.6 ± 7.6 nm (N = 311) (Fig. 1d–g)
Figure 1j shows the X-ray diffraction of the magnetite of M-H-chain ferritin (HFn) nanoparticles of Fig. 1i, confirming the crystalline ultrafine-grained magnetite in the M-HFn nanoparticles
Summary
Superparamagnetic (SP) magnetite has been found in numerous geological samples, e.g., soils, pelagic sediments, tuffs and ice sheets, usually with some distinct magnetic properties; they are of great interests in rock magnetism, environmental magnetism and paleomagnetism. Worm and Jackson (1999) studied the magnetite in the Yucca Mountain tuff samples through measurement of hysteresis loops, isothermal remanent magnetization acquisition, thermal demagnetization and frequency and temperature dependence of susceptibility. SP magnetite grains are unable to retain a remanence at room temperature, numerical simulations and experimental measurements indicate that SP magnetite in sediments is abundant evidenced by their contribution to hysteresis loops (Tauxe et al 1996) Their presence is indicative of magnetite reduction diagenesis at the Fe-redox boundary in pelagic sediments (Roberts et al 2013; Tarduno 1995) and can reflect climate-associated pedogenesis in loess and soils (Maher 2016)
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