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Magnetotactic bacteria benefit from their ability to form cellular magnetic dipoles by assembling stable single-domain ferromagnetic particles in chains as a means to navigate along Earth's magnetic field lines on their way to favorable habitats. We studied the assembly of nanosized membrane-encapsulated magnetite particles (magnetosomes) by ferromagnetic resonance spectroscopy using Magnetospirillum gryphiswaldense cultured in a time-resolved experimental setting. The spectroscopic data show that 1), magnetic particle growth is not synchronized; 2), the increase in particle numbers is insufficient to build up cellular magnetic dipoles; and 3), dipoles of assembled magnetosome blocks occur when the first magnetite particles reach a stable single-domain state. These stable single-domain particles can act as magnetic docks to stabilize the remaining and/or newly nucleated superparamagnetic particles in their adjacencies. We postulate that docking is a key mechanism for building the functional cellular magnetic dipole, which in turn is required for magnetotaxis in bacteria.

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The glass-forming ability of Fe-based metallic glasses has a direct relationship with their metalloid content. A good glass-former usually needs a metalloid content of approximately 20at.%. However, a high metalloid content causes deterioration not only in magnetic properties but also in elasticity and plasticity. Based on destabilization of the solid state we have developed a series of metalloid-free Fe-based metallic glasses of composition (Fe0.582Co0.418)100−x−yCrxZry (10⩽x⩽28 and 8⩽y⩽11). Via this destabilization the liquid state is stabilized, which results in a decreasing liquidus temperature. The mechanical and magnetic properties of the metalloid-free Fe-based metallic glass with the highest Fe and Co fractions were analyzed. The alloy of composition (Fe0.582Co0.418)80Cr10Zr10 exhibits bending elasticity and plasticity. Magnetization measurements reveal a saturation magnetization of up to 1.1T and an inverted hysteresis. The origin of this inverted hysteresis presumably lies in the inclination to decompose in a ferromagnetic iron-rich α1 phase and an antiferromagnetic chromium-rich α2 phase.

Magnetization measurements of soft-magnetic materials can be affected if they are characterized using a superconducting solenoid. Recording hysteresis loops while stepping the magnetic field can cause an apparently inverted hysteresis, if due to the pinned remanent flux in the magnet the field at the sample location exceeds the sample’s coercive field. Hysteresis loops recorded while sweeping the field can also be affected by ramping rate. The sweeping-rate dependence is caused by leakage currents resulting from the persistent switch and synchronization issues regarding collection of magnetic moment and field data. The resulting errors can be estimated by measuring paramagnetic dysprosium oxide.

Articles you may be interested inFeCoSiBNbCu bulk metallic glass with large compressive deformability studied by time-resolved synchrotron Xray diffraction Amorphous-FeCoCrZrB ferromagnets for use as high-temperature magnetic refrigerants J. Appl. Phys. 99, 08K909 (2006); 10.1063/1.2172234Effects of transition metal substitution on the glass-formation ability and magnetic properties of Fe 62 Co 9.5 Nd 3 Dy 0.5 B 25 glassy alloyThe glass-forming ability of Fe-based bulk metallic glasses is strongly correlated with the amount of metalloids they contain. Starting from a ferromagnetic and ductile Fe-based metallic glass of composition ͑Fe 0.582 Co 0.418 ͒ 80 Cr 10 Zr 10 , we were able to produce several bulk metallic glasses by alloying titanium and boron. The resulting alloys of composition ͓͑Fe 0.582 Co 0.418 ͒ 0.81 Cr 0.10 Zr 0.07 Ti 0.02 ͔ 100−x B x ͑x = 10-20 at. % ͒ exhibit a critical casting thickness of 0.5 mm, a wide undercooled liquid region ⌬T x ͑=T x − T g ͒ of 16-84 K, and ferromagnetic properties. dc magnetization measurements show an inverted hysteresis at room temperature, and small-angle neutron scattering on the ͓͑Fe 0.582 Co 0.418 ͒ 0.81 Cr 0.10 Zr 0.07 Ti 0.02 ͔ 90 B 10 bulk metallic glass reveals a power-law dependence of the differential scattering cross-section. The latter indicates a pronounced short-range order with a surface fractal dimension of 2.5. A splat-cooled sample of the same composition does not reveal this pronounced short-range order, but still an inverted hysteresis. From the scaling behavior of the magnetization curves, measured at different temperatures between 50 and 300 K for the splat-cooled sample, we find that an antagonistic internal magnetic field is present in this material. The resulting inverted hysteresis is presumably caused by interacting superparamagnetic and blocked regions.

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