Mikio Takano

Abiotic Deposition of Fe Complexes onto Leptothrix Sheaths.

Bacteria classified in species of the genus Leptothrix produce extracellular, microtubular, Fe-encrusted sheaths. The encrustation has been previously linked to bacterial Fe oxidases, which oxidize Fe(II) to Fe(III) and/or active groups of bacterial exopolymers within sheaths to attract and bind aqueous-phase inorganics. When L. cholodnii SP-6 cells were cultured in media amended with high Fe(II) concentrations, Fe(III) precipitates visibly formed immediately after addition of Fe(II) to the medium, suggesting prompt abiotic oxidation of Fe(II) to Fe(III). Intriguingly, these precipitates were deposited onto the sheath surface of bacterial cells as the population was actively growing. When Fe(III) was added to the medium, similar precipitates formed in the medium first and were abiotically deposited onto the sheath surfaces. The precipitates in the Fe(II) medium were composed of assemblies of globular, amorphous particles (ca. 50 nm diameter), while those in the Fe(III) medium were composed of large, aggregated particles (≥3 µm diameter) with a similar amorphous structure. These precipitates also adhered to cell-free sheaths. We thus concluded that direct abiotic deposition of Fe complexes onto the sheath surface occurs independently of cellular activity in liquid media containing Fe salts, although it remains unclear how this deposition is associated with the previously proposed mechanisms (oxidation enzyme- and/or active group of organic components-involved) of Fe encrustation of the Leptothrix sheaths.

Ferro-Antiferromagnetic Delta-Chain System Studied by High Field Magnetization Measurements

High field magnetization processes were measured for the single crystal of an S = 1/2 ferro-antiferromagnetic delta-chain system [Cu(bpy)(H 2 O)][Cu(bpy)(mal)–(H 2 O)](ClO 4 ) 2 . At low temperatures, the observed magnetization curve shows a rapid increase to half of the saturation moment below about 1 T and then followed by a gradual increase to fully polarized state below about 7 T. Concerning this characteristic magnetization process, we discuss the ground state of the system qualitatively based on a non-interacting two sublattice model, where a delta-chain is divided into apical free spins and remaining basal spins regarded as quasi one dimensional antiferromagnetic chain. Essential effect of ferromagnetic interaction, which connects an apical spin and basal spins, is pointed out from the comparison with theoretical curve for the non-interacting two sublattice model as well as with the numerically calculated curve. Presence of the Dzyaloshinskii–Moriya interaction is also suggested from the arguments ...

Contrasting Magnetic Behaviors in Rhodium- and Ruthenium-Doped Cubic Perovskite SrFeO3 : Nearly Ferromagnetic Metal versus Spin-Glass Insulator.

The effects of Rh and Ru doping for SrFeO3 , a helimagnetic metal with a cubic perovskite structure, are studied by magnetic and resistivity measurements. Although SrRhO3 is a paramagnetic metal and SrRuO3 is a ferromagnetic one, the Rh doping induces a nearly ferromagnetic metallic state, whereas the Ru doping induces a spin-glass insulating state. Mössbauer measurements evidence a marked difference between SrFe0.8 Rh0.2 O3 and SrFe0.8 Ru0.2 O3 in the formal valences of Fe, which are estimated to be 4+ and 3.75+, respectively. The contrasting magnetic behaviors of Rh- and Ru-doped SrFeO3 are discussed in terms of the subtle balance between the double-exchange ferromagnetism and the superexchange antiferromagnetism.