The mechanically induced structural disorder in barium hexaferrite, BaFe12O19, and its impact on magnetism

The response of the structure of the M-type barium hexaferrite (BaFe12O19) to mechanical action through high-energy milling and its impact on the magnetic behaviour of the ferrite are investigated. Due to the ability of the Fe-57 Mossbauer spectroscopic technique to probe the environment of the Fe nuclei, a valuable insight on a local atomic scale into the mechanically induced changes in the hexagonal structure of the material is obtained. It is revealed that the milling of BaFe12O19 results in the deformation of its constituent polyhedra (FeO6 octahedra, FeO4 tetrahedra and FeO5 triangular bi-pyramids) as well as in the mechanically triggered transition of the Fe3+ cations from the regular 12k octahedral sites into the interstitial positions provided by the magnetoplumbite structure. The response of the hexaferrite to the mechanical treatment is found to be accompanied by the formation of a non-uniform nanostructure consisting of an ordered crystallite surrounded/separated by a structurally disordered surface shell/interface region. The distorted polyhedra and the non-equilibrium cation distribution are found to be confined to the amorphous near-surface layers of the ferrite nanoparticles with the thickness extending up to about 2 nm. The information on the mechanically induced short-range structural disorder in BaFe12O19 is complemented by an investigation of its magnetic behaviour on a macroscopic scale. It is demonstrated that the milled ferrite nanoparticles exhibit a pure superparamagnetism at room temperature. As a consequence of the far-from-equilibrium structural disorder in the surface shell of the nanoparticles, the mechanically treated BaFe12O19 exhibits a reduced magnetization and an enhanced coercivity.DFG/SPP/1415APVV/0528-11VEGA/2/0097/1

Mechanosynthesis of nanocrystalline fayalite, Fe 2SiO 4

Nanostructured fayalite (α-Fe 2SiO 4) with a large volume fraction of interfaces is synthesized for the first time via single-step mechanosynthesis, starting from a 2α-Fe 2O 3 + 2Fe + 3SiO 2 mixture. The nonequilibrium state of the as-prepared silicate is characterized by the presence of deformed polyhedra in the interface/surface regions of nanoparticles. © 2012 The Royal Society of Chemistry

Suppression of the Cycloidal Spin Arrangement in BiFeO₃ Caused by the Mechanically Induced Structural Distortion and Its Effect on Magnetism

Bismuth ferrite (BiFeO₃) particles are prepared by a combined mechanochemical−thermal processing of a Bi₂O₃ + α-Fe₂O₃ mixture. Structural, magnetic, hyperfine, morphological and chemical properties of the as-prepared BiFeO₃ are studied using X-ray diffraction (Rietveld refinement), ⁵⁷Fe Mössbauer spectroscopy, SQUID magnetometry, electron microscopy and energy dispersive X-ray spectroscopy. It is revealed that the structure of the ferrite exhibits the long-range distortion (significantly tilted FeO₆ octahedra) and the short-range disorder (deformed FeO₆ octahedra). Consequently, these structural features result in the suppression of a space modulated cycloidal spin arrangement in the material. The latter manifests itself by the appearance of only single spectral component in the ⁵⁷Fe Mössbauer spectrum of BiFeO₃. The macroscopic magnetic behavior of the material is interpreted as a superposition of ferromagnetic and antiferromagnetic contributions with a large coercive field and remanent magnetization. Taking into account the average particle size of the as-prepared BiFeO₃ particles (∼98 nm), exceeding the typical period length of cycloid (∼62 nm), both the suppression of the spiral spin structure in the material and its partly ferromagnetic behavior are attributed to the crystal lattice distortion caused by mechanical stress during the preparation procedure

Orientia tsutsugamushi is highly susceptible to the RNA polymerase switch region inhibitor corallopyronin a In Vitro and In Vivo

Scrub typhus is a potentially lethal infection caused by the obligate intracellular bacterium; Orientia tsutsugamushi; Reports on the emergence of doxycycline-resistant strains highlight the urgent need to develop novel antiinfectives against scrub typhus. Corallopyronin A (CorA) is a novel α-pyrone compound synthesized by the myxobacterium; Corallococcus coralloides; that was characterized as a noncompetitive inhibitor of the switch region of the bacterial RNA polymerase (RNAP). We investigated the antimicrobial action of CorA against the human-pathogenic Karp strain of; O. tsutsugamushi; in vitro; and; in vivo; The MIC of CorA against; O. tsutsugamushi; was remarkably low (0.0078 μg/ml), 16-fold lower than that against; Rickettsia typhi; In the lethal intraperitoneal; O. tsutsugamushi; mouse infection model, a minimum daily dose of 100 μg CorA protected 100% of infected mice. Two days of treatment were sufficient to confer protection. In contrast to BALB/c mice, SCID mice succumbed to the infection despite treatment with CorA or tetracycline, suggesting that antimicrobial treatment required synergistic action of the adaptive immune response. Similar to tetracycline, CorA did not prevent latent infection of; O. tsutsugamushi; in vivo; However, latency was not caused by acquisition of antimicrobial resistance, since; O. tsutsugamushi; reisolated from latently infected BALB/c mice remained fully susceptible to CorA. No mutations were found in the CorA-binding regions of the β and β' RNAP subunit genes; rpoB; and; rpoC; Inhibition of the RNAP switch region of; O. tsutsugamushi; by CorA is therefore a novel and highly potent target for antimicrobial therapy for scrub typhus