We have employed soft and hard x-ray resonant magnetic scattering and polarized neutron diffraction to study the magnetic interface and the bulk antiferromagnetic domain state of the archetypal epitaxial ${\text{Ni}}_{81}{\text{Fe}}_{19}(111)/\text{CoO}(111)$ exchange biased bilayer. The combination of these scattering methods provides unprecedented detailed insights into the still incomplete understanding of some key manifestations of the exchange bias effect. We show that the several orders of magnitude difference between the expected and measured value of exchange bias field is caused by an anisotropic in-plane orientation of antiferromagnetic domains. Irreversible changes in their configuration lead to a training effect. This is directly seen as a change in the magnetic half-order Bragg peaks after magnetization reversal. The antiferromagnetic domain size is extracted from the width of the $(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ antiferromagnetic peak by both neutron and x-ray scattering and is determined to be 30 nm in size. A reduced blocking temperature as compared to the measured antiferromagnetic ordering temperature clearly corresponds to the blocking of antiferromagnetic domains. Moreover, an excellent correlation between the size of the antiferromagnetic domains, exchange bias field, and frozen-in spin ratio is found, providing a comprehensive understanding of the origin of exchange bias in epitaxial systems.
The synchrotron radiation based spectroscopies X-ray fluorescence and X-ray absorption fine structure are used to detect illness-related changes in the elemental distribution and bonding environment of metals in human nails. The effective atomic number of a collection of nails is determined using two methods, the X-ray transmittance and the scattering method, and is found equal to 7.5 +/- 0.5. X-ray fluorescence maps of the elemental distributions, recorded with a lateral resolution of 5 microm, reveal that S, Ca and Zn are distributed homogeneously while Fe tends to cluster. In the Fe-rich clusters, which have a diameter in the range 15-30 microm, the Fe concentration is 10 times higher than in the matrix. The Zn K edge X-ray Absorption Fine Structure spectra reveal that Zn, in the nails from healthy donors and patients suffering from lung diseases, is four-fold coordinated with N and S and the Zn-N and Zn-S distances are equal to 2.03 A and 2.25 A, respectively. Finally the signature of various bonds in the C-, O- and N- K near edge X-ray absorption fine structure spectra is discussed.
The intrinsic structural dynamic during the adsorption of CO2 at 195 K and N2 at 77 K on flexible porous coordination polymer Zn2(BPnDC)2(bpy) (SNU-9) was studied in situ by powder XRD. The crystal structures of as made and solvent free (activated) phases were determined by single crystal X-ray diffraction. During the structural transformation caused by activation, the rearrangement of Zn–O bonds occurs that leads to changes in coordination environment of Zn atoms. Such changes lead to the contraction of the unit cell and to decreasing unit cell volume of nearly 28% in comparison to the pristine as made structure. The solvent accessible volume of the unit cell decreases from 40.8% to 12.8%. The adsorption of CO2 and N2 on SNU-9 proceeds in a different way: the formation of intermediate phase during the CO2 adsorption could be postulated, while the transformation from narrow pore form to the open structure occurs in quasi-one-step in the case of N2 adsorption (the intermediate phase is formed only in very narrow pressure region). The transformation of the structure is guest dependent and the differences in the structures of CO2@SNU-9 at 195 K and N2@SNU-9 at 77 K were proven by Pawley and Rietveld refinements of powder XRD patterns. The structure of N2@SNU-9 is identical to this of as synthesized phase, while the structure of CO2@SNU-9 differs slightly.
Abstract Biofilms are multicellular microbial communities that encase themselves in an extracellular matrix (ECM) of secreted biopolymers and attach to surfaces and interfaces. Bacterial biofilms are detrimental in hospital and industrial settings, but they can be beneficial in agricultural contexts. An essential property of biofilms that grants them with increased survival relative to planktonic cells is phenotypic heterogeneity; the division of the biofilm population into functionally distinct subgroups of cells. Phenotypic heterogeneity in biofilms can be traced to the cellular level, however, the molecular structures and elemental distribution across whole biofilms as well as possible linkages between them remain unexplored. Mapping X-ray diffraction (XRD) across intact biofilms in time and space, we revealed the dominant structural features in Bacillus subtilis biofilms, stemming from matrix components, spores and water. By simultaneously following the X-ray fluorescence (XRF) signal of biofilms and isolated matrix components, we discovered that the ECM preferentially binds calcium ions over other metal ions, specifically, zinc, manganese and iron. These ions, remaining free to flow below macroscopic wrinkles that act as water channels, eventually accumulate and lead to sporulation. The possible link between ECM properties, regulation of metal ion distribution and sporulation across whole intact biofilms unravels the importance of molecular-level heterogeneity in shaping biofilm physiology and development. Significance Statement Biofilms are multicellular soft microbial communities that are able to colonize synthetic surfaces as well as living organisms. To survive sudden environmental changes and efficiently share their common resources, cells in a biofilm divide into subgroups with distinct functions, leading to phenotypic heterogeneity. Here, by studying intact biofilms by synchrotron X-ray diffraction and fluorescence, we revealed correlations between biofilm macroscopic architectural heterogeneity and the spatio-temporal distribution of extracellular matrix, spores, water and metal ions. Our findings demonstrate that biofilm heterogeneity is not only affected by local genetic expression and cellular differentiation, but also by passive effects resulting from the physicochemical properties of the molecules secreted by the cells, leading to differential distribution of nutrients that propagates through macroscopic length scales.
