Features of structure, magnetic state and electrodynamic performance of SrFe12-xInxO19

Indium-substituted strontium hexaferrites were prepared by the conventional solid-phase reaction method. Neutron diffraction patterns were obtained at room temperature and analyzed using the Rietveld methods. A linear dependence of the unit cell parameters is found. In³⁺ cations are located mainly in octahedral positions of 4f_VI and 12 k. The average crystallite size varies within 0.84–0.65 μm. With increasing substitution, the T_C Curie temperature decreases monotonically down to ~ 520 K. ZFC and FC measurements showed a frustrated state. Upon substitution, the average and maximum sizes of ferrimagnetic clusters change in the opposite direction. The M_r remanent magnetization decreases down to ~ 20.2 emu/g at room temperature. The M_s spontaneous magnetization and the k_eff effective magnetocrystalline anisotropy constant are determined. With increasing substitution, the maximum of the ε^/ real part of permittivity decreases in magnitude from ~ 3.3 to ~ 1.9 and shifts towards low frequencies from ~ 45.5 GHz to ~ 37.4 GHz. The maximum of the tg(α) dielectric loss tangent decreases from ~ 1.0 to ~ 0.7 and shifts towards low frequencies from ~ 40.6 GHz to ~ 37.3 GHz. The low-frequency maximum of the μ^/ real part of permeability decreases from ~ 1.8 to ~ 0.9 and slightly shifts towards high frequencies up to ~ 34.7 GHz. The maximum of the tg(δ) magnetic loss tangent decreases from ~ 0.7 to ~ 0.5 and shifts slightly towards low frequencies from ~ 40.5 GHz to ~ 37.7 GHz. The discussion of microwave properties is based on the saturation magnetization, natural ferromagnetic resonance and dielectric polarization types.

Method of surface energy investigation by lateral AFM: application to control growth mechanism of nanostructured NiFe films

A new method for the specific surface energy investigation based on a combination of the force spectroscopy and the method of nanofriction study using atomic force microscopy was proposed. It was shown that air humidity does not affect the results of investigation by the proposed method as opposed to the previously used methods. Therefore, the method has high accuracy and repeatability in air without use of climate chambers and liquid cells. The proposed method has a high local resolution and is suitable for investigation of the specific surface energy of individual nanograins or fixed nanoparticles. The achievements described in the paper demonstrate one of the method capabilities, which is to control the growth mechanism of thin magnetic films. The conditions for the transition of the growth mechanism of thin Ni₈₀Fe₂₀ films from island to layer-by-layer obtained via electrolyte deposition have been determined using the proposed method and the purpose made probes with Ni coating.

Rapid mutagenesis and purification of phage RNA polymerases

We have developed plasmid-based expression systems that encode modified forms of T7 RNA polymerase (RNAP) having 6-12 histidine residues fused to the amino terminus. The histidine-tagged RNAPs (His-T7 RNAPS) are indistinguishable from the wild-type (WT) enzyme in nearly all biochemical assays. Similar plasmids that encode His-tagged T3 and SP6 RNAPs have also been constructed. To facilitate site-directed mutagenesis of the RNAP gene, the size of the target plasmid was minimized by using T7 RNAP itself as a selectable marker. BL21 (DCAT4) cells (which carry a chromosomal copy of the chloramphenicol acetyltransferase cat gene under control of a T7 promoter) are resistant to chloramphenicol when functional T7 RNAP is expressed, thus allowing the selection and maintenance of the target plasmid in these cells. Mutagenesis is accomplished by denaturing the plasmid, annealing mutagenic DNA primers, and repairing the plasmid with T4 DNA polymerase. Two DNA primers are used: one corrects a defect in the bla gene, the other introduces the desired mutation into the RNAP gene; 30-85% of the ampicillin-resistant transformants carry the desired mutation in the RNAP gene. By using BL21 (DCAT4) cells as a recipient for transformation the functional integrity of the RNAP gene may conveniently be monitored by assessing the level of chloramphenicol resistance in vivo. Methods for rapid, simultaneous purification of multiple samples of modified (His-tagged) and conventional RNAPs are described. Together, these developments greatly enhance our ability to characterize this important class of enzymes.