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Long-range ferromagnetic order induced by a donor impurity band exchange in SnO2:Er3+ nanoparticles J. Appl. Phys. 114, 203902 (2013); 10.1063/1.4833549Role of oxygen defects on the magnetic properties of ultra-small Sn1xFexO2 nanoparticles J. Appl. Phys. 113, 17B504 (2013) One-dimensional (1D) chain-like nanocomposites, created by ensembles of nanoparticles of with diameter 13 nm, which are composed of an iron core (4 nm) and a silica protective layer, were prepared by a self-assembly process. Chain-like Fe@SiO 2 ensembles were formed due to strong magnetic dipole-dipole interactions between individual Fe nanoparticles and the subsequent fixation of the Fe particles by the SiO 2 layers. X-ray near edge absorption spectra measurements at the Fe K absorption edge confirm that the presence of a silica layer prevents the oxidation of the magnetic Fe core. Strong magnetic interactions between Fe cores lead to long-range ordering of magnetic moments, and the nanoparticle ensembles exhibit superferromagnetic characteristics demonstrated by a broad blocking Zero-field cooling (ZFC)/field-cooling distribution, nearly constant temperature dependence of ZFC magnetization, and non-zero coercivity at room temperature. Low room-temperature coercivity and the presence of electrically insulating SiO 2 shells surrounding the Fe core make the studied samples suitable candidates for microelectronic applications. V C 2014 AIP Publishing LLC.[http://dx

Despite the extensive use of boron-modified phenol−formaldehyde polymers as insulating materials in soft magnetic composites (SMCs), the structure and arrangement of the inorganic cross-linking units in these systems have not been fully elucidated. To clarify the structure, configuration, and distribution of the boron cross-links in these materials, phenol−formaldehyde resins modified by boric acid were synthesized and characterized using advanced multiplequantum 11 B− 11 B MAS NMR correlation techniques combined with the quantum chemical geometry optimizations and the subsequent 11 B NMR chemical shielding calculations. The analyses of the resulting spectra revealed a well-evolved (highdensity) phenol−formaldehyde polymer network additionally strengthened by nitrogen and boron cross-links. The boron-based cross-links were attributed to monoester (ca. 10%) and diester (ca. 90%) complexes (six-membered spirocyclic borate anions) with strictly tetrahedral coordination (B IV ). During the thermal treatment, the monoester and diester borate complexes underwent additional transformation in which the spirocyclic borate anions were more tightly incorporated into the polymer matrix via additional N-type (amino) cross-links. A 11 B− 11 B double-quantum correlation MAS NMR experiment revealed that the majority of the monoester and diester borate complexes (ca. 80%) were uniformly distributed within and effectively isolated by the polymer matrix, with an average 11 B··· 11 B interatomic distance greater than 6 Å. A non-negligible part of the spirocyclic borate anion complexes (ca. 20%), however, existed in pairs or small clusters in which the average 11 B··· 11 B interatomic distance was less than 5.5 Å. In addition, the formation of homodimers (diester−diester) was demonstrated to be preferred over the formation of heteroclusters (monoester−diester).

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Soft magnetic composites based on Fe powder and phenol-formaldehyde resin (PFR) modified with tetraethylorthosilicate are investigated in detail. The chemical synthesis of PFR, its modification with nanometersized SiO 2 particles created by sol-gel method and subsequent coating, enables a preparation of insulating PFR-SiO 2 (PFRT) layer on the surface of Fe particles. Thermal degradation and FTIR analysis of PFR and PFRT with different amount of SiO 2 was examined. Mechanical hardness and flexural strength of FePFRT composites was studied depending on the amount of nanosized-SiO 2 in the coating. SEM serves in evidence of a defectless microstructure if the coating contains at least 2% of silica particles. The morphology of Fe particles implies uniform coating without any visible exfoliation. A presence of fine SiO 2 particles was verified by TEM. The best magnetic properties were found in Fe-PFRT composite with 2% of SiO 2 in the insulating layer on behalf of its uniform arrangement and homogeneity.