Elisa Baggio Saitovitch

The microcrystalline FeSn2 alloy prepared by electric arc melting was heat-treated at 450 °C for 2 days and then rapidly quenched in water kept at room temperature. Structural analysis of the alloy, carried out using the Rietveld method for the refinement of X-ray diffractograms, showed the presence of two ordered tetragonal crystalline phases. The main phase (~97%) corresponds to the intermetallic compound FeSn2, with a C-16 type structure, space group I4/mcm, while the minority phase (~3%) corresponds to pure, unalloyed β-Sn, with a body-centered structure, space group I41/amd. The average crystallite size of the FeSn2 and β-Sn phases, calculated using the Williamson-Hall method, were 173(5) nm and 220(5) nm, respectively. In order to correlate the characteristics of local magnetism and hyperfine interactions around the atomic sites of Fe and Sn, the alloy was systematically studied using 57Fe and 119Sn Mössbauer spectroscopy. The 57Fe Mössbauer results, recorded at room temperature, show the characteristics of localized magnetism described by the magnetic hyperfine field at the 57Fe sites of the FeSn2 intermetallic (Bhf=11.07 T ), produced by the magnetic moments of the 3d electrons of the Fe atoms, whose nearest neighbors are antiferromagnetically coupled. On the other hand, the 119Sn Mössbauer results, obtained at room temperature, show a magnetic hyperfine field (Bhf trans=3.03 T) associated with magnetism induced by transferred fields. This phenomenon occurs due to the polarization of the Sn 5p orbitals, produced by the covalent hybridization of the Sn 5p orbitals with the 3d electrons of its four nearest Fe neighbors. The 119Sn Mössbauer spectrum of the sample heat-treated at 450 °C also shows the presence of pure β-Sn in a minority proportion similar to that observed by X-ray diffraction.

The miniaturisation of electronic building blocks is of uttermost importance. Efficient nanoscopic inductors and capacitors could pave the way to better energy economics and therewith more powerful processors. Topological materials which exhibit emergent electromagnetism are promising candidates for such future technologies. Due to its helical crystal structure and strong spin-orbit interaction, tellurium exhibits an experimentally verified internal inductance. By formulating the constitutive equations which describe electromagnetism in tellurium, we show how capacitive and inductive properties naturally emerge in the material. We demonstrate on the basis of AC-impedance measurements that elemental tellurium exhibits an unusually high virtual capacitance and inductance, far exceeding previously studied materials. The highest capacitance we detect in our samples is 130 mF and in one of our samples we measure an extremely impressive virtual inductance of 1.5 H. For low current densities a single piece of tellurium mimics the reactance of an RLC-circuit.

The effect of the YBCO superconducting (SC) state on the magnetic properties of as-grown YBCO/Co/IrMn heterostructures has been systematically studied using magnetometry and ferromagnetic resonance. The obtained data showed that the superconductivity of the YBCO substrate strongly affects the ferromagnetic properties of the deposited Co layer deeper (up to 50 nm) than the coherence length of the YBCO (≃4nm) by an exchange interaction between the Co magnetic moments and the superconducting pairs at the YBCO/Co interface. The interfacial exchange interaction, switched on while the YBCO enters the SC state, pins Co spins and yields an enhancement of the Co magnetic properties.

We report on the experimentally observed variation of the superconducting critical temperature (T _c ) of Ni/Bi systems as a function of the total deposited film thickness and on its explanation using a theoretical model. Two series of Ni/Bi systems have been analyzed which were obtained by depositions of Ni onto Bi in the proportions Ni3Bix (3 nm of Ni onto x nm of Bi) and NiyBi6y (y nm Ni onto 6 y nm of Bi). As shown recently, the formation of the superconducting compound NiBi₃ at Ni/Bi interfaces in the resulting NiBi₃-Bi films is thermodynamically favored by a volume contraction. Here we corroborate this result and estimate the thickness of the resulting NiBi₃ and of the remaining Bi layers for the Ni3Bix and NiyBi6y series using the laws of mass and conservation of number of atoms. We consider the resulting film as being made up of two homogeneous and uniform layers of NiBi₃ and Bi, respectively, and study this idealizing model using the Bogoliubov de Gennes (BdG) equations. It is assumed that superconductivity originates in the NiBi₃ layer and penetrates the Bi layers via a potential barrier. Our theoretical calculations predict the dependence of T _c with respect to the thicknesses of the NiBi₃ and Bi layers, and also with the strength of the potential barrier that blocks the migration of electrons from the NiBi₃ to the Bi layer. The calculations show that the superconducting gap also exists in Bi, although much weaker than in the NiBi₃ layer. We compare the predicted T _c values with the experimental data and find sufficient agreement to suggest that our model can explain the experimentally observed variation of T _c with thickness. We interpret this dependence as shape resonance oscillations which are derived from the BdG theory applied to thin superconducting films.

We report an experimental study of the bimagnetic nanocomposites CoFe₂/CoFe₂O₄. The precursor material, CoFe₂O₄ was prepared using the conventional stoichiometric combustion method. The nano-structured material CoFe₂/CoFe₂O₄ was obtained by total oxygen reduction of CoFe₂O₄ using a thermal treatment at 350 °C in H₂ atmospheres following the partial oxidation in O₂ atmospheres at 380 °C during 120; 30; 15, 10, and 5 min. The X-ray diffraction, Mössbauer spectroscopy and transmission electronic microscopy images confirmed the formation of the material CoFe₂/CoFe₂O₄. The magnetic hysteresis for the nanocomposite with different saturation magnetization (from 87 to 108 emu/g) also confirms the formation of the CoFe₂/CoFe₂O₄ with different content of CoFe₂O₄. Furthermore, the magnetic hysteresis curves for all samples presented a single magnetic behavior, suggesting the magnetic coupling between the phases of the nanocomposite. The effects of high energy milling on the magnetic properties of the precursor material and nanocomposites samples were evaluated.