Toward Versatile Sr2FeMoO6-Based Spintronics by Exploiting Nanoscale Defects.

To actualize the high spintronic application potential of complex magnetic oxides, it is essential to fabricate these materials as thin films with the best possible magnetic and electrical properties. Sr2FeMoO6 is an outstanding candidate for such applications, but presently no thin film synthesis route, which would preserve the magnetic properties of bulk Sr2FeMoO6, is currently known. In order to address this problem, we present a comprehensive experimental and theoretical study where we link the magnetic and half metallic properties of Sr2FeMoO6 thin films to lattice strain, Fe-Mo antisite disorder and oxygen vacancies. We find the intrinsic effect of strain on the magnetic properties to be very small, but also that an increased strain will significantly stabilize the Sr2FeMoO6 lattice against the formation of antisite disorder and oxygen vacancies. These defects, on the other hand, are recognized to drastically influence the magnetism of Sr2FeMoO6 in a nonlinear manner. On the basis of the findings, we propose strain manipulation and reductive annealing as optimization pathways for improving the spintronic functionality of Sr2FeMoO6.

Ca2+ solvation as a function of p, T, and pH from ab initio simulation.

First principles molecular dynamics simulations have been carried out at various temperatures and pressures starting with either Ca(2+) or CaO in a reactive volume of 63 H(2)O molecules. In the case of aqueous Ca(2+), the ion is surrounded by six H(2)O molecules in the first hydration shell at 300 K/0.3 GPa, with rare exchange between first and second hydrations shells. At 900 K/0.9 GPa, the coordination number in the first hydration shell fluctuates between six and eight, the average being 7.0. CaO immediately reacts with the surrounding H(2)O molecules to form Ca(2+) + 2OH(-). The hydroxyl ions form transient Ca(OH)(+) and Ca(OH)(2) complexes and have a mean residence time in the first coordination shell of Ca(2+) of 6 ± 4 ps at 500 K and 3 ± 3 ps at 900 K, respectively. At 500 K/0.5 GPa, the time-averaged relative concentrations of the transient Ca(2+), Ca(OH)(+), and Ca(OH)(2) species are 14%, 55%, and 29%, while at 900 K/0.9 GPa, they are 2%, 34%, and 63%.

Transport processes at alpha-quartz-water interfaces: insights from first-principles molecular dynamics simulations.

Car-Parrinello molecular dynamics (CP-MD) simulations are performed at high temperature and pressure to investigate chemical interactions and transport processes at the alpha-quartz-water interface. The model system initially consists of a periodically repeated quartz slab with O-terminated and Si-terminated (1000) surfaces sandwiching a film of liquid water. At a temperature of 1000 K and a pressure of 0.3 GPa, dissociation of H(2)O molecules into H(+) and OH(-) is observed at the Si-terminated surface. The OH(-) fragments immediately bind chemically to the Si-terminated surface while Grotthus-type proton diffusion through the water film leads to protonation of the O-terminated surface. Eventually, both surfaces are fully hydroxylated and no further chemical reactions are observed. Due to the confinement between the two hydroxylated quartz surfaces, water diffusion is reduced by about one third in comparison to bulk water. Diffusion properties of dissolved SiO(2) present as Si(OH)(4) in the water film are also studied. We do not observe strong interactions between the hydroxylated quartz surfaces and the Si(OH)(4) molecule as would have been indicated by a substantial lowering of the Si(OH)(4) diffusion coefficient along the surface. No spontaneous dissolution of quartz is observed. To study the mechanism of dissolution, constrained CP-MD simulations are done. The associated free energy profile is calculated by thermodynamic integration along the reaction coordinate. Dissolution is a stepwise process in which two Si--O bonds are successively broken. Each bond breaking between a silicon atom at the surface and an oxygen atom belonging to the quartz lattice is accompanied by the formation of a new Si--O bond between the silicon atom and a water molecule. The latter loses a proton in the process which eventually leads to protonation of the oxygen atom in the cleaved quartz Si--O bond. The final solute species is Si(OH)(4).