Transient absorption and photoluminescence are experimentally investigated in the polaronic reference system lithium niobate, LiNbO 3 (LN), with the aim to refine the microscopic model of small polaron dynamics in materials with strong electron-phonon coupling. As a unique feature, our study is performed by using two different spectroscopic methods, in crystals with dopants enhancing photorefraction or damage resistance, and over a broad temperature range from 15-400 K. Although being self-consistent for particular experimental conditions, the hitherto used microscopic polaronic models reveal inconsistencies when applied to this larger data set. We show that comprehensive modeling is unlocked by the inclusion of an additional type of polaronic state with the following characteristics: (i) strongly temperature-and dopantdependent relaxation times, (ii) an absorption feature in the blue-green spectral range, and (iii) a Kohlrausch-Williams-Watts decay shape with a temperature-dependent stretching factor β (T) showing a behavior contrary to that of small, strong-coupling polarons. The hypothesis of self-trapped excitons (STEs, i.e. bound electron-hole pairs strongly coupled to Nb 5+ and O 2− within a niobium-oxygen octahedron) and their pinning on defects as the microscopic origin of these characteristics is supported by a spectroscopic linkage of photoluminescence at low (15 K) and elevated (300 K) temperatures and explains the long-lifetime components in transient absorption as due to pinned STEs.
Magnetite (Fe3O4) is an eligible candidate for magnetic tunnel junctions (MTJs) since it shows a high spin polarization at the Fermi level as well as a high Curie temperature of 585°C. In this study, Fe3O4/MgO/Co-Fe-B MTJs were manufactured. A sign change in the TMR is observed after annealing the MTJs at temperatures between 200°C and 280°C. Our findings suggest an Mg interdiffusion from the MgO barrier into the Fe3O4 as the reason for the change of the TMR. Additionally, different treatments of the magnetite interface (argon bombardment, annealing at 200°C in oxygen atmosphere) during the preparation of the MTJs have been studied regarding their effect on the performance of the MTJs. A maximum TMR of up to -12% could be observed using both argon bombardment and annealing in oxygen atmosphere, despite exposing the magnetite surface to atmospheric conditions before the deposition of the MgO barrier.
Different aspects of ferroelectric LiTaO3 (LT) such as polaronic defects, optical response and electrical conductivity are investigated by the most recent theoretical and experimental approaches. Comparing the results with the state-of-the-art knowledge of the widely studied LiNbO3 (LN), we evaluate the general assumption that there is little difference between the aforementioned properties of LT and LN. First-principles calculations reveal the existence of point defects in LT qualitatively compatible with the polaronic picture established in LN. Though, peculiar differences with respect to the individual binding energies and polaronic deformation can be revealed. Accordingly, (sub-)picosecond transient absorption measurements show pronounced differences in the kinetics in the sub-ps time domain of small polaron formation and, even more pronounced, in the long-term evolution identified with small polaron hopping. In contrast, (sub-)ps transient luminescence, attributed to the relaxation of self-trapped excitons in LN, shows very similar kinetics. Electrical conductivity measurements are performed in air as function of temperature. Up to about 600 °C they demonstrate similar temperature dependence for the two materials, from which rather comparable activation energies can be extracted. However, in the high-temperature range from about 600 °C to 920 °C both materials show noticeable differences. The results suggest that the fundamental microscopic understanding of LN can be in part transferred to LT. However, due to differences in structure, energetic landscape and temperature behavior, discrepancies between the two materials bear a striking potential for novel applications, even at high temperatures.
Iron oxide films were reactively grown on iron buffer films, which were deposited before on MgO(001) substrates to analyze the influence of the initial iron buffer layers on the magnetic properties of the magnetite films. X-ray photoelectron spectroscopy and low energy electron diffraction showed that magnetite films of high crystalline quality in the surface near region were formed by this two-step deposition procedure. The underlying iron film, however, was completely oxidized as proved by x-ray reflectometry and diffraction. The structural bulk quality of the iron oxide film is poor compared to magnetite films directly grown on MgO(001). Although the iron film was completely oxidized, we found drastically modified magnetic properties for these films using the magnetooptic Kerr effect. The magnetite films had strongly increased coercive fields, and their magnetic in-plane anisotropy is in-plane rotated by 458 compared to magnetite films formed directly by one step reactive growth on MgO(001). V C 2015 AIP Publishing LLC.
Femtosecond-pulse-induced (E pump = 2.5 eV) picosecond infrared absorption is studied in the spectral region between 0.30 eV and 1.05 eV in LiNbO 3 :Mg. We find a noninstantaneous mid-infrared absorption peak in the time domain up to 1 ps and a broad-band, long-lived absorption (maximum at 0.85 eV, width ≈ 0.5 eV), for t > 1 ps. The modelling succeeds by considering small Nb 4+ Nb electron polaron formation along the sequence: (i) twophoton injection of hot electron-hole pairs at Nb-O-octahedra, (ii) dissociation and electron cooling by electron-phonon-scattering, and (iii) electron self-localization by strong electronphonon-coupling.
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