Browsing by Author "Schiemenz, Sandra"
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- ItemControl of Positive and Negative Magnetoresistance in Iron Oxide : Iron Nanocomposite Thin Films for Tunable Magnetoelectric Nanodevices(2020) Nichterwitz, Martin; Honnali, Shashank; Zehner, Jonas; Schneider, Sebastian; Pohl, Darius; Schiemenz, Sandra; Goennenwein, Sebastian T.B.; Nielsch, Kornelius; Leistner, KarinThe perspective of energy-efficient and tunable functional magnetic nanostructures has triggered research efforts in the fields of voltage control of magnetism and spintronics. We investigate the magnetotransport properties of nanocomposite iron oxide/iron thin films with a nominal iron thickness of 5-50 nm and find a positive magnetoresistance at small thicknesses. The highest magnetoresistance was found for 30 nm Fe with +1.1% at 3 T. This anomalous behavior is attributed to the presence of Fe3O4-Fe nanocomposite regions due to grain boundary oxidation. At the Fe3O4/Fe interfaces, spin-polarized electrons in the magnetite can be scattered and reoriented. A crossover to negative magnetoresistance (-0.11%) is achieved at a larger thickness (>40 nm) when interface scattering effects become negligible as more current flows through the iron layer. Electrolytic gating of this system induces voltage-triggered redox reactions in the Fe3O4 regions and thereby enables voltage-tuning of the magnetoresistance with the locally oxidized regions as the active tuning elements. In the low-magnetic-field region (<1 T), a crossover from positive to negative magnetoresistance is achieved by a voltage change of only 1.72 V. At 3 T, a relative change of magnetoresistance about -45% during reduction was achieved for the 30 nm Fe sample. The present low-voltage approach signifies a step forward to practical and tunable room-temperature magnetoresistance-based nanodevices, which can boost the development of nanoscale and energy-efficient magnetic field sensors with high sensitivity, magnetic memories, and magnetoelectric devices in general. Copyright © 2020 American Chemical Society.
- ItemMetallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer(Cambridge : RSC, 2021) Zalibera, Michal; Ziegs, Frank; Schiemenz, Sandra; Dubrovin, Vasilii; Lubitz, Wolfgang; Savitsky, Anton; Deng, Shihu H.M.; Wang, Xue-Bin; Advoshenko, Stanislav M.; Popov, Alexey A.We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of YxSc3−xN@C80 (x = 0–3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S1–T1 gap decrease in this row. For Sc3N@C80, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin–lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with 14N, 45Sc, and 89Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y3N to Sc3N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.
- ItemMethane as a selectivity booster in the arc-discharge synthesis of endohedral fullerenes: Selective synthesis of the single-molecule magnet Dy2TiC@C80and Its Congener Dy2TiC2@C80(Hoboken, NJ : Wiley, 2015) Junghans, Katrin; Schlesier, Christin; Kostanyan, Aram; Samoylova, Nataliya A.; Deng, Qingming; Rosenkranz, Marco; Schiemenz, Sandra; Westerström, Rasmus; Greber, Thomas; Büchner, Bernd; Greber, Thomas; Popov, Alexey A.The use of methane as a reactive gas dramatically increases the selectivity of the arc‐discharge synthesis of M‐Ti‐carbide clusterfullerenes (M=Y, Nd, Gd, Dy, Er, Lu). Optimization of the process parameters allows the synthesis of Dy2TiC@C80‐I and its facile isolation in a single chromatographic step. A new type of cluster with an endohedral acetylide unit, M2TiC2@C80, is discovered along with the second isomer of M2TiC@C80. Dy2TiC@C80‐(I,II) and Dy2TiC2@C80‐I are shown to be single‐molecule magnets (SMM), but the presence of the second carbon atom in the cluster Dy2TiC2@C80 leads to substantially poorer SMM properties.
- ItemRobust Single Molecule Magnet Monolayers on Graphene and Graphite with Magnetic Hysteresis up to 28 K(Weinheim : Wiley-VCH, 2021) Spree, Lukas; Liu, Fupin; Neu, Volker; Rosenkranz, Marco; Velkos, Georgios; Wang, Yaofeng; Schiemenz, Sandra; Dreiser, Jan; Gargiani, Pierluigi; Valvidares, Manuel; Chen, Chia-Hsiang; Büchner, Bernd; Avdoshenko, Stanislav M.; Popov, Alexey A.The chemical functionalization of fullerene single molecule magnet Tb2@C80(CH2Ph) enables the facile preparation of robust monolayers on graphene and highly oriented pyrolytic graphite from solution without impairing their magnetic properties. Monolayers of endohedral fullerene functionalized with pyrene exhibit magnetic bistability up to a temperature of 28 K. The use of pyrene terminated linker molecules opens the way to devise integration of spin carrying units encapsulated by fullerene cages on graphitic substrates, be it single-molecule magnets or qubit candidates. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
- ItemThermally Activated Delayed Fluorescence in a Y3N@C80 Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies(Weinheim : Wiley-VCH, 2017-12-8) Zalibera, Michal; Krylov, Denis S.; Karagiannis, Dimitrios; Will, Paul-Anton; Ziegs, Frank; Schiemenz, Sandra; Lubitz, Wolfgang; Reineke, Sebastian; Savitsky, Anton; Popov, Alexey A.The endohedral fullerene Y3N@C80 exhibits luminescence with reasonable quantum yield and extraordinary long lifetime. By variable-temperature steady-state and time-resolved luminescence spectroscopy, it is demonstrated that above 60 K the Y3N@C80 exhibits thermally activated delayed fluorescence with maximum emission at 120 K and a negligible prompt fluorescence. Below 60 K, a phosphorescence with a lifetime of 192±1 ms is observed. Spin distribution and dynamics in the triplet excited state is investigated with X- and W-band EPR and ENDOR spectroscopies and DFT computations. Finally, electroluminescence of the Y3N@C80/PFO film is demonstrated opening the possibility for red-emitting fullerene-based organic light-emitting diodes (OLEDs).
- ItemUsing internal strain and mass to modulate Dy⋯Dy coupling and relaxation of magnetization in heterobimetallic metallofullerenes DyM2N@C80 and Dy2MN@C80 (M = Sc, Y, La, Lu)(Cambridge : RSC, 2022) Hao, Yajuan; Velkos, Georgios; Schiemenz, Sandra; Rosenkranz, Marco; Wang, Yaofeng; Büchner, Bernd; Avdoshenko, Stanislav M.; Popov, Alexey A.; Liu, FupinEndohedral clusters inside metallofullerenes experience considerable inner strain when the size of the hosting cage is comparably small. This strain can be tuned in mixed-metal metallofullerenes by combining metals of different sizes. Here we demonstrate that the internal strain and mass can be used as variables to control Dy⋯Dy coupling and relaxation of magnetization in Dy-metallofullerenes. Mixed-metal nitride clusterfullerenes DyxY3−xN@Ih-C80 (x = 0-3) and Dy2LaN@Ih-C80 combining Dy with diamagnetic rare-earth elements, Y and La, were synthesized and characterized by single-crystal X-ray diffraction, SQUID magnetometry, ab initio calculations, and spectroscopic techniques. DyxY3−xN clusters showed a planar structure, but the slightly larger size of Dy3+ in comparison with that of Y3+ resulted in increased elongation of the nitrogen thermal ellipsoid, showing enhancement of the out-of-plane vibrational amplitude. When Dy was combined with larger La, the Dy2LaN cluster appeared strongly pyramidal with the distance between two nitrogen sites of 1.15(1) Å, whereas DyLa2N@C80 could not be obtained in a separable yield. Magnetic studies revealed that the relaxation of magnetization and blocking temperature of magnetization in the DyM2N@C80 series (M = Sc, Y, Lu) correlated with the mass of M, with DySc2N@C80 showing the fastest and DyLu2N@C80 the slowest relaxation. Ab initio calculations predicted very similar g-tensors for Dy3+ ground state pseudospin in all studied DyM2N@C80 molecules, suggesting that the variation in relaxation is caused by different vibrational spectra of these compounds. In the Dy2MN@C80 series (M = Sc, Y, La, Lu), the magnetic and hysteretic behavior was found to correlate with Dy⋯Dy coupling, which in turn appears to depend on the size of M3+. Across the Dy2MN@C80 series, the energy difference between ferromagnetic and antiferromagnetic states changes from 5.6 cm−1 in Dy2ScN@C80 to 3.0 cm−1 in Dy2LuN@C80, 1.0 cm−1 in Dy2YN@C80, and −0.8 cm−1 in Dy2LaN@C80. The coupling of Dy ions suppresses the zero-field quantum tunnelling of magnetization but opens new relaxation channels, making the relaxation rate dependent on the coupling strengths. DyY2N@C80 and Dy2YN@C80 were found to be non-luminescent, while the luminescence reported for DyY2N@C80 was caused by traces of Y3N@C80 and Y2ScN@C80