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- ItemOptical assets of in situ electro-assembled platinum black nanolayers(London : Nature Publishing Group, 2017) Stanca, S.E.; Hänschke, F.; Zieger, G.; Dellith, J.; Ihring, A.; Undisz, A.; Meyer, H.-G.Optoelectronic technology has been increasingly driven towards miniaturization. In this regard, maintaining the optical properties of the bulk materials while reducing their size is a critical need. How thin must the film be to preserve the bulk material´s optical absorbance and reflectance characteristics? This is the central question for our study of the in situ electro-assembly broad band optical absorber films of platinum in non-aqueous solution of PtCl4. By reducing the in situ constructed film to sub-visible-wavelength thicknesses, the measured reflectance in the region from the ultraviolet to the infrared remained close to that exhibited by the micrometre-width films. These platinum black films broadly absorb electromagnetic waves at a sub-incident-wavelength thickness owing to their plasmonically increased absorbance cross-section. Simulation of various incident energy electron trajectories gives insights into the electron depth through the porous platinum black of ρ = 1.6 g/cm3 and previews the optical behaviour close to the atomic thickness.
- ItemMagnetic properties of individual Co2FeGa Heusler nanoparticles studied at room temperature by a highly sensitive co-resonant cantilever sensor(London : Nature Publishing Group, 2017) Körner, Julia; Reiche, Christopher F.; Ghunaim, Rasha; Fuge, Robert; Hampel, Silke; Büchner, Bernd; Mühl, ThomasThe investigation of properties of nanoparticles is an important task to pave the way for progress and new applications in many fields of research like biotechnology, medicine and magnetic storage techniques. The study of nanoparticles with ever decreasing size is a challenge for commonly employed methods and techniques. It requires increasingly complex measurement setups, often low temperatures and a size reduction of the respective sensors to achieve the necessary sensitivity and resolution. Here, we present results on how magnetic properties of individual nanoparticles can be measured at room temperature and with a conventional scanning force microscopy setup combined with a co-resonant cantilever magnetometry approach. We investigate individual Co2FeGa Heusler nanoparticles with diameters of the order of 35 nm encapsulated in carbon nanotubes. We observed, for the first time, magnetic switching of these nanoparticles in an external magnetic field by simple laser deflection detection. Furthermore, we were able to deduce magnetic properties of these nanoparticles which are in good agreement with previous results obtained with large nanoparticle ensembles in other experiments. In order to do this, we expand the analytical description of the frequency shift signal in cantilever magnetometry to a more general formulation, taking unaligned sensor oscillation directions with respect to the magnetic field into account.
- ItemInductive flash-annealing of bulk metallic glasses(London : Nature Publishing Group, 2017) Kosiba, K.; Pauly, S.We developed a temperature-controlled inductive flash-annealing device, which heats bulk metallic glasses (BMGs) at defined rates of up to 200 K/s to a given temperature. Subsequent instantaneous quenching in water allows preserving the microstructures obtained at various stages of crystallization. One Zr-based and two CuZr-based BMGs were flash-annealed at the onset of crystallization with different heating rates in order to prepare advanced BMG-matrix composites. The highly reproducible composite microstructures contain uniformly dispersed crystals and a narrow crystal size distribution. In order to assess the limitations of the present process, which mainly originate from non-uniform inductive heating, the skin depth was calculated. It is determined to be about 2.3 mm, which enables flash-annealing of rather bulky samples. The cooling rate was estimated from the interlamellar spacing of eutectic Al-Cu alloys to be on the order of 103 K/s. This ensures that decomposition of the microstructure during quenching is prevented. The present flash-annealing procedure is applicable to a wide variety of glass-forming liquids and has a large potential for tailoring the microstructure and, consequently, the mechanical properties of BMG-matrix composites.
- ItemChemical and electrochemical synthesis of platinum black(London : Nature Publishing Group, 2017) Stanca, S.E.; Hänschke, F.; Ihring, A.; Zieger, G.; Dellith, J.; Kessler, E.; Meyer, H.-G.We present electrochemical and chemical synthesis of platinum black at room temperature in aqueous and non-aqueous media. X-ray analysis established the purity and crystalline nature. The electron micrographs indicate that the nanostructures consist of platinum crystals that interconnect to form porous assemblies. Additionally, the electron micrographs of the platinum black thin layer, which was electrochemically deposited on different metallic and semiconductive substrates (aluminium, platinum, silver, gold, tin-cooper alloy, indium-tin-oxide, stainless steel, and copper), indicate that the substrate influences its porous features but not its absorbance characteristics. The platinum black exhibited a broad absorbance and low reflectance in the ultraviolet, visible, and infrared regions. These characteristics make this material suitable for use as a high-temperature resistant absorber layer for the fabrication of microelectronics.
- ItemSignatures of a magnetic field-induced unconventional nematic liquid in the frustrated and anisotropic spin-chain cuprate LiCuSbO4(London : Nature Publishing Group, 2017) Grafe, H.-J.; Nishimoto, S.; Iakovleva, M.; Vavilova, E.; Spillecke, L.; Alfonsov, A.; Sturza, M.-I.; Wurmehl, S.; Nojiri, H.; Rosner, H.; Richter, J.; Rößler, U.K.; Drechsler, S.-L.; Kataev, V.; Büchner, B.Modern theories of quantum magnetism predict exotic multipolar states in weakly interacting strongly frustrated spin-1/2 Heisenberg chains with ferromagnetic nearest neighbor (NN) inchain exchange in high magnetic fields. Experimentally these states remained elusive so far. Here we report strong indications of a magnetic field-induced nematic liquid arising above a field of ~13 T in the edge-sharing chain cuprate LiSbCuO4 ≡ LiCuSbO4. This interpretation is based on the observation of a field induced spin-gap in the measurements of the 7Li NMR spin relaxation rate T1−1 as well as a contrasting field-dependent power-law behavior of T1−1 vs. T and is further supported by static magnetization and ESR data. An underlying theoretical microscopic approach favoring a nematic scenario is based essentially on the NN XYZ exchange anisotropy within a model for frustrated spin-1/2 chains and is investigated by the DMRG technique. The employed exchange parameters are justified qualitatively by electronic structure calculations for LiCuSbO4.
- ItemLocal melting to design strong and plastically deformable bulk metallic glass composites(London : Nature Publishing Group, 2017) Qin, Yue-Sheng; Han, Xiao-Liang; Song, Kai-Kai; Tian, Yu-Hao; Peng, Chuan-Xiao; Wang, Li; Sun, Bao-An; Wang, Gang; Kaban, Ivan; Eckert, JürgenRecently, CuZr-based bulk metallic glass (BMG) composites reinforced by the TRIP (transformation-induced plasticity) effect have been explored in attempt to accomplish an optimal of trade-off between strength and ductility. However, the design of such BMG composites with advanced mechanical properties still remains a big challenge for materials engineering. In this work, we proposed a technique of instantaneously and locally arc-melting BMG plate to artificially induce the precipitation of B2 crystals in the glassy matrix and then to tune mechanical properties. Through adjusting local melting process parameters (i.e. input powers, local melting positions, and distances between the electrode and amorphous plate), the size, volume fraction, and distribution of B2 crystals were well tailored and the corresponding formation mechanism was clearly clarified. The resultant BMG composites exhibit large compressive plasticity and high strength together with obvious work-hardening ability. This compelling approach could be of great significance for the steady development of metastable CuZr-based alloys with excellent mechanical properties.
- ItemSwitchable multiple spin states in the Kondo description of doped molecular magnets(London : Nature Publishing Group, 2017) Ray, Rajyavardhan; Kumar, SanjeevWe show that introducing electrons in magnetic clusters and molecular magnets lead to rich phase diagrams with a variety of low-spin and high-spin states allowing for multiple switchability. The analysis is carried out for a quantum spin-fermion model using the exact diagonalization, and the cluster mean-field approach. The model is relevant for a number of molecular magnets with triangular motifs consisting of transition metal ions such as Cr, Cu and V. Re-entrant spin-state behavior and chirality on-off transitions exist over a wide parameter regime. A subtle competition among geometrical frustration effects, electron itinerancy, and Kondo coupling at the molecular level is highlighted. Our results demonstrate that electron doping provides a viable mean to tame the magnetic properties of molecular magnets towards potential technological applications.
- ItemSelective mass enhancement close to the quantum critical point in BaFe2(As1−x P x )2(London : Nature Publishing Group, 2017) Grinenko, V.; Iida, K.; Kurth, F.; Efremov, D.V.; Drechsler, S.-L.; Cherniavskii, I.; Morozov, I.; Hänisch, J.; Förster, T.; Tarantini, C.; Jaroszynski, J.; Maiorov, B.; Jaime, M.; Yamamoto, A.; Nakamura, I.; Fujimoto, R.; Hatano, T.; Ikuta, H.; Hühne, R.A quantum critical point (QCP) is currently being conjectured for the BaFe2(As1−x P x )2 system at the critical value x c ≈ 0.3. In the proximity of a QCP, all thermodynamic and transport properties are expected to scale with a single characteristic energy, given by the quantum fluctuations. Such a universal behavior has not, however, been found in the superconducting upper critical field Hc2. Here we report Hc2 data for epitaxial thin films extracted from the electrical resistance measured in very high magnetic fields up to 67 Tesla. Using a multi-band analysis we find that Hc2 is sensitive to the QCP, implying a significant charge carrier effective mass enhancement at the doping-induced QCP that is essentially band-dependent. Our results point to two qualitatively different groups of electrons in BaFe2(As1−x P x )2. The first one (possibly associated to hot spots or whole Fermi sheets) has a strong mass enhancement at the QCP, and the second one is insensitive to the QCP. The observed duality could also be present in many other quantum critical systems.