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- ItemSubstrate-Independent Magnetic Bistability in Monolayers of the Single-Molecule Magnet Dy2ScN@C80 on Metals and Insulators(Weinheim : Wiley-VCH, 2020) Krylov, Denis S.; Schimmel, Sebastian; Dubrovin, Vasilii; Liu, Fupin; Nguyen, T.T. Nhung; Spree, Lukas; Chen, Chia-Hsiang; Velkos, Georgios; Bulbucan, Claudiu; Westerström, Rasmus; Studniarek, Michał; Dreiser, Jan; Hess, Christian; Büchner, Bernd; Avdoshenko, Stanislav M.; Popov, Alexey A.Magnetic hysteresis is demonstrated for monolayers of the single-molecule magnet (SMM) Dy2ScN@C80 deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy2ScN@C80 adsorbed on Au(111) were studied by STM. X-ray magnetic CD studies show that the Dy2ScN@C80 monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy2ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene-surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene-SMMs on conducting substrates and facilitates their application in molecular spintronics. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- 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
- ItemObservation of strontium segregation in LaAlO3/SrTiO3 and NdGaO3/SrTiO3 oxide heterostructures by X-ray photoemission spectroscopy(New York : American Institute of Physics, 2014) Treske, Uwe; Heming, Nadine; Knupfer, Martin; Büchner, Bernd; Koitzsch, Andreas; Di Gennaro, Emiliano; Scotti di Uccio, Umberto; Miletto Granozio, Fabio; Krause, StefanLaAlO3 and NdGaO3 thin films of different thicknesses have been grown by pulsed laser deposition on TiO2-terminated SrTiO3 single crystals and investigated by soft X-ray photoemission spectroscopy. The surface sensitivity of the measurements has been tuned by varying photon energy hν and emission angle Θ. In contrast to the core levels of the other elements, the Sr 3d line shows an unexpected splitting for higher surface sensitivity, signaling the presence of a second strontium component. From our quantitative analysis we conclude that during the growth process Sr atoms diffuse away from the substrate and segregate at the surface of the heterostructure, possibly forming strontium oxide
- ItemHigh Blocking Temperature of Magnetization and Giant Coercivity in the Azafullerene Tb 2 @C 79 N with a Single-Electron Terbium–Terbium Bond(Weinheim : Wiley-VCH, 2019) Velkos, Georgios; Krylov, Denis S.; Kirkpatrick, Kyle; Spree, Lukas; Dubrovin, Vasilii; Büchner, Bernd; Avdoshenko, Stanislav M.; Bezmelnitsyn, Valeriy; Davis, Sean; Faust, Paul; Duchamp, James; Dorn, Harry C.; Popov, Alexey A.The azafullerene Tb 2 @C 79 N is found to be a single-molecule magnet with a high 100-s blocking temperature of magnetization of 24 K and large coercivity. Tb magnetic moments with an easy-axis single-ion magnetic anisotropy are strongly coupled by the unpaired spin of the single-electron Tb−Tb bond. Relaxation of magnetization in Tb 2 @C 79 N below 15 K proceeds via quantum tunneling of magnetization with the characteristic time τ QTM =16 462±1230 s. At higher temperature, relaxation follows the Orbach mechanism with a barrier of 757±4 K, corresponding to the excited states, in which one of the Tb spins is flipped. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemSynthesis and Physical Properties of Iridium-Based Sulfide Ca1−xIr4S6(S2) [x = 0.23–0.33](Basel : MDPI, 2022) Vogl, Michael; Valldor, Martin; Piening, Roman Boy; Efremov, Dmitri V.; Büchner, Bernd; Aswartham, SaicharanWe present the synthesis and characterization of the iridium-based sulfide Ca1−xIr4S6(S2). Quality and phase analysis were conducted by means of energy-dispersive X-ray spectroscopy (EDXS) and powder X-ray diffraction (XRD) techniques. Structure analysis reveals a monoclinic symmetry with the space group C 1 2/m 1 (No. 12), with the lattice constants a = 15.030 (3) Å, b = 3.5747 (5) Å and c = 10.4572 (18) Å. Both X-ray diffraction and EDXS suggest an off-stoichiometry of calcium, leading to the empirical composition Ca1−xIr4.0S6(S2) [x = 0.23–0.33]. Transport measurements show metallic behavior of the compound in the whole range of measured temperatures. Magnetic measurements down to 1.8 K show no long range order, and Curie–Weiss analysis yields θCW = −31.4 K, suggesting that the compound undergoes a magnetic state with short range magnetic correlations. We supplement our study with calculations of the band structure in the framework of the density functional theory.
- ItemCarbon nanotubes filled with ferromagnetic materials(Basel : MDPI, 2010) Weissker, Uhland; Hampel, Silke; Leonhardt, Albrecht; Büchner, BerndCarbon nanotubes (CNT) filled with ferromagnetic metals like iron, cobalt or nickel are new and very interesting nanostructured materials with a number of unique properties. In this paper we give an overview about different chemical vapor deposition (CVD) methods for their synthesis and discuss the influence of selected growth parameters. In addition we evaluate possible growth mechanisms involved in their formation. Moreover we show their identified structural and magnetic properties. On the basis of these properties we present different application possibilities. Some selected examples reveal the high potential of these materials in the field of medicine and nanotechnology.
- ItemMagnetic Nanoparticle Chains in Gelatin Ferrogels: Bioinspiration from Magnetotactic Bacteria(Weinheim : Wiley-VCH, 2019) Sturm, Sebastian; Siglreitmeier, Maria; Wolf, Daniel; Vogel, Karin; Gratz, Micha; Faivre, Damien; Lubk, Axel; Büchner, Bernd; Sturm, Elena V.; Cölfen, HelmutInspired by chains of ferrimagnetic nanocrystals (NCs) in magnetotactic bacteria (MTB), the synthesis and detailed characterization of ferrimagnetic magnetite NC chain-like assemblies is reported. An easy green synthesis route in a thermoreversible gelatin hydrogel matrix is used. The structure of these magnetite chains prepared with and without gelatin is characterized by means of transmission electron microscopy, including electron tomography (ET). These structures indeed bear resemblance to the magnetite assemblies found in MTB, known for their mechanical flexibility and outstanding magnetic properties and known to crystallographically align their magnetite NCs along the strongest <111> magnetization easy axis. Using electron holography (EH) and angular dependent magnetic measurements, the magnetic interaction between the NCs and the generation of a magnetically anisotropic material can be shown. The electro- and magnetostatic modeling demonstrates that in order to precisely determine the magnetization (by means of EH) inside chain-like NCs assemblies, their exact shape, arrangement and stray-fields have to be considered (ideally obtained using ET). © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemElastoresistivity of Heavily Hole-Doped 122 Iron Pnictide Superconductors(Lausanne : Frontiers Media, 2022) Hong, Xiaochen; Sykora, Steffen; Caglieris, Federico; Behnami, Mahdi; Morozov, Igor; Aswartham, Saicharan; Grinenko, Vadim; Kihou, Kunihiro; Lee, Chul-Ho; Büchner, Bernd; Hess, ChristianNematicity in heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from magnetic instability were declared in RbFe2As2 and its sister compounds. Here, we report a systematic elastoresistance study of a series of isovalent- and electron-doped KFe2As2 crystals. We found divergent elastoresistance on cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven toward a Lifshitz transition. However, we conclude that none of them necessarily indicates an independent nematic critical point. Instead, the increased nematicity can be associated with another electronic criticality. In particular, we propose a mechanism for how elastoresistivity is enhanced at a Lifshitz transition.
- 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.
- ItemDiscovery, Crystal Growth, and Characterization of Garnet Eu2PbSb2Zn3O12(Weinheim : Wiley-VCH, 2020) Morrow, Ryan; Sturza, Mihai I.; Ray, Rajyavardhan; Himcinschi, Cameliu; Kern, Jonas; Schlender, Philipp; Richter, Manuel; Wurmehl, Sabine; Büchner, BerndSingle crystal specimens of previously unknown garnet Eu2PbSb2Zn3O12 were grown in a reactive PbO:PbF2 flux medium. The crystals were characterized by a combination of X-ray crystallography, magnetization measurements, and the optical techniques of Raman, photoluminescence, and UV/Vis spectroscopy. The material exhibits Van Vleck paramagnetism associated with the J = 0 state of Eu3+, which was possible to accurately fit to a theoretical model. Band structure calculations were performed and compared to the experimental band gap of 1.98 eV. The crystals demonstrate photoluminescence associated with the 4f 6 configuration of the Eu3+ ions sitting at the distorted 8-coordinate garnet A site. The title compound represents a unique quinary contribution to a relatively unexplored area of rare earth bearing garnet crystal chemistry. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.