2016, Yan, Zhijie, Song, Kaikai, Hu, Yong, Dai, Fuping, Chu, Zhibing, Eckert, Jürgen

Stress-induced viscous flow is the characteristic of atomic movements during plastic deformation of metallic glasses in the absence of substantial temperature increase, which suggests that stress state plays an important role in mechanically induced crystallization in a metallic glass. However, it is poorly understood. Here, we report on the stress-induced localized crystallization in individual shear bands of Zr60Al15Ni25 metallic glass subjected to cold rolling. We find that crystallization in individual shear bands preferentially occurs in the regions neighboring the amorphous matrix, where the materials are subjected to compressive stresses demonstrated by our finite element simulations. Our results provide direct evidence that the mechanically induced crystallization kinetics is closely related with the stress state. The crystallization kinetics under compressive and tensile stresses are interpreted within the frameworks of potential energy landscape and classical nucleation theory, which reduces the role of stress state in mechanically induced crystallization in a metallic glass.

2014, Ding, Liping, He, Shulian, Miao, Shiding, Jorgensen, Matthew R., Leubner, Susanne, Yan, Chenglin, Hickey, Stephen G., Eychmüller, Alexander, Xu, Jinzhang, Schmidt, Oliver G.

Ultrasmall SnO2 nanocrystals as anode materials for lithium-ion batteries (LIBs) have been synthesized by bubbling an oxidizing gas into hot surfactant solutions containing Sn-oleate complexes. Annealing of the particles in N2 carbonifies the densely packed surface capping ligands resulting in carbon encapsulated SnO2 nanoparticles (SnO2/C). Carbon encapsulation can effectively buffer the volume changes during the lithiation/delithiation process. The assembled SnO2/C thus deliver extraordinarily high reversible capacity of 908 mA·h·g−1 at 0.5 C as well as excellent cycling performance in the LIBs. This method demonstrates the great potential of SnO2/C nanoparticles for the design of high power LIBs.

2015, Streubel, Robert, Han, Luyang, Im, Mi-Young, Kronast, Florian, Rößler, Ulrich K., Radu, Florin, Abrudan, Radu, Lin, Gungun, Schmidt, Oliver G., Fischer, Peter, Makarov, Denys

Topological magnetic states, such as chiral skyrmions, are of great scientific interest and show huge potential for novel spintronics applications, provided their topological charges can be fully controlled. So far skyrmionic textures have been observed in noncentrosymmetric crystalline materials with low symmetry and at low temperatures. We propose theoretically and demonstrate experimentally the design of spin textures with topological charge densities that can be tailored at ambient temperatures. Tuning the interlayer coupling in vertically stacked nanopatterned magnetic heterostructures, such as a model system of a Co/Pd multilayer coupled to Permalloy, the in-plane non-collinear spin texture of one layer can be imprinted into the out-of-plane magnetised material. We observe distinct spin textures, e.g. vortices, magnetic swirls with tunable opening angle, donut states and skyrmion core configurations. We show that applying a small magnetic field, a reliable switching between topologically distinct textures can be achieved at remanence.

2019, Wu, L.S., Nikitin, S.E., Wang, Z., Zhu, W., Batista, C.D., Tsvelik, A.M., Samarakoon, A.M., Tennant, D.A., Brando, M., Vasylechko, L., Frontzek, M., Savici, A.T., Sala, G., Ehlers, G., Christianson, A.D., Lumsden, M.D., Podlesnyak, A.

Low dimensional quantum magnets are interesting because of the emerging collective behavior arising from strong quantum fluctuations. The one-dimensional (1D) S = 1/2 Heisenberg antiferromagnet is a paradigmatic example, whose low-energy excitations, known as spinons, carry fractional spin S = 1/2. These fractional modes can be reconfined by the application of a staggered magnetic field. Even though considerable progress has been made in the theoretical understanding of such magnets, experimental realizations of this low-dimensional physics are relatively rare. This is particularly true for rare-earth-based magnets because of the large effective spin anisotropy induced by the combination of strong spin–orbit coupling and crystal field splitting. Here, we demonstrate that the rare-earth perovskite YbAlO3 provides a realization of a quantum spin S = 1/2 chain material exhibiting both quantum critical Tomonaga–Luttinger liquid behavior and spinon confinement–deconfinement transitions in different regions of magnetic field–temperature phase diagram.

2015, Schropp, Andreas, Hoppe, Robert, Meier, Vivienne, Patommel, Jens, Seiboth, Frank, Ping, Yuan, Hicks, Damien G., Beckwith, Martha A., Collins, Gilbert W., Higginbotham, Andrew, Wark, Justin S., Lee, Hae Ja, Nagler, Bob, Galtier, Eric C., Arnold, Brice, Zastrau, Ulf, Hastings, Jerome B., Schroer, Christian G.

The advent of hard x-ray free-electron lasers (XFELs) has opened up a variety of scientific opportunities in areas as diverse as atomic physics, plasma physics, nonlinear optics in the x-ray range and protein crystallography. In this article, we access a new field of science by measuring quantitatively the local bulk properties and dynamics of matter under extreme conditions, in this case by using the short XFEL pulse to image an elastic compression wave in diamond. The elastic wave was initiated by an intense optical laser pulse and was imaged at different delay times after the optical pump pulse using magnified x-ray phase-contrast imaging. The temporal evolution of the shock wave can be monitored, yielding detailed information on shock dynamics, such as the shock velocity, the shock front width and the local compression of the material. The method provides a quantitative perspective on the state of matter in extreme conditions.

2015, Katukuri, Vamshi M., Nishimoto, Satoshi, Rousochatzakis, Ioannis, Stoll, Hermann, van den Brink, Jeroen, Hozoi, Liviu

With large spin-orbit coupling, the electron configuration in d-metal oxides is prone to highly anisotropic exchange interactions and exotic magnetic properties. In 5d5 iridates, given the existing variety of crystal structures, the magnetic anisotropy can be tuned from antisymmetric to symmetric Kitaev-type, with interaction strengths that outsize the isotropic terms. By many-body electronic-structure calculations we here address the nature of the magnetic exchange and the intriguing spin-glass behavior of Li2RhO3, a 4d5 honeycomb oxide. For pristine crystals without Rh-Li site inversion, we predict a dimerized ground state as in the isostructural 5d5 iridate Li2IrO3, with triplet spin dimers effectively placed on a frustrated triangular lattice. With Rh-Li anti-site disorder, we explain the observed spin-glass phase as a superposition of different, nearly degenerate symmetry-broken configurations.

2019, Zvyagin, S.A., Graf, D., Sakurai, T., Kimura, S., Nojiri, H., Wosnitza, J., Ohta, H., Ono, T., Tanaka, H.

Quantum triangular-lattice antiferromagnets are important prototype systems to investigate numerous phenomena of the geometrical frustration in condensed matter. Apart from highly unusual magnetic properties, they possess a rich phase diagram (ranging from an unfrustrated square lattice to a quantum spin liquid), yet to be confirmed experimentally. One major obstacle in this area of research is the lack of materials with appropriate (ideally tuned) magnetic parameters. Using Cs2CuCl4 as a model system, we demonstrate an alternative approach, where, instead of the chemical composition, the spin Hamiltonian is altered by hydrostatic pressure. The approach combines high-pressure electron spin resonance and r.f. susceptibility measurements, allowing us not only to quasi-continuously tune the exchange parameters, but also to accurately monitor them. Our experiments indicate a substantial increase of the exchange coupling ratio from 0.3 to 0.42 at a pressure of 1.8 GPa, revealing a number of emergent field-induced phases.

2016, Surrey, Alexander, Schultz, Ludwig, Rellinghaus, Bernd

Nanosized or nanoconfined hydrides are promising materials for solid-state hydrogen storage. Most of these hydrides, however, degrade fast during the structural characterization utilizing transmission electron microscopy (TEM) upon the irradiation with the imaging electron beam due to radiolysis. We use ball-milled MgH2 as a reference material for in-situ TEM experiments under low-dose conditions to study and quantitatively understand the electron beam-induced dehydrogenation. For this, valence electron energy loss spectroscopy (VEELS) measurements are conducted in a monochromated FEI Titan3 80–300 microscope. From observing the plasmonic absorptions it is found that MgH2 successively converts into Mg upon electron irradiation. The temporal evolution of the spectra is analyzed quantitatively to determine the thickness-dependent, characteristic electron doses for electron energies of both 80 and 300 keV. The measured electron doses can be quantitatively explained by the inelastic scattering of the incident high-energy electrons by the MgH2 plasmon. The obtained insights are also relevant for the TEM characterization of other hydrides.

2019, Hammerath, Franziska, Bali, Rantej, Hübner, René, Brandt, Mira R. D., Rodan, Steven, Potzger, Kay, Böttger, Roman, Sakuraba, Yuya, Wurmehl, Sabine

We investigated the structure-property relationship of Co2MnSi Heusler thin films upon the irradiation with He+ ions. The variation of the crystal structure with increasing ion fluence has been probed using nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM), and associated with the corresponding changes of the magnetic behavior. A decrease of both the structural order and the moment in saturation is observed. Specifically, we detect a direct transition from a highly L21-ordered to a fully A2-disordered structure type and quantify the evolution of the A2 structural contribution as a function of ion fluence. Complementary TEM analysis reveals a spatially-resolved distribution of the L21 and A2 phases showing that the A2 disorder starts at the upper part of the films. The structural degradation in turn leads to a decreasing magnetic moment in saturation in response to the increasing fluence.

2019, Reichlova, Helena, Janda, Tomas, Godinho, Joao, Markou, Anastasios, Kriegner, Dominik, Schlitz, Richard, Zelezny, Jakub, Soban, Zbynek, Bejarano, Mauricio, Schultheiss, Helmut, Nemec, Petr, Jungwirth, Tomas, Felser, Claudia, Wunderlich, Joerg, Goennenwein, Sebastian T. B.

Non-collinear antiferromagnets are revealing many unexpected phenomena and they became crucial for the field of antiferromagnetic spintronics. To visualize and prepare a well-defined domain structure is of key importance. The spatial magnetic contrast, however, remains extraordinarily difficult to be observed experimentally. Here, we demonstrate a magnetic imaging technique based on a laser induced local thermal gradient combined with detection of the anomalous Nernst effect. We employ this method in one the most actively studied representatives of this class of materials—Mn3Sn. We demonstrate that the observed contrast is of magnetic origin. We further show an algorithm to prepare a well-defined domain pattern at room temperature based on heat assisted recording principle. Our study opens up a prospect to study spintronics phenomena in non-collinear antiferromagnets with spatial resolution.