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2020


Learning Variable Impedance Control for Contact Sensitive Tasks
Learning Variable Impedance Control for Contact Sensitive Tasks

Bogdanovic, M., Khadiv, M., Righetti, L.

IEEE Robotics and Automation Letters ( Early Access ), IEEE, July 2020 (article)

Abstract
Reinforcement learning algorithms have shown great success in solving different problems ranging from playing video games to robotics. However, they struggle to solve delicate robotic problems, especially those involving contact interactions. Though in principle a policy outputting joint torques should be able to learn these tasks, in practice we see that they have difficulty to robustly solve the problem without any structure in the action space. In this paper, we investigate how the choice of action space can give robust performance in presence of contact uncertainties. We propose to learn a policy that outputs impedance and desired position in joint space as a function of system states without imposing any other structure to the problem. We compare the performance of this approach to torque and position control policies under different contact uncertainties. Extensive simulation results on two different systems, a hopper (floating-base) with intermittent contacts and a manipulator (fixed-base) wiping a table, show that our proposed approach outperforms policies outputting torque or position in terms of both learning rate and robustness to environment uncertainty.

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DOI [BibTex]

2020


DOI [BibTex]


Walking Control Based on Step Timing Adaptation
Walking Control Based on Step Timing Adaptation

Khadiv, M., Herzog, A., Moosavian, S. A. A., Righetti, L.

IEEE Transactions on Robotics, 36, pages: 629 - 643, IEEE, June 2020 (article)

Abstract
Step adjustment can improve the gait robustness of biped robots; however, the adaptation of step timing is often neglected as it gives rise to nonconvex problems when optimized over several footsteps. In this article, we argue that it is not necessary to optimize walking over several steps to ensure gait viability and show that it is sufficient to merely select the next step timing and location. Using this insight, we propose a novel walking pattern generator that optimally selects step location and timing at every control cycle. Our approach is computationally simple compared to standard approaches in the literature, yet guarantees that any viable state will remain viable in the future. We propose a swing foot adaptation strategy and integrate the pattern generator with an inverse dynamics controller that does not explicitly control the center of mass nor the foot center of pressure. This is particularly useful for biped robots with limited control authority over their foot center of pressure, such as robots with point feet or passive ankles. Extensive simulations on a humanoid robot with passive ankles demonstrate the capabilities of the approach in various walking situations, including external pushes and foot slippage, and emphasize the importance of step timing adaptation to stabilize walking.

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


Biocompatible magnetic micro‐ and nanodevices: Fabrication of FePt nanopropellers and cell transfection
Biocompatible magnetic micro‐ and nanodevices: Fabrication of FePt nanopropellers and cell transfection

Kadiri, V. M., Bussi, C., Holle, A. W., Son, K., Kwon, H., Schütz, G., Gutierrez, M. G., Fischer, P.

Adv. Mat., 32(2001114), May 2020 (article)

Abstract
The application of nanoparticles for drug or gene delivery promises benefits in the form of single‐cell‐specific therapeutic and diagnostic capabilities. Many methods of cell transfection rely on unspecific means to increase the transport of genetic material into cells. Targeted transport is in principle possible with magnetically propelled micromotors, which allow responsive nanoscale actuation and delivery. However, many commonly used magnetic materials (e.g., Ni and Co) are not biocompatible, possess weak magnetic remanence (Fe3O4), or cannot be implemented in nanofabrication schemes (NdFeB). Here, it is demonstrated that co‐depositing iron (Fe) and platinum (Pt) followed by one single annealing step, without the need for solution processing, yields ferromagnetic FePt nanomotors that are noncytotoxic, biocompatible, and possess a remanence and magnetization that rival those of permanent NdFeB micromagnets. Active cell targeting and magnetic transfection of lung carcinoma cells are demonstrated using gradient‐free rotating millitesla fields to drive the FePt nanopropellers. The carcinoma cells express enhanced green fluorescent protein after internalization and cell viability is unaffected by the presence of the FePt nanopropellers. The results establish FePt, prepared in the L10 phase, as a promising magnetic material for biomedical applications with superior magnetic performance, especially for micro‐ and nanodevices.

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link (url) DOI [BibTex]


Self-supervised motion deblurring
Self-supervised motion deblurring

Liu, P., Janai, J., Pollefeys, M., Sattler, T., Geiger, A.

IEEE Robotics and Automation Letters, 2020 (article)

Abstract
Motion blurry images challenge many computer vision algorithms, e.g., feature detection, motion estimation, or object recognition. Deep convolutional neural networks are state-of-the-art for image deblurring. However, obtaining training data with corresponding sharp and blurry image pairs can be difficult. In this paper, we present a differentiable reblur model for self-supervised motion deblurring, which enables the network to learn from real-world blurry image sequences without relying on sharp images for supervision. Our key insight is that motion cues obtained from consecutive images yield sufficient information to inform the deblurring task. We therefore formulate deblurring as an inverse rendering problem, taking into account the physical image formation process: we first predict two deblurred images from which we estimate the corresponding optical flow. Using these predictions, we re-render the blurred images and minimize the difference with respect to the original blurry inputs. We use both synthetic and real dataset for experimental evaluations. Our experiments demonstrate that self-supervised single image deblurring is really feasible and leads to visually compelling results.

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pdf Project Page Blog [BibTex]

pdf Project Page Blog [BibTex]


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Effect of the soft layer thickness of magnetization reversal process of exchange-spring nanomagnet patterns

Son, K., Schütz, G., Goering, E.

{Current Applied Physics}, 20(4):477-483, Elsevier B.V., Amsterdam, 2020 (article)

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DOI [BibTex]


{Creating zero-field skyrmions in exchange-biased multilayers through X-ray illumination}
Creating zero-field skyrmions in exchange-biased multilayers through X-ray illumination

Guang, Y., Bykova, I., Liu, Y., Yu, G., Goering, E., Weigand, M., Gräfe, J., Kim, S. K., Zhang, J., Zhang, H., Yan, Z., Wan, C., Feng, J., Wang, X., Guo, C., Wei, H., Peng, Y., Tserkovnyak, Y., Han, X., Schütz, G.

{Nature Communications}, 11, Nature Publishing Group, London, 2020 (article)

Abstract
Skyrmions, magnetic textures with topological stability, hold promises for high-density and energy-efficient information storage devices owing to their small size and low driving-current density. Precise creation of a single nanoscale skyrmion is a prerequisite to further understand the skyrmion physics and tailor skyrmion-based applications. Here, we demonstrate the creation of individual skyrmions at zero-field in an exchange-biased magnetic multilayer with exposure to soft X-rays. In particular, a single skyrmion with 100-nm size can be created at the desired position using a focused X-ray spot of sub-50-nm size. This single skyrmion creation is driven by the X-ray-induced modification of the antiferromagnetic order and the corresponding exchange bias. Furthermore, artificial skyrmion lattices with various arrangements can be patterned using X-ray. These results demonstrate the potential of accurate optical control of single skyrmion at sub-100 nm scale. We envision that X-ray could serve as a versatile tool for local manipulation of magnetic orders.

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DOI [BibTex]

DOI [BibTex]


{Tuning the magnetic properties of permalloy-based magnetoplasmonic crystals for sensor applications}
Tuning the magnetic properties of permalloy-based magnetoplasmonic crystals for sensor applications

Murzin, D. V., Belyaev, V. K., Groß, F., Gräfe, J., Rivas, M., Rodionova, V. V.

{Japanese Journal of Applied Physics}, 59(SE), IOP Publishing Ltd, Bristol, England, 2020 (article)

Abstract
Miniature magnetic sensors based on magnetoplasmonic crystals (MPlCs) exhibit high sensitivity and high spatial resolution, which can be obtained by the excitation of surface plasmon polaritons. A field dependence of surface plasmon polaritons' enhanced magneto-optical response strongly correlates with magnetic properties of MPlCs that can be tuned by changing spatial parameters, such as the period and height of diffraction gratings and thicknesses of functional layers. This work compares the magnetic properties of MPlCs based on Ni80Fe20 (permalloy) obtained from local (longitudinal magneto-optical Kerr effect) and bulk (vibrating-sample magnetometry) measurements and demonstrates an ability to control sensors' performance through changing the magnetic properties of the MPlCs. The influence of the substrate's geometry (planar or sinusoidal and trapezoidal diffraction grating profiles) and the thickness of the surface layer is examined.

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DOI [BibTex]

DOI [BibTex]


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Semi-Supervised Learning of Multi-Object 3D Scene Representations

Elich, C., Oswald, M. R., Pollefeys, M., Stueckler, J.

CoRR, abs/2010.04030, 2020 (article)

Abstract
Representing scenes at the granularity of objects is a prerequisite for scene understanding and decision making. We propose a novel approach for learning multi-object 3D scene representations from images. A recurrent encoder regresses a latent representation of 3D shapes, poses and texture of each object from an input RGB image. The 3D shapes are represented continuously in function-space as signed distance functions (SDF) which we efficiently pre-train from example shapes in a supervised way. By differentiable rendering we then train our model to decompose scenes self-supervised from RGB-D images. Our approach learns to decompose images into the constituent objects of the scene and to infer their shape, pose and texture from a single view. We evaluate the accuracy of our model in inferring the 3D scene layout and demonstrate its generative capabilities.

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link (url) [BibTex]

link (url) [BibTex]


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Specific isotope-responsive breathing transition in flexible metal-organic frameworks

Kim, J. Y., Park, J., Ha, J., Jung, M., Wallacher, D., Franz, A., Balderas-Xicohténcatl, R., Hirscher, M., Kang, S. G., Park, J. T., Oh, I. H., Moon, H. R., Oh, H.

Journal of the American Chemical Society, 142(31):13278-13282, American Chemical Society, Washington, DC, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Element-resolved study of the evolution of magnetic response in FexN compounds

Chen, Y., Gölden, D., Dirba, I., Huang, M., Gutfleisch, O., Nagel, P., Merz, M., Schuppler, S., Schütz, G., Alff, L., Goering, E.

{Journal of Magnetism and Magnetic Materials}, 498, NH, Elsevier, Amsterdam, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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The role of temperature and drive current in skyrmion dynamics

Litzius, K., Leliaert, J., Bassirian, P., Rodrigues, D., Kromin, S., Lemesh, I., Zazvorka, J., Lee, K., Mulkers, J., Kerber, N., Heinze, D., Keil, N., Reeve, R. M., Weigand, M., Van Waeyenberge, B., Schütz, G., Everschor-Sitte, K., Beach, G. S. D., Kläui, M.

{Nature Electronics}, 3(1):30-36, Springer Nature, London, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Magnetic flux penetration into micron-sized superconductor/ferromagnet bilayers

Simmendinger, J., Weigand, M., Schütz, G., Albrecht, J.

{Superconductor Science and Technology}, 33(2), IOP Pub., Bristol, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


Learning Neural Light Transport
Learning Neural Light Transport

Sanzenbacher, P., Mescheder, L., Geiger, A.

Arxiv, 2020 (article)

Abstract
In recent years, deep generative models have gained significance due to their ability to synthesize natural-looking images with applications ranging from virtual reality to data augmentation for training computer vision models. While existing models are able to faithfully learn the image distribution of the training set, they often lack controllability as they operate in 2D pixel space and do not model the physical image formation process. In this work, we investigate the importance of 3D reasoning for photorealistic rendering. We present an approach for learning light transport in static and dynamic 3D scenes using a neural network with the goal of predicting photorealistic images. In contrast to existing approaches that operate in the 2D image domain, our approach reasons in both 3D and 2D space, thus enabling global illumination effects and manipulation of 3D scene geometry. Experimentally, we find that our model is able to produce photorealistic renderings of static and dynamic scenes. Moreover, it compares favorably to baselines which combine path tracing and image denoising at the same computational budget.

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arxiv [BibTex]


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Demonstration of k-vector selective microscopy for nanoscale mapping of higher order spin wave modes

Träger, N., Gruszecki, P., Lisiecki, F., Groß, F., Förster, J., Weigand, M., Glowinski, H., Kuswik, P., Dubowik, J., Krawczyk, M., Gräfe, J.

Nanoscale, 12(33):17238-17244, Royal Society of Chemistry, Cambridge, UK, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Direct observation of spin-wave focusing by a Fresnel lens

Gräfe, J., Gruszecki, P., Zelent, M., Decker, M., Keskinbora, K., Noske, M., Gawronski, P., Stoll, H., Weigand, M., Krawczyk, M., Back, C. H., Goering, E. J., Schütz, G.

Physical Review B, 102(2), American Physical Society, Woodbury, NY, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Bandgap-adjustment and enhanced surface photovoltage in Y-substituted LaTaIVO2N

Bubeck, C., Widenmeyer, M., De Denko, A. T., Richter, G., Coduri, M., Salas-Colera, E., Goering, E., Zhang, H., Yoon, S., Osterloh, F. E., Weidenkaff, A.

Journal of Materials Chemistry A, 8(23):11837-11848, Royal Society of Chemistry, Cambridge, UK, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Fabrication and temperature-dependent magnetic properties of large-area L10-FePt/Co exchange-spring magnet nanopatterns

Son, K., Schütz, G.

{Physica E: Low-Dimensional Systems And Nanostructures}, 115, North-Holland, Amsterdam, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Research trend of metal-organic frameworks for magnetic refrigeration materials application

Kim, S., Son, K., Oh, H.

Korean Journal of Materials Research, 30(3):136-141, Materials Society of Korea, Seoul, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


Magnetic Anisotropy in Thin Layers of (Mn,Zn)Fe2O4 on SrTiO3 (001)
Magnetic Anisotropy in Thin Layers of (Mn,Zn)Fe2O4 on SrTiO3 (001)

Denecke, R., Welke, M., Huth, P., Gräfe, J., Brachwitz, K., Lorenz, M., Grundmann, M., Ziese, M., Esquinazi, P. D., Goering, E., Schütz, G., Schindler, K., Chassé, A.

Physica Status Solidi (b), 257(7):1900627, 2020 (article)

Abstract
Herein, a ferrimagnetic manganese zinc ferrite (Mn0.5Zn0.5Fe2O4) film with a thickness of 200 nm is prepared without a buffer layer on strontium titanate (001) (SrTiO3) using pulsed laser deposition. Its magnetic properties are investigated using superconducting quantum interference device (SQUID), X-ray absorption spectroscopy with subsequent X-ray magnetic circular dichroism (XMCD) and magneto-optic Kerr effect (MOKE). Hysteresis loops derived from SQUID exhibits bulk-like properties. This can further be confirmed by bulk-like XMCD spectra. In remanent magnetization, an in-plane magnetization with basically no out-of-plane component is found. The magnetic moments derived by the sum rule formalism from the XMCD data are in good agreement to the magnetization observed by SQUID and MOKE. XMCD as well as MOKE reveal an in-plane angular fourfold magnetic anisotropy with the easy direction along [110] for (Mn0.5Zn0.5)Fe2O4 on SrTiO3. The element-specific magnetic moments from XMCD show a stronger contribution of Fe to the anisotropy than of Mn and distinct contributions of the orbital moments.

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


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In situ x-ray diffraction and spectro-microscopic study of ALD protected copper films

Dogan, G., Sanli, U. T., Hahn, K., Müller, L., Gruhn, H., Silber, C., Schütz, G., Grévent, C., Keskinbora, K.

ACS Applied Materials and Interfaces, 12(29):33377-33385, American Chemical Society, Washington, DC, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


HOTA: A Higher Order Metric for Evaluating Multi-Object Tracking
HOTA: A Higher Order Metric for Evaluating Multi-Object Tracking

Luiten, J., Osep, A., Dendorfer, P., Torr, P., Geiger, A., Leal-Taixe, L., Leibe, B.

International Journal of Computer Vision (IJCV), 2020 (article)

Abstract
Multi-Object Tracking (MOT) has been notoriously difficult to evaluate. Previous metrics overemphasize the importance of either detection or association. To address this, we present a novel MOT evaluation metric, HOTA (Higher Order Tracking Accuracy), which explicitly balances the effect of performing accurate detection, association and localization into a single unified metric for comparing trackers. HOTA decomposes into a family of sub-metrics which are able to evaluate each of five basic error types separately, which enables clear analysis of tracking performance. We evaluate the effectiveness of HOTA on the MOTChallenge benchmark, and show that it is able to capture important aspects of MOT performance not previously taken into account by established metrics. Furthermore, we show HOTA scores better align with human visual evaluation of tracking performance.

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pdf [BibTex]

pdf [BibTex]


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Visual-Inertial Mapping with Non-Linear Factor Recovery

Usenko, V., Demmel, N., Schubert, D., Stückler, J., Cremers, D.

IEEE Robotics and Automation Letters (RA-L), 5, 2020, accepted for presentation at IEEE International Conference on Robotics and Automation (ICRA) 2020, to appear, arXiv:1904.06504 (article)

Abstract
Cameras and inertial measurement units are complementary sensors for ego-motion estimation and environment mapping. Their combination makes visual-inertial odometry (VIO) systems more accurate and robust. For globally consistent mapping, however, combining visual and inertial information is not straightforward. To estimate the motion and geometry with a set of images large baselines are required. Because of that, most systems operate on keyframes that have large time intervals between each other. Inertial data on the other hand quickly degrades with the duration of the intervals and after several seconds of integration, it typically contains only little useful information. In this paper, we propose to extract relevant information for visual-inertial mapping from visual-inertial odometry using non-linear factor recovery. We reconstruct a set of non-linear factors that make an optimal approximation of the information on the trajectory accumulated by VIO. To obtain a globally consistent map we combine these factors with loop-closing constraints using bundle adjustment. The VIO factors make the roll and pitch angles of the global map observable, and improve the robustness and the accuracy of the mapping. In experiments on a public benchmark, we demonstrate superior performance of our method over the state-of-the-art approaches.

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[BibTex]

[BibTex]


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How to functionalise metal-organic frameworks to enable guest nanocluster embedment

King, J., Zhang, L., Doszczeczko, S., Sambalova, O., Luo, H., Rohman, F., Phillips, O., Borgschulte, A., Hirscher, M., Addicoat, M., Szilágyi, P. A.

{Journal of Materials Chemistry A}, 8(9):4889-4897, Royal Society of Chemistry, Cambridge, UK, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Magnetic and microstructural properties of anisotropic MnBi magnets compacted by spark plasma sintering

Chen, Y., Gregori, G., Rheingans, B., Huang, W., Kronmüller, H., Schütz, G., Goering, E.

{Journal of Alloys and Compounds}, 830, Elsevier B.V., Lausanne, Switzerland, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Biocompatible magnetic micro- and nanodevices: Fabrication of FePt nanopropellers and cell transfection

Kadiri, V. M., Bussi, C., Holle, A. W., Son, K., Kwon, H., Schütz, G., Gutierrez, M. G., Fischer, P.

Advanced Materials, 32(25), Wiley-VCH, Weinheim, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Metal organic frameworks as tunable linear magnets

Son, K., Kim, R. K., Kim, S., Schütz, G., Choi, K. M., Oh, H.

Physica Status Solidi A, 217(12), Wiley-VCH, Weinheim, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Generation and characterization of focused helical x-ray beams

Loetgering, L., Baluktsian, M., Keskinbora, K., Horstmeyer, R., Wilhein, T., Schütz, G., Eikema, K. S. E., Witte, S.

Science Advances, 6(7), American Association for the Advancement of Science, 2020 (article)

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Generation and characterization of focused helical x-ray beams link (url) DOI [BibTex]

Generation and characterization of focused helical x-ray beams link (url) DOI [BibTex]


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Materials for hydrogen-based energy storage - past, recent progress and future outlook

Hirscher, M., Yartys, V. A., Baricco, M., Bellosta von Colbe, J., Blanchard, D., Bowman Jr., R. C., Broom, D. P., Buckley, C. E., Chang, F., Chen, P., Cho, Y. W., Crivello, J., Cuevas, F., David, W. I. F., de Jongh, P. E., Denys, R. V., Dornheim, M., Felderhoff, M., Filinchuk, Y., Froudakis, G. E., Grant, D. M., Gray, E. M., Hauback, B. C., He, T., Humphries, T. D., Jensen, T. R., Kim, S., Kojima, Y., Latroche, M., Li, H., Lotostskyy, M. V., Makepeace, J. W., M\oller, K. T., Naheed, L., Ngene, P., Noréus, D., Nyg\aard, M. M., Orimo, S., Paskevicius, M., Pasquini, L., Ravnsbaek, D. B., Sofianos, M. V., Udovic, T. J., Vegge, T., Walker, G. S., Webb, C. J., Weidenthaler, C., Zlotea, C.

{Journal of Alloys and Compounds}, 827, Elsevier B.V., Lausanne, Switzerland, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


{Thermal nucleation and high-resolution imaging of submicrometer magnetic bubbles in thin thulium iron garnet films with perpendicular anisotropy}
Thermal nucleation and high-resolution imaging of submicrometer magnetic bubbles in thin thulium iron garnet films with perpendicular anisotropy

Büttner, F., Mawass, M. A., Bauer, J., Rosenberg, E., Caretta, L., Avci, C. O., Gräfe, J., Finizio, S., Vaz, C. A. F., Novakovic, N., Weigand, M., Litzius, K., Förster, J., Träger, N., Groß, F., Suzuki, D., Huang, M., Bartell, J., Kronast, F., Raabe, J., Schütz, G., Ross, C. A., Beach, G. S. D.

{Physical Review Materials}, 4(1), American Physical Society, College Park, MD, 2020 (article)

Abstract
Ferrimagnetic iron garnets are promising materials for spintronics applications, characterized by ultralow damping and zero current shunting. It has recently been found that few nm-thick garnet films interfaced with a heavy metal can also exhibit sizable interfacial spin-orbit interactions, leading to the emergence, and efficient electrical control, of one-dimensional chiral domain walls. Two-dimensional bubbles, by contrast, have so far only been confirmed in micrometer-thick films. Here, we show by high resolution scanning transmission x-ray microscopy and photoemission electron microscopy that submicrometer bubbles can be nucleated and stabilized in ∼25-nm-thick thulium iron garnet films via short heat pulses generated by electric current in an adjacent Pt strip, or by ultrafast laser illumination. We also find that quasistatic processes do not lead to the formation of a bubble state, suggesting that the thermodynamic path to reaching that state requires transient dynamics. X-ray imaging reveals that the bubbles have Bloch-type walls with random chirality and topology, indicating negligible chiral interactions at the garnet film thickness studied here. The robustness of thermal nucleation and the feasibility demonstrated here to image garnet-based devices by x-rays both in transmission geometry and with sensitivity to the domain wall chirality are critical steps to enabling the study of small spin textures and dynamics in perpendicularly magnetized thin-film garnets.

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DOI [BibTex]

DOI [BibTex]


{Real-space imaging of confined magnetic skyrmion tubes}
Real-space imaging of confined magnetic skyrmion tubes

Birch, M. T., Cortés-Ortuño, D., Turnbull, L. A., Wilson, M. N., Groß, F., Träger, N., Laurenson, A., Bukin, N., Moody, S. H., Weigand, M., Schütz, G., Popescu, H., Fan, R., Steadman, P., Verezhak, J. A. T., Balakrishnan, G., Loudon, J. C., Twitchett-Harrison, A. C., Hovorka, O., Fangohr, H., Ogrin, F., Gräfe, J., Hatton, P. D.

Nature Communications, 11, pages: 1726, 2020 (article)

Abstract
Magnetic skyrmions are topologically nontrivial particles with a potential application as information elements in future spintronic device architectures. While they are commonly portrayed as two dimensional objects, in reality magnetic skyrmions are thought to exist as elongated, tube-like objects extending through the thickness of the host material. The study of this skyrmion tube state (SkT) is vital for furthering the understanding of skyrmion formation and dynamics for future applications. However, direct experimental imaging of skyrmion tubes has yet to be reported. Here, we demonstrate the real-space observation of skyrmion tubes in a lamella of FeGe using resonant magnetic x-ray imaging and comparative micromagnetic simulations, confirming their extended structure. The formation of these structures at the edge of the sample highlights the importance of confinement and edge effects in the stabilisation of the SkT state, opening the door to further investigation into this unexplored dimension of the skyrmion spin texture.

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


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Current-induced dynamical tilting of chiral domain walls in curved microwires

Finizio, S., Wintz, S., Mayr, S., Huxtable, A. J., Langer, M., Bailey, J., Burnell, G., Marrows, C. H., Raabe, J.

Applied Physics Letters, 116(18), American Institute of Physics, Melville, NY, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Highly effective hydrogen isotope separation through dihydrogen bond on Cu(I)-exchanged zeolites well above liquid nitrogen temperature

Xiong, R., Zhang, L., Li, P., Luo, W., Tang, T., Ao, B., Sang, G., Chen, C., Yan, X., Chen, J., Hirscher, M.

Chemical Engineering Journal, 391, Elsevier, Lausanne, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Ferrimagnetic skyrmions in topological insulator/ferrimagnet heterostructures

Wu, H., Groß, F., Dai, B. Q., Lujan, D., Razavi, S. A., Zhang, P., Liu, Y. X., Sobotkiewich, K., Förster, J., Weigand, M., Schütz, G., Li, X. Q., Gräfe, J., Wang, K. L.

Advanced Materials, 32(34), Wiley-VCH, Weinheim, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Room temperature ferromagnetism driven by Ca-doped BiFeO3 multiferroic functional material

Marzouk, M., Hashem, H. M., Soltan, S., Ramadan, A. A.

{Journal of Materials Science: Materials in Electronics}, 31(7):5599-5607, Springer, Norwell, MA, 2020 (article)

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DOI [BibTex]

DOI [BibTex]

2016


{Coercivity scaling in antidot lattices in Fe, Ni, and NiFe thin films}
Coercivity scaling in antidot lattices in Fe, Ni, and NiFe thin films

Gräfe, J., Schütz, G., Goering, E. J.

{Journal of Magnetism and Magnetic Materials}, 419, pages: 517-520, North-Holland, Amsterdam, 2016 (article)

Abstract
Antidot lattices can be used to artificially engineer magnetic properties in thin films, however, a conclusive model that describes the coercivity enhancement in this class of magnetic nano-structures has so far not been found. We prepared Fe, Ni, and NiFe thin films and patterned each with 21 square antidot lattices with different geometric parameters and measured their hysteretic behavior. On the basis of this extensive dataset we are able to provide a model that can describe both the coercivity scaling over a wide range of geometric lattice parameters and the influence of different materials.

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DOI Project Page [BibTex]

2016


DOI Project Page [BibTex]


Probabilistic Duality for Parallel Gibbs Sampling without Graph Coloring
Probabilistic Duality for Parallel Gibbs Sampling without Graph Coloring

Mescheder, L., Nowozin, S., Geiger, A.

Arxiv, 2016 (article)

Abstract
We present a new notion of probabilistic duality for random variables involving mixture distributions. Using this notion, we show how to implement a highly-parallelizable Gibbs sampler for weakly coupled discrete pairwise graphical models with strictly positive factors that requires almost no preprocessing and is easy to implement. Moreover, we show how our method can be combined with blocking to improve mixing. Even though our method leads to inferior mixing times compared to a sequential Gibbs sampler, we argue that our method is still very useful for large dynamic networks, where factors are added and removed on a continuous basis, as it is hard to maintain a graph coloring in this setup. Similarly, our method is useful for parallelizing Gibbs sampling in graphical models that do not allow for graph colorings with a small number of colors such as densely connected graphs.

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pdf [BibTex]


γ‐Conicein und Coniin aus Geflecktem Schierling
γ‐Conicein und Coniin aus Geflecktem Schierling

Puidokait, M., Graefe, J., Sehl, A., Steinke, K., Siehl, H., Zeller, K., Sicker, D., Berger, S.

Chemie in unserer Zeit, 50(6):382-391, 2016 (article)

Abstract
Sokrates ist das bekannteste Opfer eines 399 v.d.Z. in Athen mit dem Schierlingsbecher vollstreckten Todesurteils. Tödlich giftig wirkten darin Alkaloide wie γ‐Conicein und Coniin, die nach Mäuseurin riechen. Schaudern macht, dass dieses Gift sein Opfer allmählich bei vollem Bewusstsein lähmt und dass es kein Gegengift gibt. Auch für Tiere auf der Weide ist Gefleckter Schierling stark giftig. Daher verdrängt die Landwirtschaft den Schierling. Coniin war das erste synthetisierte Alkaloid (Ladenburg, 1886). Schierlingsgifte werden heute nicht medizinisch angewendet. Wir beschreiben die Isolierung von γ‐Conicein und Coniin aus angebautem und wildem Gefleckten Schierling und analysieren ihre Struktur spektroskopisch. Dieser Artikel gehört zur Reihe über die Isolierung und Spektroskopie von Naturstoffen und ergänzt die Kapitel des Buchs “Classics in Spectroscopy” von S. Berger und D. Sicker (Wiley‐VCH 2009). Er beruht auf studentischen Arbeiten ebenso wie auf unserem präparativen Engagement als sie betreuende Wissenschaftler.

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DOI [BibTex]

DOI [BibTex]


{Geometric control of the magnetization reversal in antidot lattices with perpendicular magnetic anisotropy}
Geometric control of the magnetization reversal in antidot lattices with perpendicular magnetic anisotropy

Gräfe, J., Weigand, M., Träger, N., Schütz, G., Goering, E. J., Skripnik, M., Nowak, U., Haering, F., Ziemann, P., Wiedwald, U.

{Physical Review B}, 93(10), American Physical Society, Woodbury, NY, 2016 (article)

Abstract
While the magnetic properties of nanoscaled antidot lattices in in-plane magnetized materials have widely been investigated, much less is known about the microscopic effect of hexagonal antidot lattice patterning on materials with perpendicular magnetic anisotropy. By using a combination of first-order reversal curve measurements, magnetic x-ray microscopy, and micromagnetic simulations we elucidate the microscopic origins of the switching field distributions that arise from the introduction of antidot lattices into out-of-plane magnetized GdFe thin films. Depending on the geometric parameters of the antidot lattice we find two regimes with different magnetization reversal processes. For small antidots, the reversal process is dominated by the exchange interaction and domain wall pinning at the antidots drives up the coercivity of the system. On the other hand, for large antidots the dipolar interaction is dominating which leads to fragmentation of the system into very small domains that can be envisaged as a basis for a bit patterned media.

mms

DOI Project Page Project Page [BibTex]

DOI Project Page Project Page [BibTex]


{Magnetic switching of nanoscale antidot lattices}
Magnetic switching of nanoscale antidot lattices

Wiedwald, U., Gräfe, J., Lebecki, K. M., Skripnik, M., Haering, F., Schütz, G., Ziemann, P., Goering, E., Nowak, U.

{Beilstein Journal of Nanotechnology}, 7, pages: 733-750, Beilstein-Institut, Frankfurt am Main, 2016 (article)

Abstract
We investigate the rich magnetic switching properties of nanoscale antidot lattices in the 200 nm regime. In-plane magnetized Fe, Co, and Permalloy (Py) as well as out-of-plane magnetized GdFe antidot films are prepared by a modified nanosphere lithography allowing for non-close packed voids in a magnetic film. We present a magnetometry protocol based on magneto-optical Kerr microscopy elucidating the switching modes using first-order reversal curves. The combination of various magnetometry and magnetic microscopy techniques as well as micromagnetic simulations delivers a thorough understanding of the switching modes. While part of the investigations has been published before, we summarize these results and add significant new insights in the magnetism of exchange-coupled antidot lattices.

mms

DOI Project Page [BibTex]

DOI Project Page [BibTex]


Map-Based Probabilistic Visual Self-Localization
Map-Based Probabilistic Visual Self-Localization

Brubaker, M. A., Geiger, A., Urtasun, R.

IEEE Trans. on Pattern Analysis and Machine Intelligence (PAMI), 2016 (article)

Abstract
Accurate and efficient self-localization is a critical problem for autonomous systems. This paper describes an affordable solution to vehicle self-localization which uses odometry computed from two video cameras and road maps as the sole inputs. The core of the method is a probabilistic model for which an efficient approximate inference algorithm is derived. The inference algorithm is able to utilize distributed computation in order to meet the real-time requirements of autonomous systems in some instances. Because of the probabilistic nature of the model the method is capable of coping with various sources of uncertainty including noise in the visual odometry and inherent ambiguities in the map (e.g., in a Manhattan world). By exploiting freely available, community developed maps and visual odometry measurements, the proposed method is able to localize a vehicle to 4m on average after 52 seconds of driving on maps which contain more than 2,150km of drivable roads.

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pdf Project Page [BibTex]

pdf Project Page [BibTex]


{Combined first-order reversal curve and x-ray microscopy investigation of magnetization reversal mechanisms in hexagonal antidot lattices}
Combined first-order reversal curve and x-ray microscopy investigation of magnetization reversal mechanisms in hexagonal antidot lattices

Gräfe, J., Weigand, M., Stahl, C., Träger, N., Kopp, M., Schütz, G., Goering, E. J., Haering, F., Ziemann, P., Wiedwald, U.

{Physical Review B}, 93(1), American Physical Society, Woodbury, NY, 2016 (article)

Abstract
The magnetization reversal in nanoscaled antidot lattices is widely investigated to understand the tunability of the magnetic anisotropy and the coercive field through nanostructuring of thin films. By investigating highly ordered focused ion beam milled antidot lattices with a combination of first-order reversal curves and magnetic x-ray microscopy, we fully elucidate the magnetization reversal along the distinct orientations of a hexagonal antidot lattice. This combination proves especially powerful as all partial steps of this complex magnetization reversal can be identified and subsequently imaged. Through this approach we discovered several additional steps that were neglected in previous studies. Furthermore, by imaging the microscopic magnetization state during each reversal step, we were able to link the coercive and interaction fields determined by the first-order reversal curve method to true microscopic magnetization configurations and determine their origin.

mms

DOI Project Page Project Page [BibTex]

DOI Project Page Project Page [BibTex]


{Switching probabilities of magnetic vortex core reversal studied by table top magneto optic Kerr microscopy}
Switching probabilities of magnetic vortex core reversal studied by table top magneto optic Kerr microscopy

Dieterle, G., Gangwar, A., Gräfe, J., Noske, M., Förster, J., Woltersdorf, G., Stoll, H., Back, C. H., Schütz, G.

{Applied Physics Letters}, 108(2), American Institute of Physics, Melville, NY, 2016 (article)

Abstract
We have studied vortex core reversal in a single submicron Permalloy disk by polar Kerr microscopy. A sophisticated lock-in-technique based on repetitive switching of the magnetic vortex core and a continuous calibration allows for a reliable determination of the switching probability. This highly sensitive method facilitates the detection of a change in the magnetic moment of the tiny magnetic vortex core which is about 1.5 × 10−17 A m2. We have investigated vortex core switching caused by excitation of the vortex core gyromode with varying frequencies and amplitudes. The frequency range in which switching occurs was found to broaden with increasing excitation amplitude, whereby the highest frequency in this range shifts stronger to higher frequencies than the lowest frequency to lower frequencies. The experimental results are in good agreement with micromagnetic simulations.

mms

DOI [BibTex]

DOI [BibTex]


{Pinned orbital moments - A new contribution to magnetic anisotropy}
Pinned orbital moments - A new contribution to magnetic anisotropy

Audehm, P., Schmidt, M., Brück, S., Tietze, T., Gräfe, J., Macke, S., Schütz, G., Goering, E.

{Scientific Reports}, 6, Nature Publishing Group, London, UK, 2016 (article)

mms

DOI [BibTex]

DOI [BibTex]


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Momentum Control with Hierarchical Inverse Dynamics on a Torque-Controlled Humanoid

Herzog, A., Rotella, N., Mason, S., Grimminger, F., Schaal, S., Righetti, L.

Autonomous Robots, 40(3):473-491, 2016 (article)

Abstract
Hierarchical inverse dynamics based on cascades of quadratic programs have been proposed for the control of legged robots. They have important benefits but to the best of our knowledge have never been implemented on a torque controlled humanoid where model inaccuracies, sensor noise and real-time computation requirements can be problematic. Using a reformulation of existing algorithms, we propose a simplification of the problem that allows to achieve real-time control. Momentum-based control is integrated in the task hierarchy and a LQR design approach is used to compute the desired associated closed-loop behavior and improve performance. Extensive experiments on various balancing and tracking tasks show very robust performance in the face of unknown disturbances, even when the humanoid is standing on one foot. Our results demonstrate that hierarchical inverse dynamics together with momentum control can be efficiently used for feedback control under real robot conditions.

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link (url) DOI [BibTex]


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Ab initio theory for ultrafast magnetization dynamics with a dynamic band structure

Müller, B. Y., Haag, M., Fähnle, M.

{Journal of Magnetism and Magnetic Materials}, 414, pages: 14-18, North-Holland, Amsterdam, 2016 (article)

mms

DOI [BibTex]

DOI [BibTex]


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High-resolution analysis of currents at low-angle grain boundaries in YBCO thin films using magnetooptics and magnetic x-ray microscopy

Ruoß, S., Stahl, C., Bayer, J., Schütz, G., Albrecht, J., Laviano, F.

{IEEE Transactions on Applied Superconductivity}, 26(3), IEEE, New York, NY, 2016 (article)

mms

DOI [BibTex]

DOI [BibTex]


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Three-dimensional character of the magnetization dynamics in magnetic vortex structures: Hybridization of flexure gyromodes with spin waves

Noske, M., Stoll, H., Fähnle, M., Gangwar, A., Woltersdorf, G., Slavin, A., Weigand, M., Dieterle, G., Förster, J., Back, C. H., Schütz, G.

{Physical Review Letters}, 117(3), American Physical Society, Woodbury, N.Y., 2016 (article)

mms

DOI [BibTex]

DOI [BibTex]


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Nanostructured materials for solid-state hydrogen storage: A review of the achievement of COST Action MP1103

Callini, E., Aguey-Zinsou, K., Ahuja, R., Ares, J. R., Bals, S., Biliskov, N., Chakraborty, S., Charalambopoulou, G., Chaudhary, A., Cuevas, F., Dam, B., de Jongh, P., Dornheim, M., Filinchuk, Y., Grbovic-Novakovic, J., Hirscher, M., Jensen, T. R., Jensen, P. B., Novakovic, N., Lai, Q., Leardini, F., Gattia, D. M., Pasquini, L., Steriotis, T., Turner, S., Vegge, T., Züttel, A., Montone, A.

{International Journal of Hydrogen Energy}, 41(32):14404-14428, Elsevier, Amsterdam, 2016 (article)

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DOI [BibTex]

DOI [BibTex]


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Magnetic X-ray microscopy at low temperatures - Visualization of flux distributions in superconductors

Stahl, C., Ruoß, S., Weigand, M., Bechtel, M., Schütz, G., Albrecht, J.

{AIP Conference Proceedings}, 1696, AIP Publishing, Melville, NY, 2016 (article)

mms

DOI [BibTex]

DOI [BibTex]