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Author: Ravishankar, Rachappa × Has files: Yes ×

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    Direct imaging of nanoscale field-driven domain wall oscillations in Landau structures
    (Cambridge : RSC Publ., 2022) Singh, Balram; Ravishankar, Rachappa; Otálora, Jorge A.; Soldatov, Ivan; Schäfer, Rudolf; Karnaushenko, Daniil; Neu, Volker; Schmidt, Oliver G.
    Linear oscillatory motion of domain walls (DWs) in the kHz and MHz regime is crucial when realizing precise magnetic field sensors such as giant magnetoimpedance devices. Numerous magnetically active defects lead to pinning of the DWs during their motion, affecting the overall behavior. Thus, the direct monitoring of the domain wall's oscillatory behavior is an important step to comprehend the underlying micromagnetic processes and to improve the magnetoresistive performance of these devices. Here, we report an imaging approach to investigate such DW dynamics with nanoscale spatial resolution employing conventional table-top microscopy techniques. Time-averaged magnetic force microscopy and Kerr imaging methods are applied to quantify the DW oscillations in Ni81Fe19 rectangular structures with Landau domain configuration and are complemented by numeric micromagnetic simulations. We study the oscillation amplitude as a function of external magnetic field strength, frequency, magnetic structure size, thickness and anisotropy and understand the excited DW behavior as a forced damped harmonic oscillator with restoring force being influenced by the geometry, thickness, and anisotropy of the Ni81Fe19 structure. This approach offers new possibilities for the analysis of DW motion at elevated frequencies and at a spatial resolution of well below 100 nm in various branches of nanomagnetism.
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    Integrated molecular diode as 10 MHz half-wave rectifier based on an organic nanostructure heterojunction
    ([London] : Nature Publishing Group UK, 2020) Li, Tianming; Bandari, Vineeth Kumar; Hantusch, Martin; Xin, Jianhui; Kuhrt, Robert; Ravishankar, Rachappa; Xu, Longqian; Zhang, Jidong; Knupfer, Martin; Zhu, Feng; Yan, Donghang; Schmidt, Oliver G.
    Considerable efforts have been made to realize nanoscale diodes based on single molecules or molecular ensembles for implementing the concept of molecular electronics. However, so far, functional molecular diodes have only been demonstrated in the very low alternating current frequency regime, which is partially due to their extremely low conductance and the poor degree of device integration. Here, we report about fully integrated rectifiers with microtubular soft-contacts, which are based on a molecularly thin organic heterojunction and are able to convert alternating current with a frequency of up to 10 MHz. The unidirectional current behavior of our devices originates mainly from the intrinsically different surfaces of the bottom planar and top microtubular Au electrodes while the excellent high frequency response benefits from the charge accumulation in the phthalocyanine molecular heterojunction, which not only improves the charge injection but also increases the carrier density.
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    Shape-Controlled Flexible Microelectronics Facilitated by Integrated Sensors and Conductive Polymer Actuators
    (Weinheim : Wiley-VCH Verlag GmbH & Co. KGaA, 2021) Rivkin, Boris; Becker, Christian; Akbar, Farzin; Ravishankar, Rachappa; Karnaushenko, Dmitriy; Naumann, Ronald; Mirhajivarzaneh, Aaleh; Medina-Sánchez, Mariana; Karnaushenko, Daniil; Schmidt, Oliver G.
    The next generation of biomedical tools requires reshapeable electronics to closely interface with biological tissues. This will offer unique mechanical properties and the ability to conform to irregular geometries while being robust and lightweight. Such devices can be achieved with soft materials and thin-film structures that are able to reshape on demand. However, reshaping at the submillimeter scale remains a challenging task. Herein, shape-controlled microscale devices are demonstrated that integrate electronic sensors and electroactive polymer actuators. The fast and biocompatible actuators are capable of actively reshaping the device into flat or curved geometries. The curvature and position of the devices are monitored with strain or magnetic sensors. The sensor signals are used in a closed feedback loop to control the actuators. The devices are wafer-scale microfabricated resulting in multiple functional units capable of grasping, holding, and releasing biological tissues, as demonstrated with a neuronal bundle.