Deposition Order Dependent Magnetization Reversal in Pressure Graded Co/Pd films

Magnetization reversal mechanisms and depth-dependent magnetic profile have been investigated in Co/Pd thin films magnetron-sputtered under continuously varying pressure with opposite deposition orders. For samples grown under increasing pressure, magnetization reversal is dominated by domain nucleation, propagation and annihilation; an anisotropy gradient is effectively established, along with a pronounced depth-dependent magnetization profile. However, in films grown under decreasing pressure, disorders propagate vertically from the bottom high-pressure region into the top low-pressure region, impeding domain wall motion and forcing magnetization reversal via rotation; depth-dependent magnetization varies in an inverted order, but the spread is much suppressed.Comment: 17 pages, 4 figure

Direct Observation of Magnetic Gradient in Co/Pd Pressure-Graded Media

Magnetometry and neutron scattering have been used to study the magnetic properties of pressure graded Co/Pd multilayers. The grading of the multilayer structure was done by varying the deposition pressure during sputtering of the samples. Magnetic depth profiling by polarized neutron reflectometry directly shows that for pressure-graded samples, the magnetization changes significantly from one pressure region to the next, while control samples sputtered at uniform pressure exhibit essentially uniform magnetic depth profiles. Complementary magnetometry results suggest that the observed graded magnetic profiles are due in part to a decrease in saturation magnetization for regions deposited at progressively higher pressure. Increased deposition pressure is shown to increase coercivity, and for graded samples, the absence of discrete steps in the hysteresis loops implies exchange coupling among regions deposited at different pressures.Comment: accepted to Journal of Applied Physic

Interfacial-Redox-Induced Tuning of Superconductivity in YBa2Cu3O7-δ.

Solid-state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here, we report a simple, scalable approach allowing for manipulation of the superconducting transition in optimally doped YBa2Cu3O7-δ (YBCO) films via a chemically driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective step toward total ionic tuning of superconductivity in oxides, an interface-induced effect that goes well into the quasi-bulk regime, opening-up possibilities for electric field manipulation

Ionic Tuning of Cobaltites at the Nanoscale

Control of materials through custom design of ionic distributions represents a powerful new approach to develop future technologies ranging from spintronic logic and memory devices to energy storage. Perovskites have shown particular promise for ionic devices due to their high ion mobility and sensitivity to chemical stoichiometry. In this work, we demonstrate a solid-state approach to control of ionic distributions in (La,Sr)CoO$_{3}$ thin films. Depositing a Gd capping layer on the perovskite film, oxygen is controllably extracted from the structure, up-to 0.5 O/u.c. throughout the entire 36 nm thickness. Commensurate with the oxygen extraction, the Co valence state and saturation magnetization show a smooth continuous variation. In contrast, magnetoresistance measurements show no-change in the magnetic anisotropy and a rapid increase in the resistivity over the same range of oxygen stoichiometry. These results suggest significant phase separation, with metallic ferromagnetic regions and oxygen-deficient, insulating, non-ferromagnetic regions, forming percolated networks. Indeed, X-ray diffraction identifies oxygen-vacancy ordering, including transformation to a brownmillerite crystal structure. The unexpected transformation to the brownmillerite phase at ambient temperature is further confirmed by high-resolution scanning transmission electron microscopy which shows significant structural - and correspondingly chemical - phase separation. This work demonstrates room-temperature ionic control of magnetism, electrical resistivity, and crystalline structure in a 36 nm thick film, presenting new opportunities for ionic devices that leverage multiple material functionalities

Vertically Graded Anisotropy in Co/Pd Multilayers

Depth-grading of magnetic anisotropy in perpendicular magnetic media has been predicted to reduce the field required to write data without sacrificing thermal stability. To study this prediction, we have produced Co/Pd multilayers with depth-dependent Co layer thickness. Polarized neutron reflectometry shows that the thickness grading results in a corresponding magnetic anisotropy gradient. Magnetometry reveals that the anisotropy gradient promotes domain nucleation upon magnetization reversal - a clear experimental demonstration of the effectiveness of graded anisotropy for reducing write-field

Solar Sails for Planetary Defense and High-Energy Missions

20 years after the successful ground deployment test of a (20 m)² solar sail at DLR Cologne, and in the light of the upcoming U.S. NEAscout mission, we provide an overview of the progress made since in our mission and hardware design studies as well as the hardware built in the course of our solar sail technology development. We outline the most likely and most efficient routes to develop solar sails for useful missions in science and applications, based on our developed ‘now-term’ and near-term hardware as well as the many practical and managerial lessons learned from the DLR-ESTEC GOSSAMER Roadmap. Mission types directly applicable to planetary defense include single and Multiple NEA Rendezvous ((M)NR) for precursor, monitoring and follow-up scenarios as well as sail-propelled head-on retrograde kinetic impactors (RKI) for mitigation. Other mission types such as the Displaced L1 (DL1) space weather advance warning and monitoring or Solar Polar Orbiter (SPO) types demonstrate the capability of near-term solar sails to achieve asteroid rendezvous in any kind of orbit, from Earth-coorbital to extremely inclined and even retrograde orbits. Some of these mission types such as SPO, (M)NR and RKI include separable payloads. For one-way access to the asteroid surface, nanolanders like MASCOT are an ideal match for solar sails in micro-spacecraft format, i.e. in launch configurations compatible with ESPA and ASAP secondary payload platforms. Larger landers similar to the JAXA-DLR study of a Jupiter Trojan asteroid lander for the OKEANOS mission can shuttle from the sail to the asteroids visited and enable multiple NEA sample-return missions. The high impact velocities and re-try capability achieved by the RKI mission type on a final orbit identical to the target asteroid‘s but retrograde to its motion enables small spacecraft size impactors to carry sufficient kinetic energy for deflection

Decentralized and Pulse-based Clock Synchronization in SpaceWire Networks for Time-triggered Data Transfers

Time-triggered communication is widely used throughout several industry do- mains, primarily for reliable and real-time capable data transfers. However, existing time-triggered technologies are designed for terrestrial usage and not directly applicable to space applications due to the harsh environment. In- stead, specific hardware must be developed to deal with thermal, mechanical, and especially radiation effects. SpaceWire, as an event-triggered communication technology, has been used for years in a large number of space missions. Its moderate complexity, her- itage, and transmission rates up to 400 MBits/s are one of the main ad- vantages and often without alternatives for on-board computing systems of spacecraft. At present, real-time data transfers are either achieved by prior- itization inside SpaceWire routers or by applying a simplified time-triggered approach. These solutions either imply problems if they are used inside dis- tributed on-board computing systems or in case of networks with more than a single router are required. This work provides a solution for the real-time problem by developing a novel clock synchronization approach. This approach is focused on being compatible with distributed system structures and allows time-triggered data transfers. A significant difference to existing technologies is the remote clock estimation by the use of pulses. They are transferred over the network and remove the need for latency accumulation, which allows the incorporation of standardized SpaceWire equipment. Additionally, local clocks are controlled decentralized and provide different correction capabilities in order to handle oscillator induced uncertainties. All these functionalities are provided by a developed Network Controller (NC), able to isolate the attached network and to control accesses

Dezentralisierte und Puls-basierte Uhrensynchronisation in SpaceWire Netzwerken für zeitgesteuerten Datentransfer

Time-triggered communication is widely used throughout several industry do- mains, primarily for reliable and real-time capable data transfers. However, existing time-triggered technologies are designed for terrestrial usage and not directly applicable to space applications due to the harsh environment. In- stead, specific hardware must be developed to deal with thermal, mechanical, and especially radiation effects. SpaceWire, as an event-triggered communication technology, has been used for years in a large number of space missions. Its moderate complexity, her- itage, and transmission rates up to 400 MBits/s are one of the main ad- vantages and often without alternatives for on-board computing systems of spacecraft. At present, real-time data transfers are either achieved by prior- itization inside SpaceWire routers or by applying a simplified time-triggered approach. These solutions either imply problems if they are used inside dis- tributed on-board computing systems or in case of networks with more than a single router are required. This work provides a solution for the real-time problem by developing a novel clock synchronization approach. This approach is focused on being compatible with distributed system structures and allows time-triggered data transfers. A significant difference to existing technologies is the remote clock estimation by the use of pulses. They are transferred over the network and remove the need for latency accumulation, which allows the incorporation of standardized SpaceWire equipment. Additionally, local clocks are controlled decentralized and provide different correction capabilities in order to handle oscillator induced uncertainties. All these functionalities are provided by a developed Network Controller (NC), able to isolate the attached network and to control accesses.Zeitgesteuerte Datenübertragung ist in vielen Industriezweigen weit verbreitet, primär für zuverlässige und echtzeitfähige Kommunikation. Bestehende Technologien sind jedoch für den terrestrischen Gebrauch konzipiert und aufgrund der rauen Umgebung nicht direkt auf Weltraumanwendungen anwendbar. Stattdessen wird spezielle Hardware entwickelt, um Strahlungseffekten zu widerstehen sowie thermischen und mechanischen Belastungen standzuhalten. SpaceWire wurde als ereignisgesteuerte Kommunikationstechnologie entwickelt und wird seit Jahren in einer Vielzahl von Weltraummissionen verwendet. Dessen erfolgreiche Verwendung, überschaubare Komplexität, und Übertragungsraten bis zu 400 MBit/s sind einige seiner Hauptvorteile. Derzeit werden Datenübertragungen in Echtzeit entweder durch Priorisierung innerhalb von SpaceWire Router erreicht, oder durch Anwendung von vereinfachten zeitgesteuerten Ansätzen. Diese Lösungen implizieren entweder Probleme in verteilten Systemarchitekturen oder in SpaceWire Netzwerken mit mehreren Routern. Diese Arbeit beschreibt eine Uhrensynchronisation, die bestimmte Eigenschaften von SpaceWire ausnutzt, um das Echtzeitproblem zu lösen. Der Ansatz ist dabei kompatibel mit verteilten Systemstrukturen und ermöglicht eine zeitgesteuerte Datenübertragung