Role of tip size, orientation, and structural relaxations in first-principles studies of magnetic exchange force microscopy and spin-polarized scanning tunneling microscopy

Using first-principles calculations based on density functional theory (DFT), we investigate the exchange interaction between a magnetic tip and a magnetic sample which is detected in magnetic exchange force microscopy (MExFM) and also occurs in spin-polarized scanning tunneling microscopy (SP-STM) experiments. As a model tip-sample system, we choose Fe tips and one monolayer Fe on W(001) which exhibits a checkerboard antiferromagnetic structure and has been previously studied with both SP-STM and MExFM. We calculate the exchange forces and energies as a function of tip-sample distance using different tip models ranging from single Fe atoms to Fe pyramids consisting of up to 14 atoms. We find that modelling the tip by a single Fe atom leads to qualitatively different tip-sample interactions than using clusters consisting of several atoms. Increasing the cluster size changes the calculated forces quantitatively enhancing the detectable exchange forces. Rotating the tip with respect to the surface unit cell has only a small influence on the tip-sample forces. Interestingly, the exchange forces on the tip atoms in the nearest and next-nearest layers from the apex atom are non-negligible and can be opposite to that on the apex atom for a small tip. In addition, the apex atom interacts not only with the surface atoms underneath but also with nearest-neighbors in the surface. We find that structural relaxations of tip and sample due to their interaction depend sensitively on the magnetic alignment of the two systems. As a result the onset of significant exchange forces is shifted towards larger tip-sample separations which facilitates their measurement in MExFM. At small tip-sample separations, structural relaxations of tip apex and surface atoms can either enhance or reduce the magnetic contrast measured in SP-STMComment: 14 pages, 13 figure

Giant Magneto-Impedance Effect in Multilayer Thin Film Sensors

Over the last couple of decades, the Giant Magneto-Impedance (GMI) effect has become a well-known phenomenon, especially for its use in magnetic field sensing applications. Discussed in this paper will be a comprehensive summary of the fundamental theory behind the GMI effect, as well as the design, fabrication, and test of multilayer thin film GMI sensors. In recent research, multilayer GMI sensors have been shown to obtain GMI sensitives ranging from 10 – 100 times more than that of currently in industry Giant Magnetoresistance (GMR) sensors, comparable to that of its bulk microwire counterpart. To investigate this, a tri-layer film stack sensor, consisting of a conductive Copper layer sandwiched between two ferromagnetic Permalloy layers, was designed and fabricated in RIT’s SMFL. Sensor performance relied heavily on two main components: structural design of the sensors (i.e. geometry and materials) and the ability to induce transverse anisotropic magnetic domain alignment. Standard CMOS processing techniques were used during fabrication to induce this transverse domain alignment. This discussion will highlight some of the challenges faced during processing and their impact on sensor performance. Despite these challenges, sensors were successfully fabricated with an added step to incorporate a Titanium seed layer beneath the first layer of Permalloy. With process modifications to consider, a maximum GMI Ratio of 0.028% and sensitivity of 0.010%/Oe for a frequency of 10 MHz was obtained. While sensor performance was less than optimal, the overall goal of qualifying and quantifying the GMI effect in multilayer thin film sensors was achieved

Structurally-driven magnetic state transition of biatomic Fe chains on Ir(001)

Using first-principles calculations, we demonstrate that the magnetic exchange interaction and the magnetocrystalline anisotropy of biatomic Fe chains grown in the trenches of the 5x1 reconstructed Ir(001) surface depend sensitively on the atomic arrangement of the Fe atoms. Two structural configurations have been considered which are suggested from recent experiments. They differ by the local symmetry and the spacing between the two strands of the biatomic Fe chain. Since both configurations are very close in total energy they may coexist in experiment. We have investigated collinear ferro- and antiferromagnetic solutions as well as a collinear state with two moments in one direction and one in the opposite direction (up-down-up-state). For the structure with a small interchain spacing, there is a strong exchange interaction between the strands and the ferromagnetic state is energetically favorable. In the structure with larger spacing, the two strands are magnetically nearly decoupled and exhibit antiferromagnetic order along the chain. In both cases, due to hybridization with the Ir substrate the exchange interaction along the chain axis is relatively small compared to freestanding biatomic iron chains. The easy magnetization axis of the Fe chains also switches with the structural configuration and is out-of-plane for the ferromagnetic chains with small spacing and along the chain axis for the antiferromagnetic chains with large spacing between the two strands. Calculated scanning tunneling microscopy images and spectra suggest the possibility to experimentally distinguish between the two structural and magnetic configurations.Comment: Accepted for publication in Physical Review

How organized is deep convection over Germany?

Deep moist convection shows a tendency to organize into mesoscale structures. To be able to understand the potential effect of convective organization on the climate, one needs first to characterize organization. In this study, we systematically characterize the organizational state of convection over Germany based on two years of cloud-top observations derived from the Meteosat Second Generation satellite and of precipitation cores detected by the German C-band radar network. The organizational state of convection is characterized by commonly employed organization indices, which are mostly based on the object numbers, sizes and nearest-neighbour distances. According to the organization index Iorg, cloud tops and precipitation cores are found to be in an organized state for 69% and 92% of the time, respectively. There is an increase in rainfall when the number of objects and their sizes increase, independently of the organizational state. Case-studies of specific days suggest that convectively organized states correspond to either local multi-cell clusters, with less numerous, larger objects close to each other, or to scattered clusters, with more numerous, smaller organized objects spread out over the domain. For those days, simulations are performed with the large-eddy model ICON with grid spacings of 625, 312 and 156?m. Although the model underestimates rainfall and shows a too large cold cloud coverage, the organizational state is reasonably well represented without significant differences between the grid spacings

Strategische Lösungen für einen Stadtverkehr in der Zukunft. Perspektiven für den öffentlichen und den Individual-Verkehr

Setzt sich die bisherige Entwicklung fort, werden unsere Städte der Zukunft durch einen MIV bestimmt sein, der sich an seinen Kapazitätsgrenzen befindet, und einen ÖV, der finanziell heruntergekommen ist. Deshalb wird ein neuer vernetzter Stadtverkehr benötigt. Die Diskussion über den Stadtverkehr der Zukunft geht vom Gegensatz zwischen ÖPNV und MIV sowie von gegebenen Stadtstrukturen aus. Aber auch Städte leben und verändern sich. Dazu zählen die Des- und Reurbanisierung im Stadtentwicklungszyklus. Aber auch die Gesellschaft, ihr Lebensstil, ihre Raumnutzungssysteme und Wettbewerbsbedingungen verändern sich. Deshalb sind isolierte Wirkungsprognosen vergebens und die Berücksichtigung von Verhaltenstendenzen dynamischer Systementwicklung unverzichtbar. Telekommunikation und elektronischer motorisierter Individualverkehr bilden neue Wachstumsbereiche. Die Problembereiche Kernstadt und Fernverkehr werden durch drastische Qualitäts¬verbesserungen (wie Fußgängerzonen und Schnellverkehre) entschärft werden. Dafür rückt der Zwischenraum zwischen Fußgängerzonen und anderen Metropolen in den Mittelpunkt. Deshalb wird erst ein Netzwerk vieler Neben- und Mittelzentren hoher Autarkie dem ÖPNV eine Zukunft verschaffen. Soll eine Teilung der Stadt in - wenn auch unterschiedlich große - Flächen extrem differenzierter Erreichbarkeitsniveaus vermieden werden, bietet sich langfristig ein Zusammenwirken von vier Systemelementen an: (1) eines vollautomatischen Massenverkehrs zwischen Zentren durch elektronisch kopplungsfähige Einheiten, (2) eines teilautomatischen ÖV mit neuartigen Bussen, (3) eines teilautomatischen privaten Verkehrs mit Einzelfahrzeugen und (4) des manuellen privaten Verkehrs mit verbesserten traditionellen Kraftfahrzeugen. Bei entsprechender Kompatibilität könnten Fahrzeuge der Kategorien (1) bis (3) die gemeinsame Infrastruktur benutzen. Maßnahmen, die schon heute solche Entwicklung zukunftsfreundlich berücksichtigen, werden genannt.If current developments continue, our cities of tomorrow will be characterised by a motorized individual transport system (MIT) at its capacity limits and a financially run-down public transport system (PT). Therefore a new integrated urban transport system will become necessary. The debate surrounding "urban future transport" is based on the conflict between MIT and PT and on existing urban structures. But cities also live and change as desurbanization and reurbanization show. The same holds true for societies, their lifestyle, land use structure and their competitive conditions. That’s why isolated forecasts of transport systems are pointless and it is essential to include and consider behavioural tendencies of dynamic systems. Telecommunication and electronic MIT are new growth areas. The problem areas of innercity and long-distance transport will be defused by dramatic quality improvements, such as pedestrian zones and express services. However, the space between pedestrian areas and other urban centres connected by fast transport services becomes of central importance. Here only a network of many subcenters and regional centres with a high degree of self-sufficiency will give PT a future. If a division of our cities into areas of extremely different levels of accessibility is to be avoided a new urban transport system is required. It consists of four system elements which interact in the long run: (1) fully automatic mass transport between conurbations by units able to dock electronically, (2) semi-automatic public transport with innovative busses (3) semi-automatic private transport by individual vehicles, and (4) manually controlled private transport in improved cars. Accordingly compatible vehicles of the categories (1) to (3) can share the common infrastructure. Development and construction activities of today are listed which can already take this future system into account

Anforderungen an zukunftsfähige Verkehrskonzepte für Berlin-Brandenburg

Zukunftsfähig können Verkehrskonzepte nur sein, wenn sie Nachfragetrends nutzen. Wichtige Tatbestände und Systemtendenzen für die Region werden in 20 Thesen präsentiert. Dabei werden kurz- und langfristige Lösungsmuster erkennbar. Mit dem Nahziel, innovative Betätigungsfelder für Planer, Fahrzeugbauer und Betreiber zu finden, wird das bereits jetzt Machbare oder sogar längst Überfällige auf vier Politikfelder (verdichtete Gebiete, ländliche Regionen, Umland von Agglomerationen, Konversionsmaßnahmen und Neuausweisung von Flächen) verdichtet. Fernziel ist ein neues Verkehrssystem als zukunftsfähige Entwicklung der Verkehrs- und Siedlungsstruktur von Berlin-Brandenburg.Transport concepts can only be sustainable if expected long-term demand trends are taken into account. Important facts and tendencies of system dynamics for the region are discussed in 20 theses. Short- and long-term solution patterns can be r ecognized. The immediate aim is to open up new fields of activity for planners, vehicle manufacturers and operators. Thereby future opportunities or even overdue changes are compressed into four policy areas. They are (1) densely populated areas, (2) rural regions, (3) the "commuter belt" outside the congested areas, (4) conversion measures, rezoning and „brownfield redevelopment“. The long-term objective is a new transport system as sustainable development of the transport and settlement structure of Berlin-Brandenburg

The Development of the German Railroad System

Dieser Aufsatz beschreibt die historische Entwicklung des deutschen Eisenbahnsystems. Das Ergebnis besteht in einem dynamischen Durchsetzungsmechanismus von Verkehrsinnovationen. Er erweist sich als verblüffend stabiles Muster der Verkehrsevolution. Unabdingbare Voraussetzung sind Grenzen der bestehenden Angebotsstruktur. Diese Grenzen werden in einer ersten Stufe durch Angebotsausweitungen der traditionellen Techniken weiter hinausgeschoben, ohne den Nachfragedruck allerdings stoppen zu können. Das Innovative (Produktinnovationen) setzt sich erst in der Krise durch. Innovative Technologien, die in Nischen vorhanden sind und auf die das System zurückgreifen kann, werden dann entscheidend. Vor dem Hintergrund stark ausgeweiteter traditioneller Technik setzt sich dann ein neues System durch und ermöglicht dabei weiteres Verkehrswachstum. In diesen Hauptprozess ist ein Nebenprozess flankierender Strukturwandlungen eingebettet.The paper describes the historical development of the German railway system. The result is a dynamic mechanism of success for transport innovations. This simplified model proves to be an astonishing stable pattern of transport evolution. The indispensable preconditions are limits of the existing system, which is unable to meet rising demand. Although at first those limits can be extended by improvements to traditional technology, these ultimately cannot cope with the demand pressures. Product innovations will only succeed in crises. It is in this crisis that innovations are able to break through. Innovative technologies that already exist in niches can be accessed by the system and become crucial. Against the background of expanded traditional technologies the new system makes the breakthrough enabling additional traffic growth. A secondary process of accompanying structural adjustments is embedded in this main process