Center for Computational Nanoscience

Vienna, Austria

Center for Computational Nanoscience

Vienna, Austria
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Weinberger P.,Center for Computational Nanoscience
Philosophical Magazine | Year: 2011

It is shown that by 'stacking' together a semi-infinite sample subsystem with a semi-infinite tip subsystem, difficulties with respect to a common Fermi level for the whole system can approximately be overcome in all those cases when the substrate (serving as lead) and the second lead are different materials. Based on this procedure by means of the spin-polarized (fully) relativistic screened Korringa-Kohn-Rostocker method and an equivalent Kubo equation, theoretical spin-polarized STM spectra for Pt(111)/Co n/Cr 15 W 22/Cu(111) with respect to an applied external magnetic field are evaluated in terms of difference conductivities as a function of the corresponding free energy. These spectra are interpreted using layer-resolved contributions to the difference conductivities in order to indicate which parts of the sample dominate changes in the tunneling current caused by changing the orientation of the magnetization. Also shown are estimates of the time-scales to switch from perpendicular to in-plane and vice versa. All investigated properties suggest that different situations apply when the number of Co layers on top of Pt(111) is increased from one to three. © 2011 Taylor & Francis.


Weinberger P.,Center for Computational Nanoscience
Philosophical Magazine | Year: 2015

The original contributions of Arthur Cayley to the Philosophical Magazine on group theory and his trees are revisited and to some extend reinterpreted. Both topics were and are of enormous importance not only in physics (group theory, graph theory), but also in quite a few other disciplines as diverse as information technology or, for example, linguistics (trees, graph theory). In order to show that these two topics originally arose from interests in the theory of permutations also Cayleys Mousetrap game is briefly mentioned. © 2015 Taylor & Francis.


Weinberger P.,Center for Computational Nanoscience
Philosophical Magazine | Year: 2012

The history of the Periodic Table of Elements covers almost 300 years: from the discovery of phosphorus in 1669 to that of Lawrencium in 1961. Before that period, only elements such as copper, silver, gold, mercury, lead and of course carbon were known, which already had accompanied the history of mankind for millennia. The first "wave" of discoveries of new metals occurred at the close of the eighteenth and the beginning of the nineteenth centuries and was almost entirely based on "wet chemistry". Because this period, in which no physics-related techniques of identification were available, seems to be now so remote, it is worthwhile to recall these discoveries in the context and style of their time by making use of the Philosophical Magazine archives. © 2012 Taylor & Francis.


Weinberger P.,Center for Computational Nanoscience
Philosophical Magazine | Year: 2013

Based on the idea that a scientific journal is also an "agora" (Greek: market place) for the exchange of ideas and scientific concepts, the history of thermodynamics between 1800 and 1910 as documented in the Philosophical Magazine Archives is uncovered. Famous scientists such as Joule, Thomson (Lord Kelvin), Clausius, Maxwell or Boltzmann shared this forum. Not always in the most friendly manner. It is interesting to find out, how difficult it was to describe in a scientific (mathematical) language a phenomenon like "heat", to see, how long it took to arrive at one of the fundamental principles in physics: entropy. Scientific progress started from the simple rule of Boyle and Mariotte dating from the late eighteenth century and arrived in the twentieth century with the concept of probabilities. Thermodynamics was the driving intellectual force behind the industrial revolution, behind the enormous social changes caused by this revolution. The history of thermodynamics is a fascinating story, which also gives insights into the mechanism that seem to govern science. © 2013 Copyright Taylor and Francis Group, LLC.


Weinberger P.,Center for Computational Nanoscience | Weinberger P.,Max Planck Institute of Microstructure Physics
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

By introducing paths on the hypersurface of the band-energy contribution (free energy) to the magnetic anisotropy energy and of corresponding differences in the zz -like elements of the electric conductivity tensor, viewed as an implicit function of the free energy, experimental spin-polarized scanning tunnel microscope (STM) data, namely, the recording of a differential current as a function of an externally applied field, can be analyzed and facilitate a direct comparison between theory and experiment. It is shown that along different paths rather different flipping times of the direction of the magnetization can occur. In particular, discussed are in terms of "theoretical experiments" horizontal and vertical movements of the tip, the influence of the magnetic properties of the tip as well as of typical samples. Furthermore, it is claimed that because of different time scales in a (presently still fictional) time-resolved mode for spin-polarized STM experiments even particular paths on these hypersurfaces could be mapped out. © 2010 The American Physical Society.


Weinberger P.,Center for Computational Nanoscience | Weinberger P.,New York University
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

In viewing layer-resolved components of physical properties as a function of multiples of the interlayer spacing and by using a Taylor expansion structural relaxation effects can be described not only for the free-energy part of the magnetic anisotropy energy but directly also for electric and magneto-optical transport properties. Examples for such a procedure are shown for the anisotropy energy of free surfaces and magnetic tunneling junctions, for tunneling currents and for permittivities, which in turn determine Kerr angles. Considered as a test case, the already well-studied magnetic anisotropy energy of Co2 /Cu (111) turns out to be in excellent agreement with very recent experiment data. © 2010 The American Physical Society.


Wieser R.,University of Hamburg | Vedmedenko E.Y.,University of Hamburg | Weinberger P.,Center for Computational Nanoscience | Wiesendanger R.,University of Hamburg
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Current driven motions of domain walls in ferromagnetic, cylindrical nanowires are investigated by solving the Landau-Lifshitz-Gilbert equation including the adiabatic and nonadiabatic spin torque terms. Depending on the type of domain wall (transverse or vortex) and on the nonadiabaticity parameter β different behavior of the domain wall motion has been found. A transverse domain wall shows a linear motion accompanied by a clock- or anticlockwise precession of the wall depending on the relation between the nonadiabaticity parameter β and the Gilbert damping α. For α=β no rotation occurs. Further, an easy way to derive the velocity equation is presented. In the case of the vortex domain wall an unexpected chirality effect has been found. Depending on the sense of rotation either a straight motion or a reversal of the rotation followed by a straight motion can be seen. Furthermore, due to the impossibility of a Walker breakdown the averaged velocity of the domain wall v is zero for all currents with β=0 while the motion is damped by the emission of spin waves for higher currents and β>α. © 2010 The American Physical Society.


Weinberger P.,Center for Computational Nanoscience
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

For magnetic double layers separated by a nonmagnetic metallic spacer the precession of the magnetization in only one magnetic slab ("spin pump") and in both of them ("spin sink") is described using the fully relativistic spin-polarized screened Korringa-Kohn-Rostocker method. It is found that both semiaxes of the ellipses that form the base planes of precessional cones can be identified with particular anisotropy energies. Slopes along particular paths of the obtained free energy hypersurfaces are then used to estimate in terms of the Landau-Lifshitz-Gilbert equation the minimal times to move from a chosen initial state to a particular final state. Furthermore, fully relativistic spin-polarized calculations by means of the Kubo equation of the corresponding zz-like elements of the conductivity tensor show that the current along the surface normal is bigger for "spin pumps" than for "spin sinks." For moderately small external fields the time scale for the individual processes is in the femtoregime. © 2011 American Physical Society.


Weinberger P.,Center for Computational Nanoscience | Weinberger P.,New York University
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The switching properties of a spin valve with perpendicular anisotropy is investigated in terms of the fully relativistic screened Korringa-Kohn-Rostoker method, the corresponding Kubo-Greenwood equation of electric transport, and the Landau-Lifshitz-Gilbert equation. In excellent agreement with very recent experimental data, it is found that (a) the inverse of the switching time is linear with respect to the critical current, (b) the ratio of critical currents for switching from parallel (P) to antiparallel (AP) and from AP to P is about 2, and (c) the tiny magnetoresistance found in experiment is a consequence of the perpendicular anisotropy. © 2010 The American Physical Society.


Weinberger P.,Center for Computational Nanoscience | Weinberger P.,New York University
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The switching properties of a prototype system containing a spin valve as well as a tunneling junction are investigated by means of a spin-polarized fully relativistic approach. Shown are the changes in the free energy and in the magnetoresistance when the orientation of the magnetization in the magnetic slabs of such a system is changed individually. It is demonstrated that the existence of a stable and a metastable state, a necessary condition for a switching device, is essentially determined by the free energy contributions of the interfaces of that magnetic slab that is part of the spin valve as well as of the tunneling junction. Furthermore, in estimating individual switching times, it is found that most of the gain in magnetoresistance occurs within a time range of about 20 ps, the time to achieve complete switching, however, being about 1.5 ns, which is close to what is found in recent experimental results. © 2011 American Physical society.

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