arXiv subjects

Peter Fischer

Publications and source records attributed to Peter Fischer.

45 records · Page 3Linked to original sources

arXiv2013

Wave modes of collective vortex gyration in dipolar-coupled-dot-array magnonic crystals

Lattice vibration modes are collective excitations in periodic arrays of atoms or molecules. These modes determine novel transport properties in solid crystals. Analogously, in periodical arrangements of magnetic vortex-state disks, collective vortex motions have been predicted. Here, we experimentally observe wave modes of collective vortex gyration in one-dimensional (1D) chains of periodic disks using time-resolved scanning transmission x-ray microscopy. The observed modes are interpreted based on micromagnetic simulation and numerical calculation of coupled Thiele equations. Dispersion of the modes is found to be strongly affected by both vortex polarization and chirality ordering, as revealed by the explicit analytical form of 1D infinite chains. A thorough understanding thereof is fundamental both for lattice vibrations and vortex dynamics, which we demonstrate for 1D magnonic crystals. Such magnetic disk arrays with vortex-state ordering, referred to as magnetic metastructure, offer potential implementation into information processing devices.

cond-mat.mes-hall↗

arXiv2011

Polarization-selective vortex-core switching by orthogonal Gaussian-pulse currents

We experimentally demonstrate low-power-consumption vortex-core switching in magnetic nanodisks using tailored rotating magnetic fields that are produced with orthogonal and unipolar Gaussian-pulse currents. Optimal width of the orthogonal pulses and their time delay are found to be determined only by the angular eigenfrequency ω_D for a given vortex-state disk of its polarization p, such that σ = 1/ω_D and Δt = πp/2ω_D, as studied from analytical and micromagnetic numerical calculations. The estimated optimal pulse parameters are in good agreements with the experimentally found results. This work provides a foundation for energy-efficient information recording in vortex-core cross-point architecture.

cond-mat.mtrl-sci↗

arXiv2010

Memory-bit selective recording in vortex-core cross-point architecture

In our earlier work [Appl. Phys. Lett. 92, 022509 (2008)], we proposed nonvolatile vortex random access memory (VRAM) based on the energetically stable twofold ground state of vortex-core magnetizations as information carrier. Here we experimentally demonstrate reliable memory bit selection and low-power-consumption recording in a two-by-two vortex-state dot array. The bit selection and core switching is made by flowing currents along two orthogonal addressing electrode lines chosen among the other crossed electrodes. Tailored pulse-type rotating magnetic fields are used for efficiently switching a vortex core only at the intersection of the two orthogonal electrodes. This robust mechanism provides reliable bit selection and information writing operations in a potential VRAM device.

cond-mat.mtrl-sci↗

arXiv2010

Tunable energy transfer between dipolar-coupled magnetic disks by stimulated vortex gyration

A wide variety of coupled harmonic oscillators exist in nature1. Coupling between different oscillators allows for the possibility of mutual energy transfer between them2-4 and the information-signal propagation5,6. Low-energy input signals and their transport with low-energy dissipation are the key technical factors in the design of information processing devices7. Here, utilizing the concept of coupled oscillators, we experimentally demonstrated a robust new mechanism for energy transfer between spatially separated dipolar-coupled magnetic disks - stimulated vortex gyration. Direct experimental evidence was obtained by time-resolved soft X-ray microscopy. The rate of energy transfer from one disk to the other was deduced from the two normal modes' frequency splitting caused by dipolar interaction. This mechanism provides the advantages of tunable energy transfer rate, low-power input signal, and low-energy dissipation for magnetic elements with negligible damping. Coupled vortex-state disks are promising candidates for information-signal processing devices that operate above room temperature.

cond-mat.mtrl-sci↗

arXiv2008

Universal criterion and phase diagram for switching a magnetic vortex core in soft magnetic nanodots

The universal criterion for ultrafast vortex-core switching between the up- and down-core bistates in soft magnetic nanodots was investigated by micromagnetic simulations along with analytical calculations. Vortex-core switching occurs whenever the velocity of vortex-core motion reaches the critical velocity that is expressed as (e.g. m/s for Permalloy), where Aex is the exchange stiffness, and is the gyromagnetic ratio. On the basis of the above results, phase diagrams for the vortex-core switching event and switching times with respect to both the amplitude and frequency of applied circularly rotating magnetic field were calculated, which offer practical guidance for implementing nanodots in vortex states into future solid-state information-storage devices.

cond-mat.mtrl-sci↗

arXiv2008

Probing the spin polarization of current by soft X-ray imaging of current-induced magnetic vortex dynamics

Time-resolved soft X-ray transmission microscopy is applied to image the current-induced resonant dynamics of the magnetic vortex core realized in a micronsized Permalloy disk. The high spatial resolution better than 25 nm enables us to observe the resonant motion of the vortex core. The result also provides the spin polarization of the current to be 0.67 +/-0.16 for Permalloy by fitting the experimental results with an analytical model in the framework of the spin-transfer torque.

cond-mat.mtrl-sci↗

arXiv2006

Fixed points of quantum gravity in extra dimensions

We study quantum gravity in more than four dimensions with renormalisation group methods. We find a non-trivial ultraviolet fixed point in the Einstein-Hilbert action. The fixed point connects with the perturbative infrared domain through finite renormalisation group trajectories. We show that our results for fixed points and related scaling exponents are stable. If this picture persists at higher order, quantum gravity in the metric field is asymptotically safe. We discuss signatures of the gravitational fixed point in models with low-scale gravity and compact extra dimensions.

hep-th↗

arXiv2006

Fixed points of quantum gravity in higher dimensions

We study quantum gravity in more than four dimensions by means of an exact functional flow. A non-trivial ultraviolet fixed point is found in the Einstein-Hilbert theory. It is shown that our results for the fixed point and universal scaling exponents are stable. If the fixed point persists in extended truncations, quantum gravity in the metric field is asymptotically safe. We indicate physical consequences of this scenario in phenomenological models with low-scale quantum gravity and large extra dimensions.

hep-th↗

arXiv2003

Single Photon Counting X-ray Imaging with Si and CdTe Single Chip Pixel Detectors and Multichip Pixel Modules

Multichip modules (MCM) with 4 single photon counting MPEC 2.3 chips bump bonded to 1.3 cm x 1.3 cm large CdTe and Si semiconductor sensors as well as to single chip pixel detectors have been successfully built and operated. The MPEC 2.3 chip provides a pixel count rate up to 1 MHz with a large dynamic range of 18 bit, 2 counters and energy windowing with continuously adjustable thresholds. Each MPEC has 32 x 32 pixels of 200 um x 200 um pixel size. For a MCM the 4 chips are arranged in a 2 x 2 array which leads to a 64 x 64 sensor pixel geometry. The MCM construction is described, and the imaging performance of the different detectors is shown. As readout system a newly developed USB system has been used.

physics.ins-det↗