arXiv subjects

J. Zweck

Publications and source records attributed to J. Zweck.

4 recordsLinked to original sources

arXiv2018

Phase programming in coupled spintronic oscillators

Neurons in the brain behave as a network of coupled nonlinear oscillators processing information by rhythmic activity and interaction. Several technological approaches have been proposed that might enable mimicking the complex information processing of neuromorphic computing, some of them relying on nanoscale oscillators. For example, spin torque oscillators are promising building blocks for the realization of artificial high-density, low-power oscillatory networks (ON) for neuromorphic computing. The local external control and synchronization of the phase relation of oscillatory networks are among the key challenges for implementation with nanotechnologies. Here we propose a new method of phase programming in ONs by manipulation of the saturation magnetization, and consequently the resonance frequency of a single oscillator via Joule heating by a simple DC voltage input. We experimentally demonstrate this method in a pair of stray field coupled magnetic vortex oscillators. Since this method only relies on the oscillatory behavior of coupled oscillators, and the temperature dependence of the saturation magnetization, it allows for variable phase programming in a wide range of geometries and applications that can help advance the efforts of high frequency neuromorphic spintronics up to the GHz regime.

physics.app-ph↗

arXiv2017

Entropy-limited topological protection of skyrmions

Magnetic skyrmions are topologically protected whirls that decay through singular magnetic configurations known as Bloch points. We have used Lorentz transmission electron microscopy to infer the energetics associated with the topological decay of magnetic skyrmions far from equilibrium in the chiral magnet Fe$_{1-x}$Co$_x$Si. We observed that the life time $\tau$ of the skyrmions depends exponentially on temperature, $\tau \sim \tau_0 \, e^{\Delta E/k_B T}$. The prefactor $\tau_0$ of this Arrhenius law changes by more than 30 orders of magnitude for small changes of magnetic field reflecting a substantial reduction of the life time of skyrmions by entropic effects and thus an extreme case of enthalpy-entropy compensation. Such compensation effects, being well-known across many different scientific disciplines, affect topological transitions and thus topological protection on an unprecedented level.

cond-mat.mes-hall↗

arXiv2017

Dynamical defects in rotating magnetic skyrmion lattices

The chiral magnet Cu$_{2}$OSeO$_{3}$ hosts a skyrmion lattice, that may be equivalently described as a superposition of plane waves or lattice of particle-like topological objects. A thermal gradient may break up the skyrmion lattice and induce rotating domains raising the question which of these scenarios better describes the violent dynamics at the domain boundaries. Here we show that in an inhomogeneous temperature gradient caused by illumination in a Lorentz Transmission Electron Microscope different parts of the skyrmion lattice can be set into motion with different angular velocities. Tracking the time dependence we show that the constant rearrangement of domain walls is governed by dynamic 5-7 defects arranging into lines. An analysis of the associated defect density is described by Frank's equation and agrees well with classical 2D-Monte Carlo simulations. Fluctuations of boundaries show surge-like rearrangement of skyrmion clusters driven by defect rearrangement consistent with simulations treating skyrmions as point particles. Our findings underline the particle character of the skyrmion.

cond-mat.mes-hall↗

arXiv2009

Characterisation of Ferromagnetic Contacts to Carbon Nanotubes

We present an investigation of different thin-film evaporated ferromagnetic materials for their suitability as electrodes in individual single-wall and multi-wall carbon nanotube-based spin devices. Various electrode shapes made from permalloy (Ni_{81}Fe_{19}), the diluted ferromagnet PdFe, and PdFe/Fe bilayers are studied for both their micromagnetic properties and their contact formation to carbon nanotubes. Suitable devices are tested in low-temperature electron transport measurements, displaying the typical tunneling magnetoresistance of carbon nanotube pseudo spin valves.

cond-mat.mes-hall↗