Search

Since magnetic vortex cores have two ground states, they are candidates for digital memory bits in future magnetic random access memory (MRAM) devices. Vortex core switching can be induced by exciting the gyrotropic eigenmode, e.g., by applying cyclic magnetic fields with typically a sub-gigahertz frequency. However, recent studies reveal that other modes exist that can be excited at higher frequencies, but still lead to switching with relatively small field amplitudes. Here, we perform a full scan of the frequency/amplitude parameter space to explore such excitation modes. The enormous amount of simulations can only be performed in an acceptable time span when the micromagnetic (CPU) simulations are drastically accelerated. To this aim, we developed MUMAX, a GPU-based software tool that speeds up micromagnetic simulations with about two orders of magnitude compared to standard CPU micromagnetic tools. By exploiting MUMAX's numerical power we were able to explore new switching opportunities at moderate field amplitudes in the frequency range between 5 and 12 GHz.

Unidirectional switching of the magnetic vortex core can be achieved in micron-sized ferromagnetic platelets by excitation of the gyrotropic mode of the vortex structure with in-plane rotating magnetic fields. Circulating fields with a switchable sense of rotation (clockwise, CW or counter clockwise, CCW) have been generated on a micrometer length scale at frequencies up to 1 GHz by two orthogonal electric RF currents with 90° phase shift flowing through crossed but not isolated striplines. Decoupling of these currents is realized by balanced symmetric RF sources. The amplitudes of the rotating magnetic fields and their spatial distributions are calculated and the stripline geometry is discussed. By taking advantage of this technique, unidirectional vortex core reversal by excitation with CW or CCW rotating magnetic fields has been observed by time resolved scanning transmission X-ray microscopy. An area with reversed magnetization, the “dip,” was observed near the vortex core before vortex core reversal.

Magnetic fusion aims at providing CO"2 free energy for the 21st century and well beyond. However, the success of the international fusion experiment ITER (currently under construction) will depend to a large degree on the value of the so-called energy confinement time. One of the most advanced tools describing the underlying physical processes is the highly scalable (up to at least 32,768 cores) plasma turbulence code GENE. GENE solves a set of nonlinear partial integro-differential equations in five-dimensional phase space by means of the method of lines, with a 4th order explicit Runge-Kutta scheme for time integration. To maximize its efficiency, the code computes the eigenspectrum of the linearized equation to determine the largest possible timestep which maintains the stability of the method. This requires the computation of the largest (in terms of its magnitude) eigenvalue of a complex, non-Hermitian matrix whose size may range from a few millions to even a billion. SLEPc, the Scalable Library for Eigenvalue Problem Computations, is used to effectively compute this part of the spectrum. Additionally, eigenvalue computations can provide new insight into the properties of plasma turbulence. The latter is driven by a number of different unstable modes, including dominant and subdominant ones, that can be determined employing SLEPc. This computation is more challenging from the numerical point of view, since these eigenvalues can be considered interior, and also because the linearized operator is available only in implicit form. We analyze the feasibility of different strategies for computing these modes, including matrix-free spectral transformation as well as harmonic projection methods.

We present a combined theoretical and experimental study, investigating the origin of the enhanced non-adiabaticity of magnetic vortex cores. Scanning transmission X-ray microscopy is used to image the vortex core gyration dynamically to measure the non-adiabaticity with high precision, including a high confidence upper bound. Using both numerical computations and analytical derivations, we show that the large non-adiabaticity parameter observed experimentally can be explained by the presence of local spin currents arising from a texture-induced emergent Hall effect. This enhanced non-adiabaticity is only present in two- and three-dimensional magnetic textures such as vortices and skyrmions and absent in one-dimensional domain walls, in agreement with experimental observations., 6 pages, 3 figures

The magnetic vortex structure, an important ground state configuration in micron and sub-micron sized ferromagnetic thin film platelets, is characterized by a curling in-plane magnetization and, in the center, a minuscule region with out-of-plane magnetization, the vortex core, which points either up or down. It has already been demonstrated that the vortex core polarity can be reversed with external AC magnetic fields, frequency-tuned to the (sub-GHz) gyrotropic eigenmode or to (multi-GHz) azimuthal spin wave modes, where reversal times in the sub-ns regime can be realized. This fast vortex core switching may also be of technological interest as the vortex core polarity can be regarded as one data bit. Here we experimentally demonstrate that unidirectional vortex core reversal by excitation with sub-100 ps long orthogonal monopolar magnetic pulse sequences is possible in a wide range of pulse lengths and amplitudes. The application of such short digital pulses is the favourable excitation scheme for technological applications. Measured phase diagrams of this unidirectional, spin wave mediated vortex core reversal are in good qualitative agreement with phase diagrams obtained from micromagnetic simulations. The time dependence of the reversal process, observed by time-resolved scanning transmission X-ray microscopy indicates a switching time of 100 ps and fits well with our simulations. The origin of the asymmetric response to clockwise and counter clockwise excitation which is a prerequisite for reliable unidirectional switching is discussed, based on the gyromode - spin wave coupling.

Spin-wave-mediated vortex core reversal is investigated using x-ray magnetic circular dichroism in a time-resolved scanning transmission x-ray microscope in combination with micromagnetic simulations. The evolution of magnetization dynamics in Permalloy disks is imaged after excitation with one-period rotating rf-field bursts. Selective unidirectional switching of the vortex polarity is achieved due to different switching thresholds for clockwise and counterclockwise excitations. A lower limit of about 200 ps for the unidirectional switching time after the onset of a one-period burst is found for the geometry of our disks ($1.6\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in diameter and 50 nm in thickness).

Micron-sized magnetic platelets in the flux-closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations, which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin-wave excitation the concept of a critical velocity as the switching condition has to be modified., Micron and submicron-sized magnetic platelets in a vortex configuration may be useful in micromagnetics and spintronics applications. Kammerer et al. show that a fast unidirectional vortex core reversal process occurs when azimuthal spin wave modes are excited at GHz frequency.