M. Zelent, J. T
obik, M. Krawczyk, K. Y. Guslienko, and M. Mruczkiewicz

Phys. Status Solidi RRL 1700259, 1-5 (2017)

Magnetic skyrmions are stable, circularly symmetric inhomogeneous magnetization configurations. These topologically protected states of nanometer size have a potential to provide useful solution for low‐power, highdensity data storage and processing. In their Letter (article no. 1700259) Zelent et al. study the influence of confinement and dipole interactions on the skyrmion stability. In the cover image the authors present the calculated energy profile of skyrmion states as a function of skyrmion size, which shows two minima. At these minima the two skyrmions of different sizes can stabilize at the same dot. These states, shown also in the image, are stable at zero magnetic field and it is feasible to switch between them in dependence on the magnetic reversal history. This result can open a new route to develop an efficient skyrmion memory, where information would be coded as a skyrmion's size. The background of the entire image is the 3D visualization of the magnetization vectors in the skyrmion configuration.

J. Rychły, J. W. Kłos and M. Krawczyk

J. Phys. D 49, 175001 (2016)

We investigated the lifetime of spin wave (SW) eigenmodes in periodic and quasiperiodic sequences of Py and Co wires. These materials differ significantly in damping coefficients, therefore, the spatial distribution of the mode’s amplitude within the structure is important for the lifetime of collective SW excitations. Modes of the lower frequencies prefer to concentrate in Py wires, because of the lower ferromagnetic resonance (FMR) frequency for this material. This inhomogeneous distribution of amplitude of modes (with lower amplitude in material of higher damping and with higher amplitude in material of lower damping) is preferable for extending the lifetime of the collective excitations beyond the volume average of lifetimes for solid materials. We established the relation between the profile of the mode and its lifetime for periodic and quasiperiodic structures. We also performed comparative studies in order to find the differences resulting from complexity of the structure and enhancement of localization in the quasiperiodic system on the lifetime of SWs.

Fig.1. (a) and (b) Lifetime of SW eigenmodes versus their frequencies for (a) Fibonacci structure—MQ and (b) periodic structure—MC, consisting of 55 wires of 91 nm width. The green (grey) horizontal series of points show the lifetime of eigenmodes obtained for solid Co (Py) homogeneous film of the same thickness and external width as considered for MC and MQ structures.

V. V. Kruglyak, C. S. Davies, V. S. Tkachenko, O. Yu. Gorobets, Yu. I. Gorobets and A. N. Kuchko

J. Phys. D 50, 094003 (2017)

We report a theoretical study of the spin-wave band spectrum of magnonic crystals formed by stacking thin-film magnetic layers, with general assumptions about the properties of the interfaces between the layers. We find that the band gaps are a ubiquitous attribute of a weakened interlayer coupling and a finite interface anisotropy (pinning). The band gaps in such systems represent a legacy of the discrete spin-wave spectrum of the individual magnetic layers periodically stacked to form the magnonic crystal rather than resulting from Bragg scattering. At the same time, magnonic crystals with band gaps due to the Bragg scattering can be described by natural boundary conditions (i.e. those maintaining continuity of the magnetization direction across the whole sample). We generalize our conclusions to systems beyond thin-film magnonic crystals and propose magnonic crystals based on the ideas of graded-index magnonics and those formed by Fano resonances as a possible way to circumvent the discovered issues.

Fig.1. A typical spectral map is shown with a set of lines ofspectra for exchange spin waves, both in coordinates k_Aa, k_Ba.The map is plotted using natural boundary conditions (see the text) for 1D magnonic crystal.

M. Mruczkiewicz and M. Krawczyk

Phys. Rev. B 94, 024434 (2016)

We study the effect of surface-induced Dzyaloshinskii-Moriya interaction (DMI) on the ferromagnetic resonance (FMR) spectrum of thickness-modulated one-dimensional magnonic crystals and isolated stripes. The DMI is found to substantially increases the intensity of absorption peaks and shifts the frequencies of the laterally quantized modes. The role of the DMI is determined by analyzing the amplitude and phase distributions of dynamic magnetic excitations calculated with frequency- and time-domain calculation methods. We propose experimentally realizable magnonic crystals and confined structures with multiple FMR absorption peaks. The frequency or magnetic field separation between FMR lines is exploited to propose a method for estimation of the DMI strength.

Fig.1. FMR frequency spectrum of the studied magnonic crystal (Co film with modulated thickness); ${\mu }_0{H}_0=100$ mT. Results of FDTD simulations for $D=0$ mJ/$m^2$ (blue dashed line) and $D=1$ mJ/$m^2$ (green solid line). The vertical lines indicate the eigenmode resonance position calculated with FDFEM. Due to the symmetry of the modes in the structure with $D=0$ only one mode around 18 GHz is observed in FDTD simulations.