Björn Obry

A micro-structured ion-implanted magnonic crystal

We investigate spin-wave propagation in a microstructured magnonic-crystal waveguide fabricated by localized ion implantation. The irradiation caused a periodic variation in the saturation magnetization along the waveguide. As a consequence, the spin-wave transmission spectrum exhibits a set of frequency bands, where spin-wave propagation is suppressed. A weak modification of the saturation magnetization by 7% is sufficient to decrease the spin-wave transmission in the band gaps by a factor of 10. These results evidence the applicability of localized ion implantation for the fabrication of efficient micron- and nano-sized magnonic crystals for magnon spintronic applications.

Interference of coherent spin waves in micron-sized ferromagnetic waveguides

We present experimental observations of the interference of spin-wave modes propagating in opposite directions in micron-sized Ni81Fe19-waveguides. To monitor the local spin-wave intensity distribution and phase of the formed interference pattern, we use Brillouin light scattering microscopy. The two-dimensional spin-wave intensity map can be understood by considering the interference of several waveguide eigenmodes with different wavevectors quantized across the width of the stripe. The phase shows a transition from linear dependence on the space coordinate near the antennas characteristic for propagating waves to discrete values in the center region characteristic for standing waves.

Temporal evolution of inverse spin Hall effect voltage in a magnetic insulator-nonmagnetic metal structure

It is demonstrated that the temporal evolution of a spin-wave induced inverse spin Hall effect voltage in a magnetic insulator–nonmagnetic metal structure is distinctly different from that of the directly excited (microwave pulse driven) spin-wave mode from which it originates. The differences in temporal behavior provide compelling evidence that incoherent secondary spin-wave modes, having a range of different characteristic lifetimes, make an important contribution to spin pumping at the insulator-metal interface.

Mode conversion by symmetry breaking of propagating spin waves

We study spin-wave transport in a microstructured Ni81Fe19 waveguide exhibiting broken translational symmetry. We observe the conversion of a beam profile composed of symmetric spin-wave width modes with odd numbers of antinodes n = 1, 3,… into a mixed set of symmetric and asymmetric modes. Due to the spatial homogeneity of the exciting field along the used microstrip antenna, quantized spin-wave modes with an even number n of antinodes across the stripe’s width cannot be directly excited. We show that a break in translational symmetry may result in a partial conversion of even spin-wave waveguide modes.

Low spin-wave damping in amorphous Co40Fe40B20 thin films

A characterization of the magnetic properties of amorphous Co40Fe40B20 thin films, developed for low damping applications in magnon spintronics, using vector network analyzer ferromagnetic resonance (VNA-FMR) and the magneto-optical Kerr effect is presented. Our films show a very weak uniaxial anisotropy and a low Gilbert damping parameter (α=0.0042). The saturation magnetization MS extracted from the FMR measurements is 1250 kA/m. The frequency dependence of the first perpendicular standing spin waves mode on the applied magnetic field is used to determine the exchange constant A for this alloy resulting in a value of 1.5×10−11 J/m. These values are discussed in comparison to literature values for different CoFeB compositions and other related alloys.

Spin-wave propagation and transformation in a thermal gradient

The influence of a thermal gradient on the propagation properties of externally excited dipolar spin waves in a magnetic insulator waveguide is investigated. It is shown that spin waves propagating towards a colder region along the magnetization direction continuously reduce their wavelength. The wavelength increase of a wave propagating into a hotter region was utilized to realize its decomposition in the partial waveguide modes which are reflected at different locations. This influence of temperature on spin-wave properties is mainly caused by a change in the saturation magnetization and yields promising opportunities for the manipulation of spin waves in spin-caloritronic applications.

Mode selective parametric excitation of spin waves in a Ni81Fe19 microstripe

We present the experimental observation of parallel parametric amplification of selected thermal spin-wave modes in a transversally magnetized Ni81Fe19 microstripe. By employing Brillouin light scattering microscopy, we identify the dominant group, i.e., the spin-wave mode that is preferentially amplified. Due to the existing spin-wave quantization in the system, it is possible to select one specific mode to be parametrically excited by changing the bias magnetic field. This gives access to transversal spin-wave eigenmodes of the stripe which are promising for spin-wave information processing and also to modes localized at the stripe edges.

Magnonic band gaps in waveguides with a periodic variation of the saturation magnetization

We present a micromagnetic analysis of spin-wave propagation in a magnonic crystal realized as a permalloy spin-wave waveguide with a spatial periodical variation of its saturation magnetization. Frequency band gaps were clearly observed in the spin-wave transmission spectra, and their origin is traced back to an overlap of individual band gaps of the fundamental and higher-order spin-wave width modes. The control of the depth, width, and position in frequency and space of the rejection band gaps by the width areas with a reduced magnetization and by the modulation level is discussed in this study.

Dissipation characteristics of quantized spin waves in nano-scaled magnetic ring structures

The spatial profiles and the dissipation characteristics of spin-wave quasi-eigenmodes are investigated in small magnetic Ni81Fe19 ring structures using Brillouin light scattering microscopy. It is found that the decay constant of a mode decreases with increasing mode frequency.

Microscopic magnetic structuring of a spin-wave waveguide by ion implantation in a Ni81Fe19 layer

We investigate the spin-wave excitation in microscopic waveguides fabricated by localized Cr+ ion implantation in a ferromagnetic Ni81Fe19 film. We demonstrate that spin-wave waveguides can be conveniently made by this technique. The magnetic patterning technique yields an increased damping and a reduction in saturation magnetization in the implanted regions that can be extracted from Brillouin light scattering measurements of the spin-wave excitation spectra. Furthermore, the waveguide performance as well as the internal field of the waveguide depend on the doping fluence. The results prove that localized ion implantation is a powerful tool for the patterning of magnon spintronic devices.