Junior Research Group Magnonics: Research Topics

Spin-Hall Oscillators

Spin Hall oscillators are modern microwave oscillators with promising future applications in communication technologies as emitters and receivers as well as in magnonics as spin wave sources. They allow for the conversion of a direct charge current in GHz oscillations of the magnetization. In the simplest case they consist of a structured ferromagnetic and nonmagnetic heavy metal layer. In submicron structures, high current densities in the range of 108 A/cm² are reached due to charge currents in the mA section. Thanks to the spin Hall effect a pure spin current is generated in the heavy metal (e.g. Pt) and can enter the interface to the ferromagnetic layer. Here, this pure spin current can lead to an antidamping torque on the magnetization, which can be large enough to compensate the intrinsic damping and therefore stabilizing one certain precession angle and frequency of the magnetization. In consequence of the nonlinearity of this auto-oscillators the frequency can be tuned by control of the spin current in other words by the control of the direct charge current.

Related publications

  • T.Hache, T.Weinhold, K.Schultheiss, J.Stigloher, F.Vilsmeier, C.Back, S.S.P.K. Arekapudi, O.Hellwig, J.Fassbender, H.Schultheiss
    "Combined frequency and time domain measurements on injection-locked, constriction-based spin Hall nano-oscillators"
    (preprint available at arXiv:1811.08133)
  • K. Wagner, A. Smith, T. Hache, J. Chen, L. Yang, E. Montoya, K. Schultheiss, J. Lindner, J. Fassbender, I. Krivorotov, H. Schultheiss
    "Injection locking of multiple auto-oscillation modes in a tapered nanowire spin Hall oscillator"
     Scientific Reportsvolume 8, Article number: 16040 (2018), DOI: 10.1038/s41598-018-34271-4

Spin waves in domain walls an other magnetic textures

Magnonics: Domain walls as wave guides ©Copyright: Dr. Schultheiß, Helmut
Schematic illustration of a 180° Néel wall carrying a magnon. The divergence of the magnetization div(M) across the width of the wall results in a magnetic field antiparallel to the magnetization. (b) Simulated domain configuration of a rectangular Py element. The color represents the m x component and the arrows display the magnetization direction in the domains. (c) Amplitude profiles of magnons excited locally at the green dot. Green bars indicate the magnon wavelengths. (d) Magnon intensities measured across the waveguide for different external magnetic fields applied parallel to the long axis of the magnon waveguide for 0.52 GHz. The domain wall can be shifted with small applied fields over micrometer  distances, therefore allowing for fine control of the magnon channel position.

Magnetic textures such as domain walls, vortices or skyrmions are promising candidates to manipulate spin waves in nanostructures. The other way around, spin waves can also manipulate the surrounding magnetic texture itself and lead for example to vortex switching or the generation of skyrmions. One focus of our research is the interaction of spin waves in thin films with Néel type domain walls. These walls have been shown to confine spin waves and can and therefore act as a wave guide which is reconfigurable by applying an external magnetic field. They may also act as a phase shifters for spin waves which are transmitting through such walls.

Related publications

  • L. Körber, K. Wagner, A. Kákay, H. Schultheiss
    "Spin-wave reciprocity in the presence of Néel walls"
    IEEE Magnetic Letters PP, 99 (2017), DOI: 10.1109/LMAG.2017.2762642
  • K. Wagner, A. Kákay, K. Schultheiss, A. Henschke, T. Sebastian, and H. Schultheiss
    "Magnetic domain walls as reconfigurable spin-wave nanochannels"
    Nature Nanotechnology 11, 432 (2016), DOI: 10.1038/NNANO.2015.339

Spin-wave propagation in time-dependent magnetic fields

The spin-wave dispersion relation depends on many parameters, for example an external magnetic field or the width of a waveguide, and can be modulated by these parameters, and spin wave properties also can be modified. For example, the spin-wave propagation modified by the spatially inhomogeneous magnetic field, and the wavelength was changed. We interested in how spin waves which are already excited behave in the time dependent magnetic field, and investigated the spin-wave propagation under the pulse field.

Spin-torque oscillators in domain walls

The idea of using spin waves as information carriers has developed the research field of magnonics, because spin waves are attractive in the points of fast processing speed and reduction of energy consumption. While magnonic logic gates have been demonstrated, significant progress was done by employing spin currents based on spin Hall effect (SHE). At an interface between a heavy metallic layer and a ferromagnetic layer, spin currents give a spin transfer torque to the magnetization in the ferromagnetic layer, which drives spin-wave auto-oscillation. We are interested in such auto-oscillation in domain walls. Since domain walls are movable and nanoscale, spin-wave oscillators in domain wall are expected to work as reprogrammable nano-sized spin-wave sources in future magnonic devices.

Related publications

  • N. Sato, K. Schultheiss, L. Körber, N. Puwenberg, T. Mühl, A.A. Awad, S.S.P.K. Arekapudi, O. Hellwig, J. Fassbender, H. Schultheiss
    "Domain wall-based spin-Hall nano-oscillators"
    (preprint available at arXiv:1812.06702)

All-Optical Switching

All Optical Switching (AOS), which means purely light induced switching of the magnetization direction, was successfully demonstrated for ferrimagnetic TbxFe100-x samples. Therefore, the magnetization direction of the sample was switched to M+ (M-) by right σ+ (left σ-) handed circular polarized femtosecond laser pulses.

Non-linear spin waves in magnetic vortices

Magnonics: Nonlinear ©Copyright: Dr. Schultheiß, Helmut

(a) Magnon spectra of a 5 μm diameter Py discs in the vortex state driven into the non-linear regime with externally applied microwaves. (b) Spatial intensity distribution magnons arising from three-magnon scattering events. (c) Temporal evolution of magnon intensities at pulsed excitation with 16 dBm pumping power. (d) Power dependence of the signal start time for the direct excitation and the split magnon modes.

Besides magnon transport we used our capabilities of time-resolved μBLS to shed light on nonlinear magnon-magnon scattering processes in magnetic microstructures with a fully quantised magnon spectrum. As a model system we studied micron-sized Py discs magnetized in the vortex state as shown in the inset in the figure above. We placed such a magnetic vortex in a loop shaped microwave antenna, which allowed us to excite magnons with a strong, symmetric out-of-plane microwave field. When measuring the magnon spectra with μBLS as a function of the applied microwave frequency (a) we observed a multitude of additional magnon peaks besides the directly excited magnons. These additional magnons are the result of three magnon scattering events, where the directly excited magnons splits into two new magnons as indicated by the dashed arrows. Due to conservation of energy, the frequencies of the split modes sum up to the excitation frequency. In stark contrast to magnon-magnon scattering events studied in continuous films in the past, the rotational symmetry of the magnetization configuration in a vortex imposes an additional conservation law during the scattering process, which is the conservation of the orbital angular momentum of the participating magnons.

Related publications

  • K. Schultheiss, R. Verba, F. Wehrmann, K. Wagner, L. Körber, T. Hula, T. Hache, A. Kakay, A.A. Awad, V. Tiberkevich, A.N. Slavin, J. Fassbender, H. Schultheiss
    "Excitation of whispering gallery magnons in a magnetic vortex"
    (preprint available at arXiv:1806.03910)