Magnetization Dynamics in Metallic Ferromagnetic Nanostructures
Funded via DFG/NSF: ‚Materials World Network‘ LI 1567/5-1
The main goal of the proposed research program is to gain quantitative understanding of the properties of spin waves in individual metallic ferromagnetic nanostructures in the presence of spin currents. Ultimately, the fundamental understanding will contribute to innovation in the emerging field of magnonics – a spinbased energy efficient processing of digital information. We propose a series of experiments that employ several onchip ferromagnetic resonance (FMR) techniques (microresonator-FMR, electrically detected FMR) as well as near-field Brillouin Light Scattering (BLS) to measure spin wave properties in magnetic nanostructures. These measurements mainly focus on understanding of (i) the effect of geometric confinement on nonlinear spin wave interactions and (ii) the effect of spin currents on spin wave dispersion and damping in ferromagnetic nanowires.
|Fig. 1: Measurements of spin wave properties of a ferromagnetic nanowire by three complementary techniques: microresonator FMR, electrically-detected FMR and nano-BLS give a complete set of parameters (frequency, damping, spatial profile, group velocity) characterizing spin waves in presence of high spin current densities. Both linear and non-linear regimes of magnetization dynamics are accessible in proposed nanostructures.
(a) Schematic of nano-BLS setup . The incident and reflected light pass through a nano-size aperture scanned above the surface of the sample (an elliptical nanomagnet in this example). The resultant BLS intensity gives the square of the local amplitude of a spin wave mode with spatial resolution of ~ 50 nm. (b) Spatial profiles of the spin wave edge mode in a 1.3´2.4mm2 elliptical nanomagnet measured at different values of the external magnetic field.
- Electrically detected Ferromagnetic Resonance
Fig. 3: This figure illustrates two types of ED-FMR setups for measurements of spectral properties of spin waves in ferromagnetic (F) nanowires. (a) A shorted coplanar strip guide is patterned parallel to the F nanowire providing spatially uniform microwave field within the wire volume. This type of drive excites long-wavelength spin waves in the nanowire. (b) Microwave drive signal is applied to a narrow nanostrip placed across the nanowire (and isolated from the nanowire by a layer of oxide). This setup provides spatially inhomogeneous driving field and will excite spin waves with shorter wave lengths. In both ED-FMR setups, the spin wave mode resonance frequency is determined as a peak (trough) of the four-point nanowire resistance as a function of either microwave frequency or a dc bias field.
- Microresonator-Ferromagnetic Resonance
Fig. 4: Schematic layout of the microstrip resonator in reflection geometry (upper panel). The lower panels show the calculated distributions of the electric high frequency field (left) and the magnetic high frequency field (right). All figures reproduced from Ref. 
 J. Jersch, V. E. Demidov, H. Fuchs, K. Rott, P. Krzysteczko, J. Münchenberger, G. Reiss, and S. O. Demokritov, Mapping of localized spin-wave excitations by near-field Brillouin light scattering, Appl. Phys. Lett. 97, 152502 (2010).
 A. Banholzer, R Narkowicz, C. Hassel, R. Meckenstock, S. Stienen, O. Posth, D. Suter, M. Farle, J. Lindner., Visualization of spin dynamics in single nanosized magnetic elements, Nanotechnology 22, 295713 (2011).
- Understanding of the effect of pure spin currents generated by spin Hall effect on energies and damping parameters of spin waves in magnetic nanostructures. Can pure spin currents excite persistent self-oscillations of magnetization?
- Understanding of the effect of spin wave spectrum quantization on nonlinear multi-magnon processes such as the Suhl instability. Do Suhl instability parameters such as threshold amplitudes become quantized in magnetic nanostructures? Can we control Suhl instability and multi-magnon scattering in general with spin currents?
- Understanding of the effect of spin-polarized current on the spin wave dispersion relation (such as spin wave Doppler shift effect). Can large spin wave Doppler shift be observed in nanostructures for short wavelength spin waves? Can spin wave frequency reduced to zero by sufficiently high spin-polarized current density? What is the nature of the ground state of magnetization for current density beyond such a critical current density?
- Understanding of the effect of domain walls on the spectrum of spin waves in nanowires. Can we control the frequency and phase of propagating spin waves with domain walls? Can we move domain walls by propagating spin waves excited in the nanowire? Can we control such processes with spin currents?
Sergej. O. Demokritov
Ilya N. Krivorotov
Frequency-domain Magnetic Resonance – Alternative Detection Schemes for Samples at the Nanoscale
M. Möller, K. Lenz, J. Lindner
Journal of Surfaces and Interfaces of Materials, in press.