Matthias Benjamin Jungfleisch
Argonne National Laboratory
New Twists For Magnons
In recent years, the exploration of magnons, the quanta of spin waves, as carriers of spin-angular momentum has flourished in spintronics. Magnon spintronics aims at developing novel functional devices that combine magnonic and electronic spin transport phenomena. Towards this end, the creation and manipulation of magnetization states by spin-orbit torques are pivotal. Magnetic insulators were mostly ignored for this particular purpose, despite their low magnetic damping. The low damping material yttrium iron garnet (Y3Fe5O12, YIG) is an ideal material for this particular purpose since it features long propagation distances and coherence times. In addition, YIG exhibits fascinating nonlinear phenomena where the superposition principle breaks down and enhanced nonlinear interactions can be observed.
Here, we demonstrate the propagation of spin-wave modes in micro-structured YIG waveguides. Spin waves propagating along the long side of the stripe are detected by means of spatially-resolved Brillouin light scattering (BLS) microscopy . Furthermore, we demonstrate electric excitation and detection of magnetization dynamics via pure spin currents by spin Hall effects in YIG/Pt micro- and nanostructures [2-4]. For this purpose we adopt a spin-transfer torque ferromagnetic resonance (ST-FMR) approach, which had previously been pioneered in all-metallic heterostructures. Besides that, we use spatially-resolved BLS spectroscopy to map the ST-FMR driven spin dynamics and reveal the formation of a strong, self-localized spin-wave intensity in YIG/Pt microstructures . This spin-wave ‘bullet’ is created due to nonlinear cross coupling of eigenmodes, which is confirmed by micromagnetic simulations. We furthermore developed a new pathway for lithographically defining patterned YIG structures with submicron dimensions, which enables to further investigate the influence of geometric confinements .
The presented results are first stepping stones toward the realization of magnon spintronics in micro- and nanoscaled devices based on magnetic insulators. These results might also lead to fascinating discoveries in nonlinear magnon physics, as well as the exploration of magnon Bose-Einstein condensates driven by spin torques in miniaturized sample geometries.
This work was supported by the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division.
 M. B. Jungfleisch et al., J. Appl. Phys 117, 17D128 (2015).
 J. Sklenar, W. Zhang, M. B. Jungfleisch et al., Phys. Rev. B 92, 174406 (2015).
 M. B. Jungfleisch et al., Phys. Rev. Lett. 116, 057601 (2016).
 M. B. Jungfleisch et al., Nano Lett., Article ASAP, DOI: 10.1021/acs.nanolett.6b02794 (2016).