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Magnon Spin Hall Effect

Fachliche Zuordnung Experimentelle Physik der kondensierten Materie
Förderung Förderung von 2012 bis 2016
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 195131915
 
Erstellungsjahr 2016

Zusammenfassung der Projektergebnisse

Magnons have a large potential for transfer and processing of spin information in spintronics which can be fulfilled only after an effective way to convert magnon flows into electron-carried spin and charge currents has been found. The most promising converters exploit a combination of two separate physical phenomena, which are the spin pumping, the spin transfer torque and the (inverse) spin Hall effect. The results presented in this Report not only allow for a more in depth understanding of the spin pumping mechanism and spin transfer torque mechanisms, but also contribute profoundly towards the utilization of magnons in micro-scaled spintronic devices. In the following, the main fundamental and applied scientific achievements obtained in the project are summarized.  It was shown that magnons of different nature (dipolar or exchange), wavelength, and velocity efficiently contribute to spin pumping and therefore can be detected in the form of a DC ISHE voltage. ISHE detection of magnons was experimentally realized for the first time for: propagating dipolar magnons [A9], long-living slow dipolar exchange magnons, and short-wavelength exchange magnons. This opens access to the combination of magnon-based data processing devices with conventional electronic circuits.  The investigation of the dependence of the spin pumping effect on the thickness of the YIG layer has shown that spin pumping induced damping is enhanced for smaller YIG film thicknesses with a tendency to saturation. This is attributed to an increase in the ratio of surface to volume and, thus, to the interface character of the spin pumping effect.  The role of nonlinear magnon dynamics on spin pumping was revealed. The spin-wave damping was found to be enhanced for high applied microwave powers due to nonlinear scattering processes representing an additional damping channel. However, it was shown that the spin pumping efficiency only weakly dependents on the applied microwave power.  It was shown that the YIG/Pt interface condition is of high importance for spin pumping based magnon to current conversion. Experimental results showed that the spin pumping efficiency can be varied by a factor of more than 150 depending on the interface conditions. Since the voltage generated by the ISHE scales with the length of the Pt electrode, optimal interface conditions are extremely essential for the utilization of spin pumping and ISHE in nano-scaled devices.  Time-resolved measurements of the spin pumping induced ISHE voltage was performed for the first time and allowed for the detection of the contribution to the spin pumping by secondary magnons. These magnons are generated as a result of the scattering of the originally excited ferromagnetic resonance uniform mode. A contribution to spin-pumping from slow dipolar-exchange spin waves was detected. The work provides convincing evidence that ISHE voltage signal delays in such structures are dominated by magnon dynamics in the magnetic insulator, rather than electron dynamics in the nonmagnetic metal.  A spatially separated spin-wave source - a spin pumping-ISHE detector arrangement was utilized in order to prove spin pumping by propagating magnons. The results suggest the employment of spin-wave buses for the transfer of information over distances of up to a centimeter between spintronic devices.  It was shown that the spin pumping effect is not sensitive to the spin-wave wavelength.  Parametric pumping was used to excite short-wavelength (down to 100 nm) exchange magnons that were consequently detected in the form of an ISHE voltage. The results are useful for the understanding of the physics of the spin pumping phenomenon and are of crucial importance for further miniaturization of the magnon-based spintronic devices as only short-wavelength exchange magnons allow signal processing on the nanoscale distance. Furthermore, the combination of the spin pumping and ISHE effects is the effective instrument for magnon detection beyond the wave number limitation of most existing methods including Brillouin light scattering spectroscopy.  It was shown that the morphology of the Fe/Pt interface strongly influences the effective spin mixing conductance [A4].  The role of the nonmagnetic materials (NM) on the effective magnetization and the effective damping parameter was studied in three-layered structures MM/NM/Pt for different nonmagnetic materials and magnetic metals (MM).  The damping of spin waves parametrically excited in the magnetic insulator YIG was controlled by a DC current passed through an adjacent Pt film. The experiment was performed on a macroscopically sized YIG(100 nm)/Pt(10 nm) bilayer of 4*2 mm2 lateral dimensions. The spin-wave relaxation frequency was determined via the threshold of the parametric instability measured by BLS spectroscopy. The application of a DC current to the Pt film leads to the formation of a spin-polarized electron current normal to the film plane due to the spin Hall effect. This spin current exerts a spin transfer torque in the YIG film and, thus, changes the spin-wave damping. Depending on the polarity of the applied DC current with respect to the magnetization direction, the damping can be increased or decreased. The magnitude of its variation is proportional to the applied current. A variation in the relaxation frequency of 7.5% is achieved for an applied dc current density of 5*1010A/m2. The SHE-STT based damping compensation below the auto-oscillatory regime can potentially be used to realize complex magnonic circuits with ultra-low effective losses  SHE/STT based generation of magnetization precession was obtained and measured by means of BLS spectroscopy in micro-scaled YIG/Pt discs. The generation has a threshold-like process. The change of the current polarity or magnetization direction resulted in the suppression of magnon thermal noise.

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