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Spin-orbitronics: Spin pumping

In the emerging field of spin-orbitronics, spin-orbit coupling (SOC) is employed in both magnetic and non-magnetic materials to generate, exploit and detect spin currents. The generation of pure spin currents in functional semiconductors is a fundamental requirement for the implementation in the next generation of spin-based devices. Besides being strategic material systems for state-of-the-art optoelectronics and high power electronics, III-nitride semiconductors like GaN and AlxGa1-xN doped with magnetic elements have been widely studied as dilute magnetic semiconductors [1-5]. Moreover, with a non-negligible Rashba spin-orbit coupling parameter of (4.5+1.0) meV Å [6] and long spin relaxation times, n-GaN:Si is an outstanding workbench for the generation and manipulation of pure spin currents.

We have reported on the generation of pure spin currents in degenerately doped wurtzite n-GaN:Si using an adiabatic spin pumping technique at room temperature and estimate the spin Hall angle θSH for this Rashba semiconductor. A Py/n-GaN:Si bilayer system is driven to a ferromagnetic resonance condition and the voltage from an induced charge emf, due to the interplay of the direct and inverse spin Hall effects, have been measured simultaneously. The measured voltage is resolved into symmetric and asymmetric components [7] due to spin Hall- and galvanomagnetic-effects. From the fundamental model of spin pumping [8,9], a spin mixing conductance of (1.38 x 1018) m-2 for the Py/ n-GaN:Si interface and a spin Hall angle θSH = 3.03 x 10-3 for wz n-GaN:Si are found [10]. The obtained value of θSH is at least one order of magnitude higher than those obtained for other semiconductors like Si, Ge, ZnO and n-GaAs [7]. We do not observe any spin pumping from an oxidised Py layer on n-GaN:Si or for a Schottky interface, as in the case for Py/u-GaN (undoped) thereby highlighting the quality of the ferromagnet/non-magnet interface. We also demonstrate explicitly the effect of the non-magnetic layer thickness on the nature of the generate emf and discuss the contributions of spurious galvanomagnetic effects, like planar Hall effect (PHE) in the Py, to the measured emf. This work paves the way to the realization of nitride based low power, non-volatile and non-dissipative spin devices e.g. spin batteries [11].

Figure 1: Spin pumping process
Figure 2: Estimated inverse spin Hall voltage


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  2. T. Devillers et al. Sci. Rep. 2, 722 (2012)
  3. T. Dietl et al. Rev. Mod. Phys. 87, 1311 (2015)
  4. R. Adhikari et al. Phys. Rev B 91, 205204 (2015)
  5. M. Rovezzi et al. Phys. Rev. B 92, 115308 (2015)
  6. W. Stefanowicz et al. Phys. Rev B 89, 205201 (2014)
  7. J. Sinova et al. Rev. Mod. Phys. 87, 1213 (2015)
  8. Y. Tserkovnyak et al. Phys. Rev. B 66, 224403 (2002)
  9. Y. Tserkovnyak et al. Rev. Mod. Phys. 77, 1375 (2005)
  10. R. Adhikari et al. Phys. Rev. B 94, 085205 (2016)
  11. A. Brataas et al. Phys. Rev. B 66, 064404 (2002)
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