Spin-orbit twisted spin waves in magnetic quantum wells

Spin-wave based transistors are an appealing alternative to the traditional charge-based transistor, since spin waves carry information with reduced dissipation compared to charge currents. However, one still has to develop efficient methods for controlling the spin waves with low energy cost, a condition not satisfied by the manipulation with magnetic fields.

Spin-orbit coupling (SOC) for conduction electrons is a quantum-relativistic interaction emerging for spin-wave control. For itinerant spins, like in a ferromagnetic metal or in a magnetic two-dimensional electron gas (2DEG), the precessing spins of a spin wave belong to conducting electrons, so the precession of the interacting spins, the charge motion, and the SOC are all interrelated.

We have addressed the challenge of disentangling the interplay between SOC and precessing itinerant spins to gain a full understanding of spin-wave control via SOC[1]. We studied spin-waves in a magnetic 2DEG, a high quality test-bed for magnetized 2D systems. Our findings can be summarized as follows: we introduce a novel "spin-orbit twist" effect: the spin-orbit coupling causes a periodical twist of the spins, resulting in a two-dimensional wave vector shift of the spin-wave dispersions and damping rate. We predict the amplitude and direction of the chiral wave-vector shift of spin waves. These theoretical predictions are validated by Raman scattering measurements. The mechanism of chirality lead us to the possibility of optically tune the electron density such as to modify and even reverse the group velocity of the spin waves.

Moreover, in itinerant spin systems, our findings show that SOC does not destroy spin-waves but offers the possibility to control both the direction and velocity of spin waves without affecting the damping rate. A similar protection of spin-waves by SOC has been recently evidenced in topological insulators [2].

[1] Perez, F. et al. PRL 117, 137204 (2016)
[2] H. Kung at al. PRL 119, 136802 (2017)