Archimedean screw in driven chiral magnets

Abstract

In chiral magnets a magnetic helix forms where the magnetization winds around a propagation vector {q}q. We show theoretically that a magnetic field B_\bot(t) \bot qB⊥(t)⊥q, which is spatially homogeneous but oscillating in time, induces a net rotation of the texture around {q}q. This rotation is reminiscent of the motion of an Archimedean screw and is equivalent to a translation with velocity v_{\text{screw}}vscrew parallel to q. Due to the coupling to a Goldstone mode, this non-linear effect arises for arbitrarily weak B_\bot(t)B⊥(t) with v_{\text{screw}} \propto |{ B_\perp}|^2vscrew∝|B⊥|2 as long as pinning by disorder is absent. The effect is resonantly enhanced when internal modes of the helix are excited and the sign of v_{\text{screw}}vscrew can be controlled either by changing the frequency or the polarization of B_\bot(t)B⊥(t). The Archimedean screw can be used to transport spin and charge and thus the screwing motion is predicted to induce a voltage parallel to q. Using a combination of numerics and Floquet spin wave theory, we show that the helix becomes unstable upon increasing B_\botB⊥, forming a `time quasicrystal’ which oscillates in space and time for moderately strong drive.