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.

Theory of charge and spin pumping in atomic-scale spiral magnets

“…The linear response results for the charge current 𝑗 𝑒 are in stark contrast to the behavior that would be expected from a classical Archimedean screw, i.e., where the current would be linear in the velocity 𝑣 = −𝜆𝜔/(2𝜋) and, thus, also in 𝜆 [21]. In contrast, the narrow plateau region close to the AFM limit with almost quantized transport is an example of almost quantized Thouless pumping.…”

“…Concentration of the Berry curvature in the SDW channels leads to constant plateaus inside the SDW gaps, since the gapless helical channels do not mediate the charge transport. Nevertheless, the gapless channels provide a route for the cancellation of Berry curvature once 𝜆 increases: In the limit of large wavelengths 𝜆 𝑎 we obtain 𝑗 𝑒 ∝ 𝜔/𝜆 ∝ −𝑣/𝜆 2 , spoiling the oversimplified picture of a classical Archimedean screw [21,25] or topological Thouless pumping [1,2]. However, to this point, we did not yet include SOC in our theory which could open additional gaps and possibly prevent the cancellation of Berry curvature.…”

“…The rotation of a magnetic spiral at angular frequency 𝜔 -equivalent to a translation at velocity 𝑣 -was predicted to pump an electrical current 𝒋 𝑒 in the direction of the 𝑸-vector, recently interpreted by del Ser et al [21] as an electromagnetic Archimedean screw. This picture agrees with simple symmetry arguments which predict | 𝒋 𝑒 | ∝ 𝜔.…”

“…1 for a helical screw, i.e., where n = 𝑸. Thus, while a rotating spin spiral naturally pumps spin into attached leads [19,20], the simultaneously activated translational mode might also pump electric charge [20,21], similar to a classical Archimedean screw pumping fluids [22][23][24][25]. However, * d.kurebayashi@unsw.edu.au Rotating at a frequency 𝜔 is equivalent to translating the indicated (continuous) screw at a velocity 𝑣 = −𝜆𝜔/(2𝜋) in the direction of the 𝑸-vector.…”

Helitronics as a potential building block for classical and unconventional computing

“…These states, which can be viewed as a labyrinth of one-dimensional topological solitons, are coming back into the focus of spintronic research due to their unique symmetry. They have been shown to work as emergent inductors [18][19][20], lead to non-reciprocal charge and spin transport [21][22][23][24], or may actively pump spin and charge as Archimedean screws [25][26][27]. Moreover, they can serve as a non-trivial background for other topological excitations such as skyrmions [28][29][30] or dislocations [31][32][33][34].…”