Archimedean Screw in Driven Chiral Magnets

Abstract

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

arXiv comment12 pages, 9 figures. Supplementary: 7 pages, 1 figure. V2: added references, added videos showing dynamics of driven system. V3: corrected mistake in Sec. VI: transport, and eq. F7 in App.F. In the limit $v_F\tau \ll q^{-1}$ current grows as ${\tau }^2$, rather than being independent of $\tau$ as previously claimed. Changes in v4: minor revisions following referee recommendations; typos corrected

References in corpus (22)

• The design and verification of Mumax3
• Topological Hall effect in the A-phase of MnSi
• Artificial Brownian motors: Controlling transport on the nanoscale
• Spin Transfer Torques in MnSi at Ultra-low Current Densities
• Emergent electrodynamics of skyrmions in a chiral magnet
• Keldysh Field Theory for Driven Open Quantum Systems
• Long wavelength helimagnetic order and skyrmion lattice phase in Cu2OSeO3
• Spin-Wave Modes and Their Intense Excitation Effects in Skyrmion Crystals
• Observation of magnetic excitations of skyrmion crystal in a helimagnetic insulator
• Collective spin excitations of helices and magnetic skyrmions: review and perspectives of magnonics in non-centrosymmetric magnets
• Observation of time quasicrystal and its transition to superfluid time crystal
• LaBonte's method revisited: An effective steepest descent method for micromagnetic energy minimization
• Driving magnetic skyrmions with microwave fields
• Bose-Einstein Condensation of Quasi-Particles by Rapid Cooling
• Time crystal platform: from quasi-crystal structures in time to systems with exotic interactions
• Symmetry breaking, slow relaxation dynamics, and topological defects at the field-induced helix reorientation in MnSi
• Helical magnetic structure and the anomalous and topological Hall effects in epitaxial B20 Fe${}_{1-y}$Co${}_y$Ge films
• Experimental Spin Ratchet
• Band structure of helimagnons in MnSi resolved by inelastic neutron scattering
• Linearly polarized GHz magnetization dynamics of spin helix modes in the ferrimagnetic insulator Cu${}_2$OSeO${}_3$
• Creation of nanometric magnetic skyrmions by global application of circularly polarized microwave magnetic field
• Weak spin-orbit interactions induce exponentially flat mini-bands in magnetic metals without inversion symmetry