Archimedean screw in driven chiral magnets

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.

Modified spin-wave theory applied to one-dimensional Heisenberg ferromagnet with the nearest-neighbor and next-nearest-neighbor exchange anisotropies

The modified spin-wave theory is used to investigate the one-dimensional Heisenberg ferromagnet with the nearest-neighbor (NN) and next-nearest-neighbor (NNN) exchange anisotropies. The ground-state and low-temperature properties of the system are studied within the self-consistent method. It is found that the effect of the NN anisotropy on the thermodynamic quantities is stronger than that of the NNN anisotropy in the low-temperature region. The anisotropy dependence behaviors (such as the power, exponential and linear laws) are obtained for the position and the height of the maximum of the specific heat and its coefficient, as well as the susceptibility coefficient. The specific heat and its coefficient both display the low-temperature double maxima which are induced by the anisotropies and the NNN interaction. In the very low temperatures the specific heat and the susceptibility behave severally as T[Formula: see text] and T[Formula: see text] at the critical point J2/J1 = −0.25, where J1 and J2 are the NN and NNN interactions, respectively.