We present a micromagnetic study on the current-induced domain-wall motion in cylindrical Permalloy nanowires with diameters below 50~nm. The transverse domain walls forming in such thin, round wires are found to differ significantly from those known from flat nanostrips. In particular, we show that these domain walls are zero-mass micromagnetic objects. As a consequence, they display outstanding dynamic properties, most importantly the absence of a breakdown velocity generally known as the Walker limit. Our simulation data are confirmed by an analytic model which provides a detailed physical understanding. We further predict that a particular effect of the current-induced dynamics of these domain walls could be exploited to measure the nonadiabatic spin-transfer torque coefficient.

We report on a micromagnetic study on domain wall (DW) propagation in ferromagnetic nanotubes. It is found that DWs in a tubular geometry are much more robust than ones in flat strips. This is explained by topological considerations. Our simulations show that the Walker breakdown of the DW can be completely suppressed. Constant DW velocities above 1000 m/s are achieved by small fields. A different velocity barrier of the DW propagation is encountered, which significantly reduces the DW mobility. This effect occurs as the DW reaches the phase velocity of spin waves (SWs), thereby triggering a Cherenkov-like emission of SWs.

In magnonics, spin waves are conceived of as electron-charge-free information carriers. Their wave behavior has established them as the key elements to achieve low power consumption, fast operative rates, and good packaging in magnon-based computational technologies. Hence, knowing alternative ways that reveal certain properties of their undulatory motion is an important task. Here, we show using micromagnetic simulations and analytical calculations that spin-wave propagation in ferromagnetic nanotubes is fundamentally different than in thin films. The dispersion relation is asymmetric regarding the sign of the wave vector. It is a purely curvature-induced effect and its fundamental origin is identified to be the classical dipole-dipole interaction. The analytical expression of the dispersion relation has the same mathematical form as in thin films with the Dzyalonshiinsky-Moriya interaction. Therefore, this curvature-induced effect can be seen as a “dipole-induced Dzyalonshiinsky-Moriya-like” effect.

Field-driven domain wall (DW) propagation in ferromagnetic nanotubes displays unusual effects, as revealed by a micromagnetic study. The left-right symmetry of the DW dynamics is broken, yielding markedly different DW mobilities for opposite propagation directions. This asymmetry arises from the tubular geometry and its impact on the DW structure. Also, the Walker breakdown field and velocity are found to be asymmetric for opposite directions. In certain cases, the breakdown can even be suppressed in one or both directions. Topological constraint requires a vortex-antivortex pair mediated breakdown, contrary to the single (anti)vortex in flat strips. This results in a higher breakdown velocity.

We report on the Cherenkov-type excitation of spin waves (SWs) in ferromagnets. Our micromagnetic simulations show that a localized magnetic field pulse moving sufficiently fast along the surface of a ferromagnet generates a SW boom, with a Mach-type cone of propagating wave fronts. The SWs are formed when the velocity of the source exceeds the propagation speed of SWs. Unlike the single cone of the usual Cherenkov effect, we find that the magnetic Mach cone consists of two wave fronts with different wave numbers. In patterned thin strips, this magnetic analog of the Cherenkov effect should enable the excitation of SWs with well-defined and velocity-dependent frequency. It thereby provides a promising route towards tunable SW generation, with important potential for applications in magnonic devices.

The Dzyaloshinskii-Moriya interaction (DMI) can cause spin-wave (SW) nonreciprocality in extended magnetic films. Here, we report a correlated effect for SWs propagating in a waveguide, namely, a longitudinally magnetized nanowire. Due to the shape confinement, SWs traveling in the wire are quasiquantized along the width direction. In the presence of the DMI, the wave-front lines become oblique with respect to the propagation direction, unlike any ordinary wave pattern. The tilting direction of the wave front is determined by the sign of the DMI constant, manifesting the chiral nature of the DMI. Another remarkable SW property revealed is that the two dynamical magnetization components that jointly characterize a SW mode exhibit different spatial profiles. Mathematically, these extraordinary effects can be attributed to the first-order spatial derivative brought into the wave equation by the DMI.