Magnus Induced Magnetic Diode Effect in Skyrmion Systems

TL;DR

This work identifies a magnetic analog of a diode in skyrmion transport, where a fixed drive exhibits nonreciprocal velocity when the magnetic field is reversed due to the Magnus force. The authors realize this in a sawtooth-channel geometry with opposite wall asymmetries and analyze both spin-transfer torque and spin-orbit torque driving using atomistic Landau-Lifshitz-Gilbert simulations with $J=1$ meV and $D=0.2J$. They show STT can reverse the velocity and alter its magnitude upon field reversal, while SOT can preserve direction but strongly suppress velocity, producing near-perfect magnetic diode regimes; the effect strengthens with current and Gilbert damping and persists across parameter ranges. The results suggest a new, field-tunable nonreciprocal transport element for skyrmion-based devices and imply similar Magnus-induced diodes could exist in other systems.

Abstract

We show that skyrmions can exhibit what we call a magnetic diode effect, where there is a nonreciprocal response in the transport when the magnetic field is reversed. This effect can be achieved for skyrmions moving in a channel with a sawtooth potential on one side and a reversed sawtooth potential on the other side. We consider the cases of both spin-transfer torque (STT) and spin-orbit torque (SOT) driving. When the magnetic field is held fixed, the velocity response of the skyrmion is the same for current applied in either direction for both STT and SOT driving, so there is no current diode effect. When the magnetic field is reversed, under STT driving the velocity of the skyrmion reverses and its absolute value changes. Under SOT driving, the velocity remains in the same direction but drops to a much lower value, resulting in negative differential conductivity. For a fixed current, we find a nonreciprocal skyrmion velocity as a function of positive and negative applied fields, in analogy to the velocity-current curves observed in the usual diode effect. The nonreciprocity is generated by the Magnus force, which causes skyrmions to interact preferentially with one side of the channel. Since the channel sides have opposite asymmetry, a positive magnetic field can cause the skyrmion to interact with the "hard" asymmetry side of the channel, while a negative magnetic field causes the skyrmion to interact with the easy asymmetry side. This geometry could be used to create new kinds of magnetic-field-induced diode effects that can be harnessed in new types of skyrmion-based devices.

Figures (8)

• Figure 1: Illustration of the simulation system. Grey regions are composed of high PMA defects with $K_D=5J$. (a, c) A $Q=-1$ skyrmion is stabilized with a $+\hat{\bf z}$ magnetic field ${\bf H}$. (b, d) A $Q=+1$ skyrmion is stabilized with a $-\hat{\bf z}$ magnetic field ${\bf H}$. An STT current is applied in (a, b) and an SOT current is applied in (c,d). The blue (red) lines and arrows indicate the skyrmion motion under positive (negative) currents $j$. The skyrmions are colored according to their local $m_z$ values.
• Figure 2: (a) Average skyrmion velocity $\langle v_x\rangle$ vs applied current $j_\text{STT}$ with STT driving under positive, $\mu H=+0.5D^2/J$ (black), and negative, $\mu H=-0.5D^2/J$ (red), magnetic fields, for a system with $\alpha=0.3$. (b) The corresponding absolute average skyrmion velocity $|\langle v_x\rangle|$ vs applied current $j_\text{STT}$ under positive, $\mu H=+0.5D^2/J$ (black), and negative, $\mu H=-0.5D^2/J$ (red), magnetic fields.
• Figure 3: Average skyrmion velocity $\langle v_x\rangle$ vs applied current $j_\text{SOT}$ with SOT driving under positive, $\mu H=+0.5D^2/J$ (black), and negative, $\mu H=-0.5D^2/J$ (red), magnetic fields, for a system with $\alpha=0.3$.
• Figure 4: Image of skyrmion motion from the initial position at the center of the channel to the final trapped position in the corner of the well for the system from Fig. \ref{['fig:3']} with SOT driving at $j_\text{SOT}=2\times 10^{10}$ A m$^{-2}$ and positive magnetic field $\mu H = +0.5 D^2/J$. The colorbar indicates the passage of time but is cut off at $t=3$ ns for visualization purposes; once trapped, the skyrmion remains pinned for the duration of the simulation up to $t=200$ ns.
• Figure 5: Average skyrmion velocity $\langle v_x\rangle$ vs applied magnetic field $\mu H$ for a system with $\alpha=0.3$ and STT driving at $j_\text{STT}=+2.5\times10^{10}$ A m$^{-2}$ (blue), $j_\text{STT}=+1.5\times10^{10}$ A m$^{-2}$ (red), and $j_\text{STT}=+0.5\times10^{10}$ A m$^{-2}$ (black).