Gate-controlled skyrmion and domain wall chirality

Abstract

Magnetic skyrmions are localized chiral spin textures, which offer great promise to store and process information at the nanoscale. In the presence of asymmetric exchange interactions, their chirality, which governs their dynamics, is generally considered as an intrinsic parameter set during the sample deposition. In this work, we experimentally demonstrate that a gate voltage can control this key parameter. We probe the chirality of skyrmions and chiral domain walls by observing the direction of their current-induced motion and show that a gate voltage can reverse it. This local and dynamical reversal of the chirality is due to a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we attribute to ionic migration of oxygen under gate voltage. Micromagnetic simulations show that the chirality reversal is a continuous transformation, in which the skyrmion is conserved. This control of chirality with 2-3 V gate voltage can be used for skyrmion-based logic devices, yielding new functionalities.

Fig. 1. Skyrmion chirality reversal.

a Schematic representation of the Ta/FeCoB/TaO_x trilayer with additional ZrO₂ oxide and transparent ITO electrode for gate voltage application. b Schematic cross section of the sample: the top-Ta wedge induces an oxidation gradient at the top interface, leading to c a iDMI sign crossover as directly measured by BLS vs. top-Ta thickness. Error bars of ± 100 MHz on the frequency difference Δf, represented on the right axis, are due to the setup. d, e CIM monitored during 4 s under p-MOKE microscope at the star location shown on b for zero gate voltage and g, h for V_g = + 3.5 V, applied on ITO (the dark rectangular region). The in-plane current density (J ≃ 5 × 10⁹ A/m²) is represented by the white arrow and the out of plane magnetic field is μ₀H_ext ≃ 80 μT. d, e In the initial state, skyrmions move in the direction of the current (encircled skyrmion moving along the red arrow), indicating CW chirality (D < 0), schematically represented in f. g, h Under the positive gate voltage, an inversion of the skyrmion motion occurs (encircled skyrmion moving along the blue arrow), indicating a CCW chirality (D > 0), as represented in i.

Fig. 2. Persistent and reversible chirality switch.

In the region close to iDMI sign inversion (star in Fig. 1b), the current density J (black arrow) induces a motion of DWs (red/blue arrows for a motion along/opposite to the current density), as observed by p-MOKE microscopy after switching off the gate voltage. a–e observation of DW motion under zero gate voltage and μ₀H_ext ≃ 30 μT, after sequential 90s-long voltage pulses. f Schematic representation of the applied voltage as a function of time. Initially a DWs have CW chirality; after a positive gate voltage pulse (b), chirality is reversed to CCW under the ITO electrode; after a negative gate voltage pulse (c), CW chirality is recovered ; after a positive gate voltage pulse (d), chirality has switched again to CCW; e after waiting ~ 2 h with zero gate voltage applied, the initial CW chirality is recovered. g Schematics of the effect of gate voltage pulses on interface oxidation, DW chirality and iDMI.

Fig. 3. Analytical model: stability of skyrmions in FeCoB during iDMI inversion induced by the gate voltage.

Analytical calculation of energy difference (in units of k_BT_300K) between skyrmion and uniform state for FeCoB as a function of skyrmion radius. Solid orange, dashed green, and dash-dotted gray lines correspond, respectively, to negative, zero and positive iDMI, associated with a progressive anisotropy variation under the gate voltage, as experimentally measured. Owing to the small iDMI value in FeCoB, the slight change of equilibrium radius (depicted by symbols) is mostly due to the anisotropy variation.

Fig. 4. Micromagnetic simulations of chirality switch.

Simulated stable states show a gradual transition between a CW Néel skyrmion at D = −0.5 mJ/m² and c CCW Néel skyrmion at D = 0.5 mJ/m² via b a stable Bloch skyrmion state at D = 0. d Angle ξ between the in-plane magnetic moments and the radial direction of the magnetic moments at the domain-wall center and e radius of the skyrmion as a function of the iDMI value. The helicity and radius of the skyrmion corresponding to a–c images are shown, respectively, in d, e by the star, square and triangle symbols.