Facilitating electrical and laser-induced skyrmion nucleation with a dipolar-field enhanced effective DMI

TL;DR

This study demonstrates that engineering layer-resolved DMI signs in Ir/Co/Pt multilayers, by aligning the dipolar field with the DMI across the stack, yields an enhanced effective DMI that markedly boosts skyrmion nucleation density for both current-driven and laser-induced methods while leaving threshold conditions largely unaffected. The enhanced stack shows approximately a 2.5-fold increase in $|D_{ ext{eff}}|$ compared with the reduced stack, and skyrmion densities can be up to 20-fold higher under current pulses and about 4-fold higher under laser pulses in the saturated regime. These findings reveal that dipolar-field engineering provides a robust route to control the nucleation and stability of chiral magnetic textures in multilayers, independent of major changes to $M_s$ or $K_u$. The work also suggests time-resolved experiments to directly probe the dipolar-DMI mechanism and motivates extended multilayer spin-dynamics modeling to capture long-range dipolar effects in layer-resolved DMI systems.

Abstract

We demonstrate experimentally how the nucleation of skyrmions in an Ir, Co, and Pt based magnetic multilayer is affected by introducing a layer dependent sign for the Dzyaloshinskii-Moriya interaction (DMI). In one stack, the bottom half of the stack is given a positive DMI and the top half a negative DMI, and as a result, the in-plane component of the dipolar field is aligned parallel to the effective field of the DMI in every layer, enhancing the effective DMI. We show that this enhanced DMI facilitates the nucleation and stability of skyrmions using both current-driven and laser-induced skyrmion nucleation. In the devices with an enhanced effective DMI, the density of nucleated skyrmions is greater by up to a factor 20 and skyrmions can be observed in stronger magnetic fields - suggesting that their stability is also improved. These results show that skyrmion nucleation depends strongly on the magnitude of the effective DMI in a magnetic multilayer and that the dipolar field within such a multilayer presents an effective route towards controlling the effective DMI, and thereby, the nucleation of chiral magnetic textures.

Figures (4)

• Figure 1: Sketches of the different multilayer structures. (a) Uniform$+$ stack. All the layers in this stack have a positive DMI sign. (b) Uniform$-$ stack. Same as (a), but with the opposite DMI sign. (c) Enhanced stack. Here the magnetic layers in the top half of the stack have a negative DMI and the layers in the bottom half a positive DMI, as a result the DMI effective field and the dipolar field are aligned in all layers. (d) Reduced stack. Opposite DMI signs compared to the Enhanced stack in (c), now the DMI effective field and the dipolar field are antiparallel in all layers. Thick arrows represent magnetization and follow the DMI effective field, the long white arrows represent the dipolar field $\mu_{0} H_{\textrm{d}}$. The top and bottom half of each schematic represent multiple repeats with the same layer order.
• Figure 2: Characterization of the different magnetic multilayers. (a) The saturation magnetization $M_{\textrm{s}}$ and (b) uniaxial anisotropy $K_{\textrm{u}}$ measured using SQUID-VSM. (c) The effective DMI strength $\lvert D_{\textrm{eff}} \rvert$ determined by first measuring the equilibrium domain width and then using a model for the expected equilibrium domain width by Lemesh et al.Lemesh2017 to calculate $\lvert D_{\textrm{eff}} \rvert$. The horizontal lines in (a) - (c) show the average of the values measured for the two uniform stacks. (d) The expected contribution of the dipolar field to the effective DMI calculated using MuMax³ simulations (green and purple) and an analytical model by Lemesh et al.Lemesh2018a (blue and red).
• Figure 3: Effect of the enhanced and reduced effective DMI on the current- and laser-induced nucleation of skyrmions. In (a) - (l) we show MFM scans of the magnetization in the devices fabricated from the enhanced and reduced multilayer stack after applying a single nucleation pulse. (a) - (c): Enhanced stack after current-induced nucleation, the current densities are: $J =$0A.m^-2, $J =$7.14e11A.m^-2, and $J =$7.77e11A.m^-2, respectively. (d) - (f): Reduced stack after current-driven nucleation, the current densities are: $J =$0A.m^-2, $J =$6.97e11A.m^-2, and $J =$7.55A.m^-2. In (a) through (f) the current flows from top to bottom. (g) - (i): Enhanced stack after laser-induced nucleation, the fluences are: $F =$8.9mJ.cm^-2, $F =$11.6mJ.cm^-2, and $F =$15.0mJ.cm^-2, respectively. (j) - (l), Reduced stack after laser-induced nucleation, the fluences are: $F =$6.9mJ.cm^-2, $F =$9.1mJ.cm^-2, and $F =$10.1mJ.cm^-2. In (m) and (n), we summarize the results from the current-driven and laser-induced skyrmion nucleation, respectively. For each current density we plot the average skyrmion density in the scanned area, for both the enhanced (blue dots) and reduced stack (red diamonds). The solids lines in (m) and (n) are fits to the data using \ref{['eq:error_function']}.
• Figure 4: Dependence of the number of skyrmions after nucleation on the strength of the bias field applied during nucleation. (a) Current-induced nucleation (b) laser-induced nucleation. The lines are a guide to the eye.