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

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.

💡 Research Summary

The authors investigate a novel route to control the effective Dzyaloshinskii‑Moriya interaction (DMI) in magnetic multilayers by exploiting the internal dipolar field. In conventional Ir/Co/Pt stacks the interfacial DMI has a fixed sign, which determines the handedness of Néel‑type domain walls and skyrmions. By constructing a multilayer in which the bottom half consists of Pt/Co/Ir repeats (positive DMI) and the top half of Ir/Co/Pt repeats (negative DMI), the dipolar field generated by the up‑ and down‑domains aligns parallel to the DMI effective field throughout the whole stack. This alignment enhances the net DMI, whereas the opposite stacking order makes the dipolar field antiparallel to the DMI field, reducing the net DMI.

Four stacks were fabricated: Uniform+ (all positive DMI), Uniform‑ (all negative DMI), Enhanced (positive DMI in the bottom three repeats, negative DMI in the top three), and Reduced (the reverse). All samples share the same seed and cap layers, and their saturation magnetization (Ms) and uniaxial anisotropy (Ku) are essentially identical, as confirmed by SQUID‑VSM measurements. The effective DMI strength |Deff| was extracted from equilibrium domain widths measured by magnetic force microscopy (MFM) and interpreted using the Lemesh et al. model. The Enhanced stack exhibits a |Deff| that is 2.5 × larger than that of the Reduced stack, confirming that the dipolar‑field engineering successfully modifies the DMI without altering other magnetic parameters.

Skyrmion nucleation was probed by two independent stimuli: (i) 50 ns current pulses delivered through a narrow line, and (ii) single 70 fs laser pulses (800 nm wavelength) focused onto the device. In both cases the Enhanced stack shows a dramatically higher nucleation density—up to a factor of 20—compared with the Reduced stack. Moreover, skyrmions in the Enhanced stack survive at higher out‑of‑plane magnetic fields, indicating improved stability. The threshold current density and laser fluence required for nucleation change only weakly with DMI, suggesting that the primary effect of the enhanced DMI is to lower the energy barrier for skyrmion formation and to increase the domain‑wall energy reduction that stabilizes the textures.

Micromagnetic simulations (MuMax3) and an analytical dipolar‑field model were employed to quantify the contribution of the dipolar field to the effective DMI. The simulated values match the experimentally extracted |Deff|, reinforcing the interpretation that the internal dipolar field can be harnessed as an effective DMI‑boosting (or suppressing) mechanism.

The work demonstrates three key advances: (1) a practical method to engineer the effective DMI in multilayers by simply reversing the stacking order of identical trilayers, thereby exploiting the intrinsic dipolar field; (2) the ability to enhance skyrmion nucleation efficiency for both electrical and ultrafast optical excitations, despite their vastly different timescales; and (3) clear evidence that DMI engineering can be decoupled from changes in Ms or Ku, allowing systematic studies of DMI‑dependent phenomena. These findings open a pathway toward low‑energy skyrmion‑based memory and logic devices, where reliable nucleation and robust stability are essential. Future directions include extending the dipolar‑field‑enhanced DMI concept to other material systems, refining quantitative models of dipolar‑DMI interaction, and integrating the approach with device architectures that require deterministic skyrmion creation and deletion.