Magnetic skyrmions with unconventional helicity polarization in a van der Waals ferromagnet

Skyrmion helicity, which defines the spin swirling direction, is a fundamental parameter that may be utilized to encode data bits in future memory devices. Generally, in centrosymmetric ferromagnets, dipole skyrmions with helicity of -π/2 and π/2 are degenerate in energy, leading to equal populations of both helicities. On the other hand, in chiral materials where the Dzyaloshinskii-Moriya interaction (DMI) is prevailing and the dipolar interaction is negligible, only a preferred helicity is selected by the type of DMI. However, whether there is a rigid boundary between these two regimes remains an open question. Herein, we report the observation of dipole skyrmions with unconventional helicity polarization in a van der Waals ferromagnet Fe5-δ GeTe2 . Combining magnetometry, Lorentz transmission electron microscopy, electrical transport measurements, and micromagnetic simulations, we demonstrate that the short-range superstructures in Fe5-δ GeTe2 results in a localized DMI contribution, which breaks the degeneracy of the opposite helicities and leads to the helicity polarization. Therefore, the helicity feature in Fe5-δ GeTe2 is controlled by both the dipolar interaction and DMI that the former leads to Bloch-type skyrmions with helicity of ±π/2 whereas the latter breaks the helicity degeneracy. This work provides new insights into the skyrmion topology in van der Waals materials.