Domain Wall Skyrmions: Examining Domain Wall Substructures in [Pt/(Co/Ni)M/Ir]N Multi-Layers

Abstract

Magnetic domain wall (DW) Skyrmions are 360° transitions of the internal magnetization in a Dzyaloshinskii DW. These structures are analogous to vertical Bloch lines (VBLs), which were once considered for magnetic memory storage. Although they have been predicted theoretically [1], DW skyrmions have only recently been observed experimentally [2]. Here we performed a systematic study of multiple iterations of [Pt/(Co/Ni)M/Ir]N multi-layers to formulate a phase diagram of magnetic excitations related to domain wall substructures, including DW skyrmions, via Lorentz transmission electron microscopy (LTEM). [Pt/(Co/Ni)M/Ir]N multi-layers were deposited by magnetron sputtering onto Si3N4 TEM membranes [3]. LTEM was performed using an aberration-corrected FEI Titan G2 80-300. In low DMI samples, VBLs are formed over all thicknesses, but are only seen to form pile-ups (i.e. n-π VBLs) in the case of ultrathin samples. We attribute this to the formation of hybrid DWs in thicker samples where the magnetization varies along the z-axis, thereby, creating low energy paths to annihiliation of 2π VBLs. Likewise, only pure Dzyaloshinskii DWs are observed in thicker samples with high DMI. It is only for lower thicknesses in this strong DMI regime that DW skyrmions are observed. Like their VBL counterparts, DW Skyrmions are found to have a pinning effect on the domain wall as seen through the in-situ application of a perpendicular magnetic field. The stability and dynamic response of DW skyrmions are examined further via micromagnetics. The energy barrier to annihilation was calculated using M3 [4] and a GNEB method and is observed to decrease with increased DMI strength. Most notably an energy barrier to annihilation of >60 kBT was found for D<1.0 mJ/m2 at room temperature giving confidence in the stability of DW Skyrmions for use in magnetic memory devices. Details regarding their dynamic response to electric current will be discussed. [5]