Shifting and Pinning of a Magnetic Vortex Core in a Permalloy Dot by a Magnetic Field

Magnetic pinning in thin films seems to be a major research subject in the near future, as it is involved in all switching processes which include a movement of a domain wall or a magnetic vortex. We used Lorentz transmission electron microscopy and vortex pinning at artificial pinning sites to investigate the pinning behavior of magnetic vortices for the first time with high spatial resolution.

Figure 1

In a second preparation step a hole array was introduced in the already patterned Permalloy disks. First, the sample was coated with PMMA, the hole pattern was transferred via $e$-beam lithography and the resist developed (a). Then Titanium was deposited onto the specimen, tilted by 45° (b). This results in an incomplete coating, which leads to a selective etching of the holes during the ion etching process (c). After removal of the residual PMMA and Ti, the hole pattern was transferred to the disks as well as to the electron transparent substrate (d). A suitable alignment of the two preparation steps leads to central holes in the disks.

Figure 2

The line scan method is shown, which allows the determination of the vortex core position. The outer (a) and inner (b) edges of the particle can be seen as sharp peaks in the signal, while the vortex appears as a change of the signal’s sign.

Figure 3

The position of the vortex in a circular dot with $3\mu \mathrm{m}$ diameter in an applied external field is depicted, the dashed line shows a linear fit in the zero field region, the gray area is the associated 5% confidence interval.

Figure 4

A vortex configuration was prepared in a circular dot with $3\mu \mathrm{m}$ diameter and a 300 nm central hole. The vortex was shifted by an external field towards the center from the right (a) or the left (b) side, respectively. In the central hole the vortex became pinned, before the hole boundary was actually reached, the vortex persisted pinned over a certain field range, until in the opposite field the vortex was released.

Figure 5

The vortex is moved through a circular dot with a diameter of $1\mu \mathrm{m}$. In this high-resolution measurement jumps in the vortex position are visible which can be attributed to microscopic pinning sites. The enlargement of the gray area shows this very clearly.

Figure 6

Schematic representation of possible stable pinning sites for a vortex within a fictive crystallite arrangement. The double arrows give the local anisotropies within the crystallites, which are sought to be met as closely as possible by the internal structure of the vortex (black arrows indicate the local magnetization direction within the vortex, the gray scaled areas in the background represent predominantly down or up oriented regions). The vortex position is optimized to match the given local anisotropies with the least distortion and increase in the energies involved. The vortex position in (a) is energetically more favorable compared to that in (b), as the magnetization in one crystallite (second from left) corresponds to the respective anisotropy. Thus, pinning at the configuration (a) is expected.