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Magnetic structures and data storage

Chiral magnetic structures was the central topic of the thesis of Mark de Jong. But what is a magnetic structure?

In most magnets, one side of the magnet is the north pole, and the other side is the south pole. A physicist would say that the magnetization of the material is uniform and points towards the north pole throughout the material.

De Jong looked at very thin layers made of magnetic metals such as cobalt, where this magnetization does not have to be uniform. The north pole can be at the top of the layer or at the bottom, and this can vary on a very small length scale of about a hundred nanometers, creating a magnetic structure in the material. In this structure, data is encoded as follows: if the magnetization points upwards, it is a '1', and if it points downwards, it is a '0'.

Between these regions of upward and downward magnetization, the so-called domains, there are narrow regions where the magnetization rotates from one direction to the other, called domain walls. If the rotation direction for each wall in the magnetic structure is the same, we call the structure chiral. This property, chirality, turns out to be important for the stability of the domains, for the formation of these domains, and for the ways we can influence and move these domains with an electric current. Chirality is therefore crucial for future data storage technologies that use such magnetic structures.

Skyrmion energy

De Jong’s research contributes to the understanding of chiral magnetic textures in several ways.

For example, he showed that the energy required to create a small circular magnetic domain with a chiral domain wall, a so-called skyrmion, decreases when the material is irradiated with Ga+ ions.

These ions collide with the atoms in the material, causing an increase in the degree to which the atoms of the different layers are mixed. This affects the magnetic properties and ultimately lowers the energy needed to create such a domain.

One of the nice features of this technique is that you can irradiate locally, with a resolution of just tens of nanometers. This means that in the future, devices could be made where skyrmions are formed only in designated areas.

Electron microscopy

In addition to research on creating skyrmions, De Jong also used a beautiful electron microscopy technique called SEMPA (Scanning Electron Microscopy with Polarization Analysis).

With this technique, it’s possible to not only measure the number of electrons, as in a normal electron microscope, but also the direction of the local magnetization with high resolution.

De Jong used this, for example, to measure the magnetization in the domain walls of an Ir|Co bilayer, which is very difficult with other techniques because the walls are so thin.

Using his fabrication technique, there was uncertainty about the chirality of the magnetization in this system, but with SEMPA, De Jong and his colleagues were able to determine this chirality directly.

In short, in this thesis, De Jong looked at different ways to visualize and manipulate very small chiral magnetic textures.