In-house synthesis and characterisation methods

The synthesis of new materials is central to our research in order to be in control in all parts of the Materials research spiral (see figure on the right).

We use two techniques called magnetron sputter deposition and molecular beam epitaxy (MBE) to fabricate thin films, layers with thicknesses on the nanometre scale, of crystalline or amorphous materials. These thin films often have physical properties which differ significantly from those of thick bulk materials.

Layers of different materials can be deposited with thickness control down to a single layer of atoms. By simultaneous deposition of different materials from several sources we can produce alloys and phases that do not exist naturally. For sputter deposition it is also possible to admit a reactive gas into the sputtering chamber to produce compounds such as oxides or nitrides. These deposition processes are extremely versatile allowing us to tailor our samples to our research questions.

Self-assembly is the spontaneous formation of structures on all length scales. Examples range from atoms forming galaxies containing millions of stars to cells growing into highly complex biological structures. The crucial parameter controlling self-assembly is the specific interaction between building blocks.

Self-assembly on intermediate length scales, tens of nanometres up to millimetres, allows to develop materials with new properties interesting for many applications. The most relevant interactions between building blocks for our research are van der Waals forces, hydrophobic, and magnetic and electrostatic interactions. The first two in particular are comparable to thermal excitations at room temperature, which might cause chemical bonds to break and reform resulting, for example in phase transitions.

In our group, we focus on fundamental understanding of self-assembly of colloids as well as viscoelastic properties. We study the first one by tuning magnetic and electrostatic interactions between nano- and micrometre sized particles. This way we control the self-assembly process and design materials in a bottom-up approach. We also drive self-assembled systems out of equilibrium and study the response. By combing stress-strain measurements with scattering methods we try to understand how elastic and flow properties of materials are related to the structure of the building blocks down to atomic length scales.

The magneto-optical Kerr effect (MOKE) provides a convenient way to obtain the changes in magnetic moment at different fields and temperatures. We have several experimental setups using this effect.

Hysteresis loops, with shapes strongly dependent on e.g. anisotropy or interlayer coupling, are conveniently measured at room temperature or under other conditions, depending on which setup is used. For investigations of e.g. magnetic phase transitions, a set-up with extremely low background fields (less than 10 μT) is used. Here we can measure both AC susceptibility and low-field loops at different temperatures below 300 K. The sensitivity allows studies of even just a few monolayers of magnetic material.

At the other extreme, some samples need higher fields to reach saturation. The Vector MOKE setup combines three configuration modes with different wavelengths (spectroscopic MOKE; all elements respond differently to the incident light), a relatively high field, and a temperature range 4 – 500 K. Simultaneous measurement of the first harmonic (ellipticity) and second harmonic (rotation) of the modulated reflected light allows determination of the scaling of magnetisation with field in the vicinity of the ordering temperature as well as separation of the contribution from different elements in multilayered structures.