Mapping 3D orientation fields on the nanoscale
Orientation fields exist across many material systems – from local crystallographic orientations in crystals, to ordering in biological materials, and even magnetic ordering of spins in ferro- and antiferromagnetic materials. The macroscopic properties of a system are strongly linked to the local nanoscale orientation fields that can form domains of uniform orientation, domain – or grain – boundaries, and even topological textures. Being able to map such orientations on the nanoscale is key to understanding – and designing! – a wide variety of material systems.
To make the visualisation of such nanoscale orientation fields possible, researchers from the Max Planck Institute for Chemical Physics of Solids, the University of Oxford, the ETH Zurich, EPFL and the Paul Scherrer Institute have developed a new technique called XL-DOT – X-ray linear dichroic orientation tomography. This technique harnesses a resonant X-ray effect called linear dichroism – in which the scattering of the X-rays depends strongly on the orientation of the electric field of linearly polarised X-rays with the orientation fields in a material. By combining this effect with nanoscale spatial resolution imaging, three-dimensional tomographic measurements and new algorithms to analyse the data, they obtain direct access to three-dimensional orientation fields, on the nanoscale, in three-dimensional space. This work has just been published in Nature.
crystallographic and topological defects”, Apseros et al.
In the first demonstration of the technique, the researchers targeted a porous sample of the catalyst vanadium pentoxide (V2O5) that was formed of many small crystallites. Using XL-DOT, they mapped the crystal orientation in 3D, revealing a complex orientation distribution. The 3D orientation field is shown in the figure, where on the left-hand side, the morphology can be seen in the scalar tomogram of the electron density of the material – and on the right-hand side, the local orientation distribution is represented by rods, coloured by their in-plane orientation. Within the 3D distribution, it was possible to identify single crystal grains, as well as the presence of different types of grain boundaries – twin, twist and tilt. Particularly interesting was the observation of smoothly winding topological textures in the orientation – so-called comet and trefoil structures, similar to those you find in your fingerprints. By tracking these topological textures in 3D through the sample, it was possible to see how they interact with volume defects – i.e. voids – within the sample. Indeed, these volume defects were found to promote the creation and annihilation of the topological defects, providing insight into how different types of material defects interact.
With this new capability to map nanoscale orientation fields, the researchers are particularly excited about what comes next! The technique can immediately be applied to a number of materials, allowing for different scientific questions to be addressed. For example, as well as mapping crystallographic microstructure, it should become possible to map ferroic order in condensed matter systems – of particular interest for our institute. In this way, one could imagine mapping the 3D configuration of the Néel vector in antiferromagnets, something that until now has not been possible!
As well as looking at the static configuration of materials, seeing how they react to stimuli can help us understand how they behave. Combining this new technique with operando measurements will allow one to track the evolution of defects or domain configurations in both crystallographic microstructure and ferroic materials in response to the application of magnetic fields, or strain.
