Imaging oxygen beyond the diffraction limit in ferroelectric ultra-thin films

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

Some materials can spontaneously generate a magnetic or electrical field within themselves, and are known as ferromagnetic or ferroelectric materials. The fields usually organise themselves into a domain structure, with different regions having fields that have different polarity. Technologically, this has many uses since we can organise and switch the domains - in devices such as a hard disc drive, or a ferroelectric random access memory. When these materials are produced in the form of a thin layer the domain size matches the layer thickness - which is very useful since it allows both a higher density of domains to be packed into thinner films and less power to switch them. However, we have discovered that when ferroelectric films are made extremely thin - around ten atomic layers or less - they exhibit very different behaviour with fields that curl, twist and change in complicated ways. We would like to understand how and why the materials behave in this strange way, both for the limitations on conventional devices and also to see if we can exploit these effects, e.g. for devices that could switch between multiple states rather than just two.
The only way to 'see' such tiny structures is to use transmission electron microscopy, which is now capable of atomic resolution and can be used to map the picometer displacements of the atoms that produce the electric fields. This travel grant will develop a new collaboration between electron microscopists at Warwick who have been working on these materials and the world-leading group of Prof. Rosenauer in Bremen, Germany, who has developed a new electron microscopy technique to image the atoms in a material that has strong advantages for these measurements. The visits will allow the two groups to exchange expertise, establish their different techniques in each other's institutions, work together to understand these materials, and expand the work into new fields.

Planned Impact

We expect to achieve an immediate impact by publishing the study to evaluate and compare imaging techniques for ferroelectrics that will result from this work. Longer term, it is clear that application of the I-STEM technique to materials that have a close relationship between their properties and the picometre-scale displacements of atoms in their unit cells will lead to new science, developing the field of functional oxide materials in particular. We also expect to have wider impact across the electron microscopy community by establishing the I-STEM technique in the UK and acting as a centre for distribution of the technique on JEOL machines.

Publications

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Description During the development of this grant we worked to implement a new technique of electron microscopy, ISTEM, to observe oxygen in some materials. This technique was developed by a German group in certain type of microscope and we wanted to extrapolate the technique to ours.
We discovered that although it was possible to implement the techique on our sytem, the complex setup due to the different optics made the experiment very time consuming. Even simple and necessary things like changing the magnification of the image was difficult. One of our key aims was to publish a study comparing our usual way of imaging oxygen atoms with this new method, but a competing group published a paper on exactly this theme while we were still working to collect data. Given the complexity of implementation we think that there is no significant benefit in using ISTEM on our microscope.
Exploitation Route ISTEM is a powerful technique to image oxygen atoms if you have the appropriate electron microscope for this technique. For systems like the one at Warwick, the level of complication to get a single image should be considered before implementation. You can get very good results with other techniques such as Annular Bright Field in these systems.
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