All That Matters

Domains in a ferroelectric material, where electric charges have a different orientation. Here, there are two separate sets of domains. The cross-hatched patterns indicate domains in the plane, the rounder shapes are domains where the polarization points out of the plane. Reprinted by permission from Macmillan Publishers Ltd. Nature Materials 7, 209-215 (2008).

Insulators might seem pretty boring materials for an electronic device such as a computer memory, because by the very nature of their definition, they don’t conduct any electrical current. But some insulators show some pretty intriguing properties. Amongst them are the so-called ferroelectrics.

Dipoles in a ferroelectric. During switching, positive and negative charges interchange.

A ferroelectric is a material where positive and negative electrical charges, are permanently separated along a common direction. These are the positive and negative ions that make up the crystal. Their order leads to an overall electrical polarization of the material. This can only happen in an insulator, because if the crystal would enable electrical charges to move around the separated plus and minus charges could be compensated easily by such movements of electrons.

In some special materials, ferroelectricity and magnetism occur simultaneously. These are known as multiferroics, and I blogged about their potential applications before. In particular, the dipoles in a ferroelectric can be switched by an electric field, which makes them attractive for electronic applications as ferroelectrics can be used to permanently store information as a new form of computer memory.

But how can the electric polarization in a ferroelectric be switched? There are two options. One mechanism is similar to what happens in a magnet. If an electric field is applied, new domains with a polarization aligned in direction of the external field form (see figure below). These domains gradually replace the old ones. This process is abrupt, because as the new domains expand, the ions in the crystal swap places in a single process.

The second possibility of switching electric polarization is a continuous mechanism. There, the positive and negative ions move slowly in opposite direction. First, the electric polarization weakens, vanishes, and then builds up again in opposite direction. This process occurs without the involvement of any domains. Of these two processes, the domain-based switching is far more favourable, which is why the switching process without domains hasn’t been observed before. Two independent papers now both claim to have seen switching without domains. Continue reading…

Photo of the electronic device used to measure the high electron mobility in ZnO. Reprinted by permission from Macmillan Publishers Ltd. Nature Materials (2010). doi:10.1038/nmat2874

Even if you don’t know much about this compound, everybody is familiar with zinc oxide (ZnO). It is a white powder used as the UV-light absorbing component in many sun lotions (and the part of the sun lotion that leaves those white marks on clothes), as an antibacterial agent in some baby powders, in rubbers where it promotes vulcanisation, or as white pigment in colours, and for many other products.

And even now ZnO remains one of the most widely studied oxygen-containing oxide compounds — and this is not going to change anytime soon. In a paper that is now published in my journal Nature Materials, Atsushi Tsukazaki, Masashi Kawasaki and colleagues from Tohoku University in Japan accomplished growing ZnO to such high purity that its electrons can move at extremely high speeds. The material is so clean that it shows quantum effects that are only known from a few very pure compounds. Darrell Schlom from Cornell University in the USA, who works on the growth of oxides, is quite enthusiastic: “Oxides have the unfortunate stigma of being associated with dirt, bricks, and toilet bowls. I love this rags to riches story because it shows that oxides can be clean, so clean that with ZnO they have broken into the most exclusive and elite club that was reserved for just half a dozen of the world’s cleanest materials. This is the greatest achievement of the year for oxides!”

Indeed, the ZnO thin films that Tsukazaki and colleagues grew are so clean that electrons in them move so fast that the researchers were able to observe the so-called Fractional Quantum Hall Effect (FQHE), a first for any oxide compound. The FQHE is a sign that electrons are in quantum states that can be used in quantum computing, and by showing the FQHE, ZnO has established itself as a candidate material for such schemes.

Continue reading…