Dr. Šmejkal, what is the problem on which your research focuses?
Dr. Libor Šmejkal: Many fascinating and technologically important nanoelectronics components rely on ferromagnetism. A notorious example is a magnetic hard-drive read head in computer. The problem is that the ferromagnetism produces stray fields which limit the scaling of devices and can disturb other parts of the electronics circuits. Also, they can destroy the states with low dissipation promising for next generation nanoelectronics devices. Ferromagnets are also often highly metallic what hinders to separate the low dissipation states from the other conventional states which do dissipate energy and limit the efficiency of the device.
So, on one hand, you need them to generate the effect, and on the other hand, they are an obstacle. People observed the dissipationless limit of Hall effect only either at very low temperatures at like minus 200-something degrees of Celsius or at extremely huge applied magnetic fields. You can imagine that you can’t have a huge fridge or magnet in your computer or some medical applications.
Interestingly, in the 1930s another class of magnets was discovered, which are called antiferromagnets. Antiferromagnets have antiparallel moments in the crystals. If you zoom in to the atomic sites in the material, the moments are not all oriented in one direction, as in ferromagnets, but they are like a checkerboard: Up, down, up, down, and so on (marked by red and blue in Fig 1).
However, for many decades scientist believed, that due to their alternating moment structure they will not produce effects present in ferromagnets. And that brings us to our discovery of "ferromagnetic effects" in antiferromagnetic substances. We were asking the question: Is it even possible to suppress the unwanted dipolar fields without killing the magnetic effects itself? Can we find some systems which are better for applications?
We found out that if we distribute in a checkerboard antiferromagnet cage of nonmagnetic atoms in a tricky way, we can keep the material antiferromagnetic, but they will start to have magnetic effects! Practically, we were simulating the relativistic quantum-mechanical effects of such materials on supercomputers (Fig 2). And we found that, in contrast to what is known, a new form of magnetization densities and in trun also Hall current arises in these materials!
When you look at the conventional magnets like iron, the magnetization densities are very symmetric, they look like balloons around the atoms. Also, many conventional antiferromagnets have highly spherical magnetisation densities (see Fig.1 left). Unfortunately, the magnetic effects precisely compensate by the oppositely polarized spherical magnetisation clouds. Surprisingly, we have discovered that the magnetization densities are shaped more like dumbbells in our new class of antiferromagnets. This new distribution of magnetic density does not lead to compensation of the magnetic effects such as the Hall current as can be intuitively understood from the anisotropic shapes shown in Fig. 1 right .
However, at the same time, the magnetization densities integrate to zero so there are no stray fields which is exactly what we wanted to have. All the great magnetic effects and moving electrons without dissipation at room temperature without stray fields! The next step is to push these electrons to the edges because then we could remove the states which remain in the bulk, which are highly conductive and decrease the efficiency. Then we would be very close to the dream of having only these dissipation-less states at the room temperature in a perfectly compensated antiferromagnetic substance.