Putting the pressure on magnetism
Applying pressure to metal compounds with electronic and magnetic properties of technological interest provides a uniquely powerful way of tuning those properties so as to understand them better. Through record-breaking high-pressure measurements, we have been able to track the fascinating evolution of conductivity and magnetism in an interesting layered compound as the layers of atoms are pushed together.
Metals by definition conduct electricity – that is because the outer electrons of metal atoms are held rather loosely and are free move through the metal. Another characteristic of electrons is that they also have a quantum property called spin, the source of magnetism. If the spins all align in one direction (ferromagnetism), as they do are in some metals, then this creates the bulk magnetic force seen in everyday iron magnets. One of the most exciting areas of modern physics is the study of the interplay of magnetism and conductivity as seen in a range of materials consisting of metals chemically combined with non-metals (which are normally insulating) such as oxygen, sulfur and phosphorus. These compounds often show a range of intriguing electronic and magnetic behaviours, some being conducting only over a particular temperature or pressure range; some becoming superconductors at low temperatures – that is, conducting electricity without resistance. Some compounds are antiferromagnetic, which means that adjacent spins are alternately oppositely aligned.
A significant aspect is that the geometrical layout of the atoms in these materials – the crystal structure – mediates their electronic and magnetic properties. This becomes very clear in those compounds that have a layered structure, where the space between the layers of atoms is greater than that between atoms within the layers. The result is that the electromagnetic interactions can be different along the layers than between them. Such materials are interesting, not only because they provide probes of exotic electronic behaviour, but also because, in single-layer form, they may lead to the next generation of nano-electronic devices that work right at the quantum level.
We have been studying the magnetic behaviour of one particular compound, iron phosphorus trisulfide (FePS3), which consists of honeycomb layers of magnetic iron atoms with large gap between them. This is an insulator that is normally antiferromagnetic both along and across the layers. We were interested in seeing what happened when the layers were gradually compressed under very high pressures. Our previous experiments had shown that when pressure is applied to the material, the spacing between the layers becomes so reduced that the distances between atoms becomes comparable in all three dimensions – the inter-layer gap closes to form a fully 3D crystal. The compound also starts to behave like a metal, as the electrons become free to move in all directions.
We followed how the magnetism changed with increasing pressure. First, the antiferromagnetic arrangement of spins across the layers changed to ferromagnetic as they were squeezed closer together. Then, as the interatomic spacings became the equivalent in all directions, the magnetic alignments began to uncouple such that they survived only over short distances. This is probably due to the increased mobility of the conduction electrons disrupting local atomic environments, and the spins interacting more strongly with those above and below them. This remained the case up to room temperature, in contrast to the low temperature (73K) required to induce magnetism under normal pressure.
We would like to know more about this emerging magnetic behaviour under high pressure. In the analogous selenium compound, FePSe3, superconductivity is seen at high pressure. We think that it might also be observed in FePS3 at higher pressures or lower temperatures. Such findings are significant in understanding the mysterious relationship between superconductivity and magnetism in superconducting materials of technological importance.