Perhaps the most basic classification of a material is according to whether or not it conducts electricity in response to an applied electric field. However, our understanding of why the metallic state is preferred over the insulating one, or vice versa, is far from complete. This is particularly true for various groups of materials that transform between the two states by displaying a metal to insulator transition (MIT) in which the electrons go from being delocalised to fully localised. It is believed that the MIT in materials in which correlations between electrons are particularly strong arises from collective quantum effects. This makes this group of strongly correlated electron materials - typically oxides containing transition metals - an attractive subject for studies from a fundamental point of view. There is also interest in exploring their use in applications as the MITs can be tuned to be exquisitely sensitive to changes in external control parameters (temperature, electric or magnetic fields, etc.). Most recently, attention has focussed on so called correlated spin-orbit materials in which the electrons are also strongly relativistic, leading to an enhanced coupling of the two basic flavours of magnetism, spin and orbital.
In a recent paper in Nature Communications an international team from the USA, Germany, China and the LCN reports a comprehensive study of the electronic and magnetic properties of a prototypical correlated spin-orbit material undergoing an MIT. Using a combination of neutron and X-ray scattering techniques, the team were able to solve the magnetic structure and measure the nature of the magnetic excitations in Cd2Os2O7 (see figure). Their results suggest that the MIT in Cd2Os2O7 lays outside of the standard paradigm of correlated electron materials, with the basic driving mechanism of the MIT being the formation of the unusual all in/all out magnetic structure.
Stuart Calder, a staff scientist from Oak Ridge National Laboratory and former PhD student from the LCN comments: "The material is a rare example of where magnetic ordering and the MIT are directly related. We were able to use both neutron and X-ray scattering to gain direct access to the way the spins order, how they interact and how the electrons are arranged in the magnetic ions. The results show, surprisingly, a strong degree of spin-orbit coupling associated with exotic states and they indicate that a reformulation beyond the established mechanisms used to describe MITs is required in Cd2Os2O7."
Figure: The magnetic structure and excitations in Cd2O2O7 revealed by neutron and X-ray scattering. A section of the crystal structure of Cd2Os2O7 shows the position of the osmium S=3/2 moments. The low-temperature magnetic structure corresponds to either all the osmium magnetic moments pointing into or out from the centre of the corner-shared tetrahedra. The red arrow indicates an excitation of the all in/all out magnetic structure formed by flipping one of the osmium spins. Image courtesy of Oak Ridge National Laboratory.
Link to Nature Communications article: Spin-orbit-driven magnetic structure and excitation in the 5d pyrochlore Cd2Os2O7