Designing quantum phases in metal-organic magnets

Summary by Jem Pitcairn

Figure 1. Left: the structure of CrCl₂(Pym), showing the alternating spin chains weakly coupled by the organic molecules. Right: Inelastic neutron scattering (INS) measurements and calculations which allowed us to measure the strength of the magnetic interactions.

The spins in typical magnetic materials form 3D ordered arrays as they are cooled, like a liquid solidifying. Some magnets however, form 1D chains or 2D sheets that do not freeze into a static ordered arrangement but stay instead in a dynamic liquid-like state. These states can give rise to unusual quantum phenomena, some of which possess properties thought to be key to next-generation quantum technologies. One such quantum phenomenon is the “Haldane phase”, which formed the basis of the 2016 Nobel Prize in Physics and is the prototypical example of “topological matter” (see this explanation using coffee and doughnuts). The Haldane phase can occur in infinite chains of magnetic spins which prefer to alternate directions (so-called antiferromagnetic interactions) only when each spin is made from an even number of unpaired electrons (integer spin, S). While the Haldane phase is predicted to occur for all spins consisting of an even number of electrons, it has been only been actually seen in chains made from two electron spins (S = 1) and remains elusive for all larger even numbers (S >= 2). This is because the more electrons per spin, the more stringent the requirements on the other magnetic interactions in the material.

Motivated by this challenge we have made a new compound CrCl₂(pyrimidine), and investigated its magnetic properties. We found it was formed from infinite CrCl₂ chains bridged by organic pyrimidine molecule into 2D sheets. Our magnetic measurements confirmed that there were four unpaired electrons per Cr and so we decided to investigate how close this material was to the Haldane phase. We used neutron scattering (at ISIS Neutron and Muon Source and the Institut Laue Langevin) to measure that the interactions much stronger through the CrCl₂ chain than through the organic connector: making this family of materials a good family to search for this Haldane phase. We found upon further investigation we found that CrCl₂(pyrimidine) undergoes normal 3D magnetic ordering, but the ability to make analogous compounds with tunable properties by swapping the organic molecule suggest it could be used to tune the magnetic interactions towards ideal conditions for the Haldane phase.

This work was carried out in collaboration with the University of Birmingham, ISIS Neutron and Muon Source and the Institut Laue Langevin.