Nature inspired complex magnetic order

Summary by Maddie Geers

Nature is a deep-rooted source of inspiration for innovations, a notion continually woven into our everyday lives. Magnesium silicate (MgSiO₃) is one of the most abundant minerals on Earth and about 20 years ago it was discovered to undergo a structural transition. In most of the Earth’s mantle, magnesium silicate adopts the perovskite structure, however, plunging to the lower depths of the Earth’s mantle it evolves to have the post-perovskite structure. The high temperatures and pressures needed for this transition to occur (1200 K and 120 GPa) make studying this compound very challenging, so identifying compounds that exhibit the post-perovskite structure at atmospheric pressure is a useful task. One method to achieve this is by replacing the atomic oxygen anions with molecular anions, for example dicyanimide (NCNCN^–) or thiocyanate (NCS^–) ligands.

This work explores the properties of three thiocyanate compounds: CsM(NCS)₃, where M = Ni²⁺, Mn²⁺ and Co²⁺, which, through X-ray diffraction measurements, are found to adopt the post-perovskite structure in ambient conditions. Since the transition metals (M) have unpaired electrons–meaning the ions possess a permanent magnetic moment—when the materials are cooled these moments interact with neighbouring moments forcing them to “lock” into an ordered magnetic structure. The magnetic moments now have specific orientations, which can be revealed using neutron diffraction experiments. In this paper we determined the magnetic structures of CsNi(NCS)₃ and CsMn(NCS)₃. Both compounds have magnetic structures that are non-collinear: the magnetic moments are arranged with orientations that are neither parallel or anti-parallel to other moments in the structure. These non-collinear orderings likely originate from the post-perovskite arrangement as structural connectivity plays a role in dictating the magnetic interactions experienced by the transition metal ions. Materials with non-collinear magnetic structures have potential future applications in memory storage devices, so compounds such as these molecular post-perovskites provide potential avenues of exploration to unearth the physics sought after for future information technologies.

This work was carried out in collaboration with the Institut Laue Langevin and the Advanced Materials Research Group at the University of Nottingham.