High pressure neutron studies reveal evolution of magnetism through an insulator-to-metal transition

Through record-breaking high-pressure measurements at the ILL, it has been possible for the first time to directly track the evolution of magnetism through an insulator-to-metal transition in an exciting family of low-dimensional magnetic insulators.

Such materials, in which magnetic interaction is broadly limited to within the two-dimensional planes of transition metal ions, provide a versatile playground in which to explore both the fundamental properties of magnetism and electrical conduction as well as exhibiting a range of behaviour useful for the development of future devices.

FePS₃ is a material similar to graphite in that it may be delaminated down a single monolayer. It differs to graphite in that it is intrinsically magnetic, presenting some intriguing physical properties and technological possibilities.  Previous experiments have shown that as pressure is applied to FePS₃, the material undergoes a series of structural transitions. The higher pressure of these involves a collapse of the inter-planar spacing causing the material to become more three-dimensional in nature. It is this transition which drives the material from being electrically insulating to become metallic.

Up to now, the question of what happens to the long-range magnetic order in the system as the material metallises has only been possible to probe by indirect techniques. Using specially designed double-toroidal sintered diamond anvils on the D20 beamline at the ILL, neutron diffraction measurements were performed up to a maximum pressure of 18.3 GPa, far above what has previously been possible for non-specialist users.

Through the evolution of the primary magnetic scattering feature, the magnetic order is seen to evolve with the structural transitions, first switching the inter-planar coupling from anti- to ferromagnetic, and then with the collapse of the inter-planar spacing, long-range order is lost and is replaced with a previously unobserved form of short-range magnetic order.

This result is of great interest due to the observation of superconductivity under pressure in the related compound FePSe₃. Previous indirect measurements by x-ray emission spectroscopy have reported a crossover to a zero spin state accompanying metallisation in both compounds. The direct contradiction by this neutron study raises questions regarding the coexistence of magnetism and superconductivity in these low-dimensional materials, or the potential for such superconductivity to be magnetically mediated. Superconductivity in these materials is exciting as an alternate pathway to explore this phenomenon outside of more complex materials such as the similarly layered high T_c cuprates.

This was the first user experiment not performed by the cell developers to make use of this new high-pressure set up at the ILL, and the stunning success of the measurements mark excellent progress that further demonstrates the viability of this apparatus for more widespread use in scattering experiments at the institute.