Quantum magnets under pressure

The demonstration that applied pressure can substantially change – rather than merely tweak – the properties of a metal–organic quantum magnet indicates a route to designing quantum materials with tailored properties.

Quantum magnets have evolved over the past decade into systems of large interest in fundamental and, increasingly, in applied physics. Their exceptional properties, including non-magnetic ground states, fractionalised excitations and topological order, have become candidate resources for quantum information and computing. Application in spintronic devices are possible if spin interactions can be controlled.

It has long been known that pressure effects, whether physical, chemical or realised by growing thin films on mismatched substrates, can be used to control magnetic interactions. Usually this pressure control takes the form of a fine-tuning over a percent-level range. In some metal–organic materials, however, the combination of a soft lattice and strong structural anisotropy can lead to changes at the 10-% level over accessible pressure ranges.

Writing in Physical Review Letters this week [1], Björn Wehinger (University of Geneva & Laboratory for Neutron Scattering and Imaging at PSI) and colleagues report continuous pressure effects at the 100-% level. This giant pressure dependence of magnetic interaction was demonstrated on the metal–organic material [CuF₂(H₂O)₂]₂pyrazine in experiments performed at the US National High Magnetic Field Laboratory in Tallahassee together with detailed experimental and theoretical analysis at the PSI's NUM division and at SLS, at the University of Geneva, the University of Bern and the Université de Sherbrook (Canada).

At zero pressure, this material realises a quasi-two-dimensional spin-½ square-lattice Heisenberg antiferromagnet. By detailed susceptibility measurements over a wide range of high pressures and extreme applied fields, the team showed that the dominant exchange parameter is reduced continuously by a factor of 2 on compression. The giant pressure dependence is accompanied by a second qualitative change, in the form of a discontinuous dimensionality switching: a phase transition occurs above 18 kbar, inducing an orbital re-ordering that transforms the square lattice into a system of weakly coupled chains. The magnetic exchange can thus be switched from quasi-two to quasi-one dimensional. By combining detailed x-ray and neutron diffraction studies with quantitative modelling using spin-polarized density functional theory and Quantum Monte Carlo simulations, Wehinger and co-workers explained the evolution of the magnetic orbitals and hence the microscopic mechanisms for both phenomena. This extraordinary pressure dependence of magnetic interactions is important in fundamental science for the study of quantum many-body effects in low-dimensional quantum magnets and is expected to have direct applications in the design of antiferromagnetic spintronic devices using piezoelectric heterostructures.