Competing, or frustrated interactions between atoms, molecules, electrons, etc are ubiquitous in a wide variety of physical systems. They arise in molecular crystals and in superconducting materials, they are involved in the phenomenology of protein folding in biology and are believe to stabilize an exotic thin “nuclear pasta” crust near the surface of neutron stars, providing the setting for dramatic and possibly observable astronomical events occurring at the surface of those stars. The theoretical understanding of physical systems with competing interactions is at present very much incomplete. Very often, competing interactions give rise to intriguing thermodynamic phenomena that challenge a full microscopic understanding but invite comparison to other systems. It turns out that magnetic materials and models of magnetic systems have long provided physicists with an exquisite platform to unravel the broad principles that govern collective phenomena in nature. In a recent Nature Communications paper, Gingras and collaborators exposed in a spin ice material — a system in which the microscopic atomic magnetic moments experience frustrated interactions, and closely mimic the behaviour of the protons in common water ice — an heretofore unnoticed analogy with classical gases and introduced for the first time the notion of “special temperatures” in frustrated ferromagnets, in analogy to the conventional gas-liquid transition. This work provides a useful setting to characterize magnetic materials subject to competing interactions. One hopes that the ideas fleshed out in this paper can be useful to understand other class of complex materials with complex competing microscopic interactions.
