Pressure-induced crystallization of a spin liquid

Liquids are expected to crystallize at low temperature. The only exception is helium, which can remain liquid at 0 K, owing to quantum fluctuations1,2. Similarly, the atomic magnetic moments (spins) in a magnet are expected to order at a temperature scale set by the Curie–Weiss temperature θCW (ref. 3). Geometrically frustrated magnets represent an exception. In these systems, the pairwise spin interactions cannot be simultaneously minimized because of the lattice symmetry4. This can stabilize a liquid-like state of short-range-ordered fluctuating moments well below θCW (refs 5–7). Here we use neutron scattering to observe the spin liquid state in a geometrically frustrated system, Tb2Ti2O7, under conditions of high pressure (∼9 GPa) and low temperature (∼1 K). This compound is a three-dimensional magnet with θCW = -19 K, where the negative value indicates antiferromagnetic interactions. At ambient pressure Tb2Ti2O7 remains in a spin liquid state down to at least 70 mK (ref. 8). But we find that, under high pressure, the spins start to order or ‘crystallize’ below 2.1 K, with antiferromagnetic order coexisting with liquid-like fluctuations. These results indicate that a spin liquid/solid mixture can be induced by pressure in geometrically frustrated systems.