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 fluctuations 1 , 2 . Similarly, the atomic magnetic moments (spins) in a magnet are expected to order at a temperature scale set by the Curie–Weiss temperature θ CW (ref. 3 ). Geometrically frustrated magnets represent an exception. In these systems, the pairwise spin interactions cannot be simultaneously minimized because of the lattice symmetry 4 . This can stabilize a liquid-like state of short-range-ordered fluctuating moments well below θ CW (refs 5–7 ). Here we use neutron scattering to observe the spin liquid state in a geometrically frustrated system, Tb 2 Ti 2 O 7 , under conditions of high pressure (∼9 GPa) and low temperature (∼1 K). This compound is a three-dimensional magnet with θ CW = -19 K, where the negative value indicates antiferromagnetic interactions. At ambient pressure Tb 2 Ti 2 O 7 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.