TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature T(N)<41 K is accompanied by ferroelectric order for T<28 K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3.

As liquids crystallize into solids on cooling, spins in magnets generally form periodic order. However, three decades ago, it was theoretically proposed that spins on a triangular lattice form a liquidlike disordered state at low temperatures. Whether or not a spin liquid is stabilized by geometrical frustration has remained an active point of inquiry ever since. Our thermodynamic and neutron measurements on NiGa2S4, a rare example of a two-dimensional triangular lattice antiferromagnet, demonstrate that geometrical frustration stabilizes a low-temperature spin-disordered state with coherence beyond the two-spin correlation length. Spin liquid formation may be an origin of such behavior.

Neutron scattering is used to investigate spin correlations in ultrapure single crystals of the S=1 triangular lattice NiGa(2)S(4). Despite a Curie-Weiss temperature of Θ(CW)=-80(2)  K, static (τ>1  ns) short-range (ξ(ab)=26(3)  Å) incommensurate order prevails for T>1.5  K. The incommensurate modulation Q(0)=(0.155(3),0.155(3),0), Θ(CW), and the spin-wave velocity (c=4400  m/s) can be accounted for by antiferromagnetic third-nearest-neighbor interactions J(3)=2.8(6)  meV and ferromagnetic nearest-neighbor coupling J(1)=-0.35(9) J(3). Interplane correlations are limited to nearest neighbors and weakened by an in-plane field. These observations show that the short-range ordered glassy phase that has been observed in a number of highly degenerate systems can persist near the clean limit.

No abstract

We present experimental evidence for the unconventional coherent state in two dimensions, found in the geometrically frustrated antiferromagnet NiGa 2 S 4 with a triangular Bravais lattice. Despite strong antiferromagnetic coupling, S = 1 spins only show nanoscale order at low temperatures. Thermodynamic measurements, however, indicate a gapless linearly dispersive mode and suggest coherence beyond the two-spin correlation length.