Magnetic soft modes in the distorted triangular antiferromagnet α-CaCr ₂O ₄

In this Letter, we explore the phase diagram and excitations of a distorted triangular lattice antiferromagnet. The unique two-dimensional distortion considered here is very different from the "isosceles"-type distortion that has been extensively investigated. We show that it is able to stabilize a 120° spin structure for a large range of exchange interaction values, while new structures are found for extreme distortions. A physical realization of this model is α-CaCr ₂O ₄, which has a 120° structure but lies very close to the phase boundary. This is verified by inelastic neutron scattering which reveals unusual rotonlike minima at reciprocal space points different from those corresponding to the magnetic order.