Magnetic Soft Modes in the Distorted Triangular Antiferromagnet alpha CaCr2O4

Abstract

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 CaCr2O4, 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. DOI 10.1103 PhysRevLett.109.127203 PACS numbers 75.30.Et, 61.05.F , 75.10.Hk, 75.30.Ds In frustrated antiferromagnets AF , the magnetic interactions compete and it is impossible to satisfy them all simultaneously. This competition causes degenerate ground states to occur, and the system fluctuates between them, suppressing long range magnetic order to temperatures well below the Curie Weiss temperature [1]. Most frustrated magnets do eventually order, and in the case of isotropic or Heisenberg interactions a compromise is often reached where the magnetic interactions are all partially satisfied by a noncollinear arrangement of the magnetic moments. The triangular lattice Heisenberg antiferromagnet TLHAF is one of the simplest examples of an extended frustrated magnetic system; however, open questions remain concerning its physical properties. For spin 1, different spin liquid phases have been proposed [2,3], while spin 1 2 and large spin systems order in a helical structure where the angle between nearest neighbor spins is 120 . Introduction of antiferromagnetic next nearest neighbor interactions modifies the excitations and drives the magnetic structure from helical to collinear when the ratio of next nearest to nearest neighbor interactions is greater than the critical value of 0.125 [4]. In the collinear phase, quantum fluctuations lift the classical degeneracy and select collinear stripe order via the mechanism of order by disorder [5]; they also significantly alter the spin wave excitation spectra around certain regions of the Brillouin zone [4]. Most physical realizations of triangular lattices are distorted. The distortion usually constitutes a shortening of the neares