New nonlinear modes in planar antiferromagnets: A study of polarized-neutron, inelastic scattering of (${\mathrm{CD}}_3$${)}_4$${\mathrm{NMnCl}}_3$ in a transverse field

Neutron inelastic scattering combined with neutron-polarization analysis has been used to observe new, nonlinear excitations in the planar antiferromagnet (${\mathrm{CD}}_3$ ${)}_4$ ${\mathrm{NMnCl}}_3$ (TMMC) at low temperatures and in the presence of a large, transverse magnetic field. The new excitations result from a field-induced nonlinear coupling of in-plane and out-of-plane magnons.