Spin-wave directional anisotropies in antiferromagnetic Ba 3 NbFe 3 Si 2 O 14

Ba3NbFe3Si2O14 (langasite) is structurally and magnetically single-domain chiral with the magnetic helicity induced through competing symmetric exchange interactions. Using neutron scattering, we show that the spin waves in antiferromagnetic langasite display directional anisotropy. On applying a time-reversal symmetry breaking magnetic field along the c axis, the spin-wave energies differ when the sign is reversed for either the momentum transfer ±→Q or applied magnetic field ±μ0H. When the field is applied within the crystallographic ab plane, the spin-wave dispersion is directionally isotropic and symmetric in ±μ0H. However, a directional anisotropy is observed in the spin-wave intensity. We discuss this directional anisotropy in the dispersion in langasite in terms of a field-induced precession of the dynamic unit cell staggered magnetization resulting from a broken twofold symmetry. Directional anisotropy, often referred to as nonreciprocal responses, can occur in antiferromagnetic phases in the absence of the Dzyaloshinskii-Moriya interaction or other effects resulting from spin-orbit coupling.