Methods to control the direction of excitations, analogous to diodes in electric circuits, have been sought after to create devices. An example of anisotropic excitations in bulk materials are spin waves in low crystallographic symmetry crystals where magnons propagating in differing directions have dissimilar velocities. Such excitations have been defined as nonreciprocal given that the motion in one direction differs from that in the opposite in the presence of broken time-reversal symmetry such as in the presence of an applied magnetic field. Ba3NbFe3Si2O14 (Langasite) is structurally and magnetically single-domain chiral with the magnetic helicity induced through competing symmetric exchange interactions. We show that the spin-waves in Langasite display directional anisotropy. On applying a time-reversal symmetry breaking magnetic field along the c-axis, the spin-wave energies are different when sign is reversed for either the momentum transfer ±Q or applied magnetic field ±μ0H. We discuss this directional anisotropy in the dispersion in Langasite in terms the broken twofold symmetry of the underlying P321 space group. Langasite provides an example where directional anisotropy, often referred to as nonreciprocal, can occur in antiferromagnetic phases in the absence of the Dzyaloshinskii-Moriya interaction or other effects resulting from spin-orbit coupling.
C. Stock et al. Phys. Rev. B 100, 134429 (2019)
Dr. Jitae Park
Dr. Dominic Hayward