Spin dynamics of a canted antiferromagnet in a magnetic field

The spin dynamics of a canted antiferromagnet with a quadratic spin-wave dispersion near q=0 is shown to possess a unique signature. When the anisotropy gap is negligible, the spin-wave stiffness D_sw(q,B)=(w _q-B)/q² depends on whether the limit of zero field or zero wave vector is taken first. Consequently, D_sw is a strong funtion of the magnetic field at a fixed wave vector. Even in the presence of a sizable anisotropy gap, the field dependence of the extrapolated q=0 gap energy, distinguishes a canted antiferromagnet from a phase-separated mixture containing both ferromagnetic and antiferromagnetic regions.