Neutron scattering study of magnetic anisotropy in the tetragonal antiferromagnet Bi2CuO4

We present a comprehensive study of magnon excitations in the tetragonal easy-plane antiferromagnet Bi2CuO4 using inelastic neutron scattering and spin-wave analyses. The nature of low-energy magnons, and hence the anisotropy in this material, has been controversial. We show unambiguously that the low-energy magnon spectrum consists of a gapped and a gapless mode, which we attribute to out-of-plane and in-plane spin fluctuations, respectively. …