Spin dynamics study and experimental realization of tunable single-ion anisotropy in multiferroic Ba 2 CoGe 2 O 7 under external magnetic fields

We report a spin-wave study on multiferroic ${\mathrm{Ba}}_{2}{\mathrm{CoGe}}_{2}{\mathrm{O}}_{7}$ under magnetic fields up to 12 T using low-energy inelastic neutron scattering. In-plane transverse $({T}_{1})$ spin-wave modes are highly dispersive along $(h00)$ and rather flat but strong in intensity along $(30l)$. In addition, two dispersive electromagnon modes have been observed around 3.5 meV. Dispersion of the out-of-plane transverse modes $({T}_{2})$ under fields reveals that the single-ion anisotropy constant decreases with increasing magnetic field, which is consistent with the linear spin-wave theory. Our results imply that the field-dependent single-ion anisotropy plays a crucial role in determining the characteristics of ${T}_{2}$ and electromagnon modes in the three-dimensional anisotropic spin-wave spectrum.