Spin dynamics study and experimental realization of tunable single-ion anisotropy in multiferroic Ba2CoGe2 O7 under external magnetic fields

Abstract

We report a spin-wave study on multiferroic Ba2CoGe2O7 under magnetic fields up to 12 T using low-energy inelastic neutron scattering. In-plane transverse (T1) 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 (T2) 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 T2 and electromagnon modes in the three-dimensional anisotropic spin-wave spectrum.