Spin-driven multiferroics is a research field which explores nontrivial phenomena emerged by spin-orbit coupling (SOC). The effect of SOC in quasi-particle spectra has been investigated in high-energy range; electromagnon, hybridization of magnon and phonon, and renormalization of optical magnon. The effect of SOC to acoustic magnon in low-energy range which plays important roles in bulk properties, however, has not been studied. Here we demonstrate that the acoustic magnon in multiferroics Ba₂MnGe₂O₇ [1-3] is hybridized with orbitals by measuring the temperature variations of magnetization curves and neutron scattering spectra. The temperature dependence of the anisotropy energy is scaled not by the spin moment but by the electric polarization. The observation evidences renormalization of orbital effect into a spin-nematic interaction and the hybridized magnon in an emergent field. Our finding exhibits that the spin-flop transition field is a sensitive probe for SOC, which could be a key for search of new magnetoelectric materials.
[1] H. Murakawa et al., Phys. Rev. B 85, 174106 (2012).
[2] T. Masuda et al., Phys. Rev. B 81 100402(R) (2010).
[3] Y. Iguchi et al., Phys. Rev. B 98, 064416 (2018).