Nontrivial Temperature Dependence Of Magnetic Anisotropy In Multiferroic Ba2mnge2o7

We measured the temperature dependences of the static magnetization and the spin excitation in the squarelattice multiferroic Ba2MnGe2O7. An anisotropy gap of the observed low-energy mode is scaled by electric polarization rather than a power of the sublattice moment. Spin-nematic interaction in the effective spin Hamiltonian, which is equivalent to interaction of the electric polarization, is responsible for the easy-axis anisotropy. The nontrivial behavior of the anisotropy gap can be rationalized as change in the hybridized d-p orbital with temperature, leading to the temperature dependence of the spin-nematic interaction.