Doping influence of spin dynamics and magnetoelectric effect in hexagonal Y$_{0.7}$Lu$_{0.3}$MnO$_3$

Inelastic neutron scattering experiments were performed to study spin waves and their correlation with the magnetoelectric effect in Y$_{0.7}$Lu$_{0.3}$MnO$_3$. The Mn trimerization distortion has been suggested to play a key role in determining the magnetic structure and the magnetoelectric effect in YMnO$_3$ and LuMnO$_3$. In Y$_{0.7}$Lu$_{0.3}$MnO$_3$, our INS study reveals a much smaller in-plane (hexagonal \textit{ab}-plane) anisotropy gap that coincides with a weaker in-plane dielectric anomaly at $T_N$. Since both the smaller in-plane anisotropy gap and the weaker in-plane dielectric anomaly are coupled to a weaker Mn trimerization distortion in Y$_{0.7}$Lu$_{0.3}$MnO$_3$ comparing to YMnO$_3$ and LuMnO$_3$, we conclude that the Mn trimerization is responsible for the magnetoelectric effect and multiferroic phenomenon in Y$_{1-x}$LuxMnO$_3$.