Universal exchange-driven phonon splitting

We report on a linear dependence of the phonon splitting on the non-dominant exchange coupling $J_{nd}$ in the antiferromagnetic monoxides MnO, Fe$_{0.92}$O, CoO and NiO, and in the highly frustrated antiferromagnetic spinels CdCr$_{2}$O$_{4}$, MgCr$_{2}$O$_{4}$ and ZnCr$_{2}$O$_{4}$. For the monoxides our results directly confirm the theoretical prediction of a predominantly exchange induced splitting of the zone-centre optical phonon [1,2]. We find the linear relation $\hbar\Delta\omega = \beta J_{nd} S^2$ with slope $\beta$ = 3.7. This relation also holds for a very different class of systems, namely the highly frustrated chromium spinels. Our finding suggests a universal dependence of the exchange-induced phonon splitting at the antiferromagnetic transition on the non-dominant exchange coupling [3].\\[4pt] [1] S. Massidda et al., Phys. Rev. Lett. 82, 430 (1999).\\[0pt] [2] W. Luo et al., Solid State Commun. 142, 504 (2007).\\[0pt] [3] Ch. Kant et al., arxiv:1109.4809.