Indexed on: 13 Jun '17Published on: 13 Jun '17Published in: arXiv - Physics - Strongly Correlated Electrons
The multiferroic hexagonal rare-earth manganites h-REMnO$_3$ display a diverse array of magnetic phenomena, as many contain both rare-earth and Mn magnetic moments which interact in a low symmetry crystal structure. Of these materials h-HoMnO$_3$ (HMO) possesses the largest rare-earth magnetic moment and is therefore ideal for investigating magnetic exchange in hexagonal manganites. In this work, time-domain THz spectroscopy experiments uncover evidence for exceptionally strong and unconventional Ho-Mn spin interactions in HMO through careful examination of an antiferromagnetic resonance (AFR) of the Mn sublattice. Experiments reveal the AFR to split asymmetrically in magnetic field with substantially different $g$-factors for the high and low energy branches of this excitation. The temperature dependence reveals this asymmetry to be related to the Ho sublattice magnetization. Furthermore, we uncover a drastic renormalization of the $g$-factors of the Mn AFR at the Ho spin ordering temperature, confirming strong Ho-Mn coupling. Theoretical calculations demonstrate these results are not explained by conventional exchange mechanisms and therefore suggest a new Ho-Mn spin interaction in HMO. Our results provide a paradigm for the optical study of such spin interactions in other hexagonal manganites.