Visualizing ferro-rotational domains and domain walls using scanning nonlinear optical spectroscopy and microscopy*

Ferroic orders have attracted growing interest over the past few decades. Among these include the ferro-rotational (FR) order, invariant under both time reversal and spatial inversion operations. Despite theoretical predictions of its wide presence and outstanding significance, the FR order is experimentally challenging to investigate due to its nontrivial conjugate coupling fields. Investigations of FR order domains and domain walls are critical not only to characterizing physical properties of the FR order but also in determining, for example, the multiferroic states in many type-II multiferroic candidates. Here, we report our efforts in using a series of second harmonic generation (SHG) techniques to investigate the FR domains and domain walls in NiTiO₃. We use a SHG rotational anisotropy (RA) technique to survey a NiTiO₃ sample and show the presence of two FR domain states that both and individually lack mirror symmetries but are related to each other by a vertical mirror operation. We then use scanning SHG microscopy to visualize the spatial distribution of the two FR domain states along with their corresponding domain walls. Finally, we take SHG RA measurements both across and along the domain walls and demonstrate the mirror symmetry is preserved at the domain walls, while broken within domains.