Phase transition and domain formation in ferroaxial crystals

Abstract

The ferroaxial order, which is characterized by a rotational structural distortion in a crystal, has been recently proposed as one of ferroic orders. Though the domain formation is a characteristic feature in ferroic materials, there has been little study done concerning that for the ferroaxial order. Here, we investigate ferroaxial domains that are formed through a ferroaxial transition in two representative ferroaxial materials, ${\mathrm{NiTiO}}_{3}$ and $\mathrm{RbFe}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}$. We spatially resolve their domain structures using an optical method based on electric-field-induced optical rotation, that is, electrogyration (EG). In ${\mathrm{NiTiO}}_{3}$, multidomains are constructed when crystals undergo a ferroaxial transition and the domain size depends on the cooling rate around the transition temperature. Furthermore, the ferroaxial domain structure obtained by the EG measurement is well matched with that by scanning x-ray diffraction. $\mathrm{RbFe}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}$ also exhibits multidomain states in which domain patterns are different each time a crystal undergoes a ferroaxial transition. In addition, the temperature dependence of the EG signal well obeys that of the order parameter of a first-order phase transition. These results ensure the effectiveness of the EG effect to elucidate the nature of ferroaxial order.