Peculiarities of optical absorption spectra of NdFe${}_3$(BO${}_3$)${}_4$ crystal in magnetically ordered state and at the transition to spiral magnetic phase

We study experimentally and theoretically optical absorption spectra of neodymium ferroborate in the temperature range $6-32$ K. We show that the temperature dependences of integral intensities of absorption lines demonstrate qualitatively different behavior for $\sigma$- and $\pi$-polarizations of the light. We suggest that this behavior may be caused by the appearance of a spiral magnetic structure in the compound under study. To describe the temperature dependences of lines intensities we propose a theoretical model which takes into account the Dzyaloshinskii-Moriya interaction between Nd${}^{3+}$ and Fe${}^{3+}$ ions.