Optical Linear Dichroism in the ab-Plane of NdFe3(BO3)4 Ferroborate

Abstract

The high-resolution optical absorption spectra of NdFe₃(BO₃)₄ single crystals have been recorded at temperatures from 4 to 40 K in the IR range of f–f transitions in a Nd³⁺ ion. Light linearly polarized at different angles to the C₂ axes in the basal plane has been passed along the trigonal C₃ axis. Below the temperature of magnetic moment ordering into a collinear antiferromagnetic structure (T_N ≈ 30 K), dichroism, that is, the absorption versus polarization dependence, arises. The temperature and angular dependences of dichroism indicate that the magnetic moments of iron are directed along the C₂ axes up to about 17 K, the number of domains with variously directed C₂ axes being different. The mechanism of linear dichroism has been discussed. Below 17 K, a smooth transition to the helicoidal magnetic phase has been observed, with the collinear phase coexisting with the helicoidal one. Data presented in this article contradict the earlier concept of magnetic moments fluctuating in the low-temperature phase near the C₂ axis within the ±10° interval.