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

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 C-2 axes in the basal plane has been passed along the trigonal C-3 axis. Below the temperature of magnetic moment ordering into a collinear antiferromagnetic structure (T-N approximate to 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 C-2 axes up to about 17 K, the number of domains with variously directed C-2 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 C-2 axis within the +/- 10 degrees interval.