Magneto-optical activity of f–f transitions in ErFe3(BO3)4 and ErAl3(BO3)4 single crystals

• Experimental temperature dependences of f–f transitions magneto-optical activity.
• Magneto-optical activity of f–f transitions does not follow the Curie–Weiss law.
• Magneto-optical activity of f–f transitions strongly depends on the local environment.
• The vibronic transition with the large natural optical activity was revealed.
• Two nonequivalent absorbing centers connected with the helicoidal chains.

Absorption, magnetic circular dichroism and natural circular dichroism spectra of ErFe3(BO3)4 and ErAl3(BO3)4 single crystals were measured as a function of temperature in the range of 90–293 K. It was found out that magneto-optical activity of the same f–f transitions in the studied crystals substantially differed and their temperature dependences did not follow the Curie–Weiss law in contrast to the properties of allowed transitions. The observed phenomena were accounted for by the nature of f–f transitions allowance. Properties of the transition 4I15/2→4S3/2 were studied in detail. In particular, the Zeeman splitting and the natural optical activity of the absorption lines composed of the transition were determined. The vibronic line with the very large natural optical activity was revealed and identified. Two nonequivalent Er3+ ion positions with the opposite chirality were found out in one of the excited states. Polarization properties of the 4I15/2→4S3/2 transition in the ErFe3(BO3)4 crystal have shown that the local symmetry of Er3+ ion in this crystal in the range of 90–293 K is lower than the D3 one. From the heat capacity measurements it was revealed, that the first order structural phase transition to lower symmetry occurred in ErFe3(BO3)4 at 433–439 K.