Paramagnetic magneto-optical activity of f-f transitions in HoFe3(BO3)4 and HoAl3(BO3)4 single crystals

Absorption and magnetic circular dichroism (MCD) spectra of HoFe3(BO3)4 and HoAl3(BO3)4 single crystals were measured as a function of temperature in the range of 90–293 K. Based on integrals of f-f absorption and МСD bands, temperature dependences of the paramagnetic magneto-optical activity (MOA) of f-f transitions were obtained for the first time. It was revealed that MOA of the same f-f transitions in the studied crystals were substantially different 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 as a consequence of the nature of f-f transitions allowance. In temperature dependences of the MOA of two transitions the singularities were revealed, which indicated to some local structural distortions in the corresponding excited states.