  Polarized absorption spectra of single-crystal Nd0.5Gd0.5Fe3(BO3)4 were
studied in the region of transition 4I9/2-(4G5/2+2G7/2) in Nd3+ ion as a
function of temperature (2-34 K) and magnetic field (0-65 kOe). Spectra of
natural circular dichroism were measured in the range of 5-40 K. It was found
out that the local magnetic properties in vicinity of the excited ion
substantially depend on its state. In particular, a weak ferromagnetic moment
appears in some excited states. It was found out that selection rules for
electron transitions in magnetically ordered state substantially deviate from
those in paramagnetic state of the crystal. They are different for different
transitions and they are very sensitive to orientation of the sublattice
magnetic moment relative to the light polarization. In the spectrum of the
natural circular dichroism, the transition is revealed which is not observed in
the absorption spectrum.
