Abstract: | Influence of the partial substitution of paramagnetic Fe3+ ions by diamagnetic
Ga3+ ions in the trigonal crystal
GdFe3 (BO3)4 on its optical and magnetic properties is
studied and discussed in connection with problems common for all antiferromagnets
containing 3d
5 ions. Polarized optical absorption spectra and
linear birefringence of GdFe3 (BO3)4 and
GdFe2.1Ga0.9 (BO3)4 single crystals have been
measured in the temperature range 85–293 K. Specific heat temperature dependence (2–300 K)
and structure of GdFe2.1Ga0.9 (BO3)4 crystal
have been also studied. As a result of substitution of 30% Fe to Ga the Neel temperature
diminishes from 38 till 16 K, the strong absorption band edge shifts on
860 cm-1 (0.11 eV) to higher energy and the
d-d transitions intensity decreases substantially
larger than the Fe concentration does. Strong absorption band edge is shown to be due to
Mott-Hubbard transitions. Correlation between position of the strong absorption band edge
and the Neel temperature of antiferromagnets has been revealed. Properties of the doubly
forbidden d-d transitions in the studied crystals and
in other antiferromagnets are explained within the framework of the model of the
exchange-vibronic pair absorption, which is theoretically analyzed in detail. The model
permitted us to determine the connection between parameters of
d-d absorption bands (intensity, width and their
temperature dependences), on the one hand, and the exchange, spin-orbit and
electron-lattice interactions, on the other hand. |