Light absorption anomalies in the magnetic phase transition of FeCo3

The behavior of exciton and exciton-magnon light absorption bands in magnetic fields up to 300 kOe (H∥C3) is studied in monocrystals of antiferromagnetic (TN = 38°K) rhombohedric (R3¯c) dielectric FeCo3. In the region of the magnetic phase transition (H1 < H < H2, H1 = 148 kOe, H2= 176 kOe) to the state with parallel orientation of the magnetizations of both sublattices (S1∥S2) a number of anomalies were observed: an abrupt change in exciton line splitting in a field H1 and further unusual behavior of this line at H > H1; significant attenuation of integral intensity of the exciton-magnon absorption band upon reaching H1 and almost complete extinction of absorption in fields H > H2. The energy of exciton line components and the integral coefficient of exciton-magnon absorption are calculated in the molecular field model. It is shown that, in the critical field range where a multisublattice structure is realized, three types of ion may be distinguished, to each of which there corresponds a certain excitation energy value. Within the framework of this calculation the behavior of the exciton line component frequencies and the integral coefficient of exciton-magnon absorption correlate completely with crystal magnetization, and are explained by anomalies of the magnetic phase transition in FeCo3.

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