Colossal magneto-optical conductivity in doped manganites

We show that the current carrier density collapse in doped manganites, whic h results from bipolaron formation in the paramagnetic phase, leads to a co lossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition. As with the colossal ma gnetoresistance itself, the corresponding magneto-optical effect is explain ed by the dissociation of bipolarons into polarons owing to the exchange in teraction with the localized Mn spins in the ferromagnetic phase. The effec t is positive at low frequencies and negative in the high-frequency region. The present results agree with available experimental observations. [S0163 -1829(99)02233-X].