A Theory for a Self-Trapped Antiferromagnetic Polaron at Finite Temperatures

A self trapped antiferromagnetic polaron at finite temperatures is studied in detail in the region where the band width E 0 is larger than the s – f exchange energy I S . Fluctuation effect is taken into account to the order of ( I S / E 0 ) 2 . Within this approximation, the extended molecular field theory is applicable for the localized spins. A large magnetic polaron can exist in which the localized spins are aligned nearly ferromagnetically. With increasing temperature, the fluctuation effect reduces the effective field on the localized spins, making the effective radius of the polaron larger and the polaron itself unstable. The overall feature of the magnetic polaron in the narrow band limit is also investigated on the simplified model.