Self-trapped magnetic polaron in electron-doped CaMnO3

We study the energetics of the self-trapped magnetic polaron (electron plus the distorted local magnetization cloud) in electron-doped manganites, e.g., Ca1-xLaxMnO3, with small x and at zero temperature. A single electron moving in a cubic lattice of antiferromagnetic t(2g) core spins, as appropriate for the manganites, is examined, taking into account the effects of the nearest- and the next-nearest-neighbour hoppings and the Anderson-Hasegawa double-exchange, as well as the Jahn-Teller interaction. We compute the ground-state energy and the wavefunction of the system using a set of self-consistent equations. While we show that the next-nearest-neighbour hopping significantly reduces the binding energy of the magnetic polaron, this reduction is not enough to destabilize the self-trapped state. The ground state of the polaron is found to be a seven-site ferromagnetic region, comprising the central spin and the six nearest neighbours, with a net magnetic moment of approximately 7 mu(B), in qualitative agreement with the interpretation of Neumcier and Cohn of their experimental magnetization data (Neumeier and Cohn 2000 Phys. Rev. B 61 14319). We argue that the polaron should exhibit an activated hopping as seen in the experiments, and estimate an activation energy of about 40 meV.