Ferromagnetic and antiferromagnetic correlations in a pair of manganese impurities

Abstract

Mixed valent manganese ions play an important role in the magnetoresistance of LaMnO3-based systems. We describe each Mn impurity with the Mn4- represented by a spin S = 3/2 (three localized 3d-t2g electrons with their spins ferromagnetically coupled) and the Mn3 configuration having an additional localized 3d-eg electron to form a total spin (S + 1/2). The eg electron hybridizes with conduction electrons (representing the crystal), excluding the multiple occupancy of the eg level at each site. This gives rise to a quadrupolar Kondo effect (orbital singlet), which interferes with the usual spin Kondo effect. We consider a pair of such manganese ions and allow the eg electrons to hop between the two sites. Hence, bonding and antibonding levels are formed giving rise to the ferromagnetic (F) double-exchange mechanism, which competes with the antiferromagnetic (AF) superexchange between the t2g spins of the two impurities. In the intermediate valent regime we calculate the ground state energy, the valence, the population of the bonding and antibonding states, the charge and the quadrupolar susceptibilities, and the response to a charge imbalance between the two sites as a function of the energy of the eg level for parallel and antiparallel t2g spins using a mean-field slave-boson formulation. The susceptibilities are a measure for the coupling to phonons, i.e. Jahn-Teller distortions or small polarons. A crossover from AF to F to AF correlated groundstates is found as a function of the eg level energy for reasonable model parameters.