Magnetic phase diagrams of manganites in the electron doping region

Abstract

The paper reviews the physical properties of the R1−xAxMnO3 manganites (R=La, Pr, Nd, Sm, etc., A=Ca, Sr, Ba) in the region of their electron doping where the divalent atom concentration x in the compound lies in the interval 0.5<x<1.0. Experimental magnetic phase diagrams of the most well-studied compounds and the results of theoretical calculations of these diagrams made in the tight-binding approximation within the degenerate double-exchange model for T=0 are presented. The experimental section of the review deals primarily with neutron diffraction studies of the magnetic and crystal structures of the manganites, and the theoretical part, with the relation between their magnetic and orbital structures. The review describes, in considerable detail, the method of calculation of the energy spectrum ɛ(k) and of the total carrier energy for all possible magnetic and orbital configurations of the system corresponding to the translation symmetry of the lattice. The theoretical analysis is carried out separately for two models of the crystal structure, with two and four manganese atoms in the unit cell. All equilibrium magnetic and orbital configurations of the four-sublattice manganite model were determined by minimizing the total energy of the system with respect to the directions of the local manganese magnetic moments and orbital states of the eg electrons. It is shown that, by using the effective Hamiltonian of the degenerate double-exchange model for the eg electrons, which takes into account the eg level splitting, and the Heisenberg Hamiltonian of the localized t2g electrons, one can describe the diversity of the magnetic phases, the sequence of their alternation with increasing x, and the correlation between the spin and orbital degrees of freedom, which are observed in most electron-doped manganites.