Spin canting in three-dimensional perovskite manganites

We reexamine the double exchange (DE) theory of the canted spin arrangement in three-dimensional perovskite manganites R_1-xD_xMnO₃ (R=trivalent rare-earth ion, D=divalent ion). Our approach is based on the multiple-scattering theory, takes into account the twofold degeneracy of the e_g levels, and allows us to address the problem of local stability of the collinear and spin-canted states with respect to rotations of the spin magnetic moments. We argue that the metallic antiferromagnetism is compatible with the DE picture if the former goes along with the cubic symmetry breaking. There is an important self-stabilizing mechanism inherent to the DE physics itself, when the anisotropic antiferromagnetic (AFM) ordering acts as the main symmetry breaker and produces dramatic changes of the electronic structure, which appear to be sufficient to explain the local stability of this AFM state. As the result, the anisotropic AFM ordering can be stabilized, and eventually become the ground state of the DE model. This explains the rich magnetic phase diagram of doped manganites at large x, which typically includes the isotropic ferromagnetic ordering and a number of anisotropic AFM structures of the layered and chainlike types. We stress that this is a typical behavior of the DE systems. Parameters controlling the local stability of these phases are magnetic-state dependent. This may lead to the phase coexistence in certain areas of x, and eventually to a phase separation. We also expect a number of stable spin-canted configurations which can compete with the phase separation. The main ideas of the model analysis are illustrated in the first-principles calculations for the virtual-crystal alloy La_1-xBa_xMnO₃ in the local-spin-density approximation.