Magnetic phase diagram of layered manganites in the highly doped regime

The naturally layered colossal magnetoresistive manganites La2-2xSr1+2xMn2O7 exhibit an extremely varied range of magnetic and electronic behavior over a very narrow composition range between x = 0.3 and x = 0.5. The successful synthesis in our laboratories of compounds with nominally greater than 50% Mn4+ concentration has now allowed the study of this region of the phase diagram. Here we present detailed neutron diffraction measurements of these compounds with doping levels 0.5 < x < 1.0. As predicted by simple theories, the type-A layered antiferromagnetic (AF) structure is found at x similar to 0.5 and the type-G rocksalt AF structure at x = 1.0. Between these two extremes is found a C-type structure (ferromagnetic rods parallel to b coupled antiferromagnetically to all neighboring rods) stabilized by orbital ordering of y(2) states. Also in this Mn4+-rich regime is found a region in which no long-range magnetic order is observed. We discuss how semiempirical models can explain the variety of magnetic structures and how structural trends as a function of doping corroborate the unifying notion of a shift from in-plane to axial orbital occupation as the Mn4+ concentration is decreased. (C) 2001 American Institute of Physics.