Abstract
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∼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 y2 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.
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