Layered CMR manganites: Structure, properties, and unconventional magnetism

Neutron powder diffraction studies of the layered compounds R{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}, (R = La, Pr, Nd), RSr{sub 2}Mn{sub 2}O{sub 7} (R = Pr, Nd), and La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} show that the degree of distortion of the MnO{sub 6} octahedra do not correlate with the appearance of a metal-insulator (MI) transition in these compounds. Instead, the in-plane Mn-O bond length appears to be a better indicator of the electronic behavior. Detailed bulk magnetization studies on single crystal La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} show that there are three magnetic regimes as a function of temperature: paramagnetic insulator, short-range ordered (SRO) ferromagnet, and long-range ordered (LRO) ferromagnet. Scaling analysis indicates that a 2D finite-size XY model is an appropriate description of the magnetic state in the SRO regime