Interplay of spin and orbital ordering in the layered colossal magnetoresistance manganite La2-2xSr1+2xMn2O7 (0.5 <= x <= 1.0)

The crystallographic and magnetic phase diagram of the n = 2 layered mangan ite La2-2xSr1+ 2xMn2O7 in the region x greater than or equal to0.5 has been studied using temperature-dependent neutron powder diffraction. The magnet ic phase diagram reveals a progression of ordered magnetic structures gener ally paralleling that of three-dimensional (3D) perovskites with similar el ectronic doping: A (0.5 less than or equal tox less than or equal to0.66)-- >C (0.75 less than or equal tox less than or equal to0.90)-->G (0.90 less t han or equal tox less than or equal to1.0). However, the quasi-2D structure amplifies this progression to expose features of manganite physics uniquel y accessible in the layered systems: (i) a ''frustrated'' region between th e A and C regimes where no long-range magnetic order is observed; (ii) magn etic polytypism arising from weak interbilayer magnetic exchange in the typ e-C regime; and (iii) a tetragonal-to-orthorhombic phase transition whose t emperature evolution directly measures ordering of d(3y2-r2) orbitals in th e a-b plane. This orbital-ordering transition is precursory to type-C magne tic ordering, where ferromagnetic rods lie parallel to the b axis. These ob servations support the notion that e(g) orbital polarization is the driving force behind magnetic spin ordering. Finally, in the crossover region betw een type-C and type-G states, we see some evidence for the development of l ocal type-C clusters embedded in a type-G framework, directly addressing pr oposals of similar short-range magnetic ordering in highly doped La1-xCaxMn O3 perovskites.