Hole-concentration-induced transformation of the magnetic and orbital structures in Nd_1-xSr_xMnO₃

We have peformed neutron diffraction measurements on melt-grown polycrystalline samples of Nd_1-xSr_xMnO₃(0.49 < x < 0.75). A systematic transformation of the crystalline and magnetic structures of this system was observed. This can be consistently explained by the change of the character of the Mn e_gorbitals. When increasing X, the Nd_1-xSr_xMnO₃system exhibits an evolution from a metallic ferromagnetic state, to a metallic A-type antiferromagnetic (AFM), and then to an insulating C-type AFM state. The CE-type charge-ordered AFM state was observed only in the vicinity of x = 1/2 and it coexists with the A-type AFM state for x ≳ 1/2, indicating that the energy difference between these two states is very small. We also found that the MnO₆octahedra are apically compressed in the CE-type and A-type AFM states due to the d(3x₂-r₂)/d(3y₂-r₂) or d(x₂-y₂) orbital ordering, whereas they are apically elongated by the rod-type d(3z₂-r₂) orbital ordering in the C-type AFM state. Finally, a selective broadening of Bragg peaks was observed in the C-type AFM phase and its x dependence strongly suggests the onset of charge ordering for either x=4/5 or x=3/4.