Abstract
We have peformed neutron diffraction measurements on melt-grown polycrystalline samples of ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(0.49<~x<~0.75).$ 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 ${e}_{g}$ orbitals. When increasing X, the ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ system 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 $x\ensuremath{\gtrsim}1/2,$ indicating that the energy difference between these two states is very small. We also found that the ${\mathrm{MnO}}_{6}$ octahedra are apically compressed in the CE-type and A-type AFM states due to the ${d(3x}^{2}\ensuremath{-}{r}^{2}{)/d(3y}^{2}\ensuremath{-}{r}^{2})$ or ${d(x}^{2}\ensuremath{-}{y}^{2})$ orbital ordering, whereas they are apically elongated by the rod-type ${d(3z}^{2}\ensuremath{-}{r}^{2})$ 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 $x=3/4.$
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