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The mixed-valence manganites Pr 1− x Na x MnO 3 have been investigated by neutron diffraction, electric transport and magnetic measurements. Similarly to related systems with divalent alkali earths, the increasing monovalent sodium substitution decreases the Jahn–Teller deformation of the MnO 6 octahedra, lowers the resistivity and changes gradually the magnetic ordering from the layered type antiferromagnetism ( x =0) through spin-canted arrangements ( x ∼0.05) to the pure ferromagnetism (0.10⩽ x ⩽0.15) with T C ∼125 K. The samples with ferromagnetic ground state are not metallic below T C but show appreciable magnetoresistive effects in a broad temperature region. The electronic localization at low temperatures is further enhanced in the sample with the maximum sodium content x ∼0.2. Electron and neutron diffraction evidences that Pr 0.8 Na 0.2 MnO 3 exhibits a commensurate charge and orbital ordering of the Mn 3+ /Mn 4+ (1:1) kind below T co =215 K, followed with a transition to the antiferromagnetic arrangement of pseudo-CE type at T N =175 K, analogous to that of previously studied Pr 0.65 Ca 0.35 MnO 3 . In addition, Pr 0.8 Na 0.2 MnO 3 undergoes below ∼50 K a spin reorientation and, simultaneously, ferromagnetic clusters in the charge-ordered matrix are formed. By application of external field of 2–5 T below T co , the insulating charge-ordered antiferromagnet is transformed to a metallic ferromagnetic state which is persistent below ∼60 K, i.e. temperature close to the spin reorientation transition. |
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