Electron−Hole Asymmetry in the Rare-Earth Manganates: A Comparative Study of the Hole- and the Electron-Doped Materials

Properties of the hole-doped Ln1-xAxMnO3 (Ln = rare earth, A = alkaline earth, x < 0.5) are compared with those of the electron-doped compositions (x > 0.5). Charge ordering is the dominant interaction in the latter class of manganates unlike ferromagnetism and metallicity in the hole-doped materials. Properties of charge-ordered (CO) compositions in the hole- and electron-doped regimes, Pr0.64Ca0.36MnO3 and Pr0.36Ca0.64MnO3, differ markedly. Thus, the CO state in the hole-doped Pr0.64Ca0.36MnO3 is destroyed by magnetic fields and by substitution of Cr3+ or Ru4+ (3%) in the Mn site, while the CO state in the electron-doped Pr0.36Ca0.64MnO3 is essentially unaffected. It is not possible to induce long-range ferromagnetism in the electron-doped manganates by increasing the Mn−O−Mn angles up to 165 and 180° as in La0.33Ca0.33Sr0.34MnO3; application of magnetic fields and Cr/Ru substitution (3%) do not result in long-range ferromagnetism and metallicity. Application of magnetic fields on the Cr/Ru-doped, electron-doped manganates also fails to induce metallicity. These unusual features of the electron-doped manganates suggest that the electronic structure of these materials is likely to be entirely different from that of the hole-doped ones, as verified by first-principles linearized muffin-tin orbital calculations.