Electron-hole asymmetry in the rare-earth manganates: a comparative study of the hole- and the electron-doped materials

Abstract

Properties of the hole-doped Ln_1−xA_xMnO₃ (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, Pr_0.64Ca_0.36MnO₃ and Pr_0.36Ca_0.64MnO₃, differ markedly. Thus, the CO state in the hole-doped Pr_0.64Ca_0.36MnO₃ 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 Pr_0.36Ca_0.64MnO₃ 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 La_0.33Ca_0.33Sr_0.34MnO₃; 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.