Ordering Phenomena in Transition Metal Oxides

Impact of cationic orbital and charge ordering upon magnetic and transport properties of manganites   

Orbital and charge ordering in Ln_1-xCa_xMnO₃ manganites (Ln = Pr, Nd) is mainly governed by the Jahn Teller effect of Mn³⁺, leading to a CE-type antiferromagnetic insulating structure. An important issue deals with the knowledge of the mechanism which contributes to the melting of this AFM state into a FM metallic-state, leading to the CMR effect. In this respect, the study of the substitution of very small amounts of barium for calcium and of various trivalent elements such as Ga and Cr for manganese is of interest since it induces ferromagnetism. In the present study, we show that a ferromagnetic ground state can be induced by Ba, Ga or Cr doping for a crucial composition x = 0.43, whatever Ln = Pr or Nd. The study of the magnetization vs magnetic field at 2.5 K evidences the existence of ultrasharp multisteps which are also observed for resistivity. A martensic like scenario based on structural phase separation, is proposed to explain the latter phenomena.