Influence of Mn-site doping on charge and orbital ordering in La 1 / 3 Ca 2 / 3 Mn 1 − y M y O 3 manganites ( M = Ni , Ga )
Résumé
The influence of Ni and Ga doping on orbital and charge ordering (CO) in La1/3 Ca2/3 Mn1−y My O3 (M: Ni, Ga, and 0≤y≤0.1) has been investigated by combining magnetic and transport measurements with observation and analysis in transmission electron microscopy (TEM). Both types of doping cause a gradual decrease of TCO—the temperature of the charge ordering transition—while the temperature dependences of the magnetization and electric resistivity remain typical of the CO state. In the undoped parent sample, direct observations by TEM reveal the extended commensurate striped superstructure with q-vector 1/3a* over the whole temperature range <200 K. In Ni-doped samples, the long-range superstructure is kept but has an incommensurate character with reduced q-vector by 18%–20% for Ni concentration y=0.03. The extent of the superstructure parameter modulation is in agreement with the resulting decrease in the concentration of Mn3+ ions due to their substitution by Ni²+. Unlike doping by Ni, the incorporation of Ga on the Mn site completely suppresses the formation of the extended striped superstructure. Analysis of the obtained data allows us to conclude about the crucial role of the orbital-orbital (super)exchange interaction (rather than the Jahn-Teller interaction) in the stabilization of striped superstructure in the Mn-site-doped La1/3 Ca2/3 MnO3 system.