Double exchange and orbital correlations in electron doped manganites

A double exchange model for degenerate eg orbitals with intraand inter-orbital interactions has been studied for the electron doped manganites A1−xBxMnO3 (x > 0.5). We show that such a model reproduces the observed phase diagram and orbital ordering in the intermediate bandwidth regime and the Jahn-Teller effect, considered to be crucial for the region x < 0.5, does not play a major role in this region. Brink and Khomskii have already pointed this out and stressed the relevance of the anistropic hopping across the degenerate eg orbitals in the infinite Hund’s coupling limit. From a more realistic calculation with finite Hund’s coupling, we show that inclusion of interactions stabilizes the C-phase, the antiferromagnetic metallic A-phase moves closer to x = 0.5 while the ferromagnetic phase shrinks. This is in agreement with the recent observations of Kajimoto et. al. and Akimoto et. al. PACS Nos. 75.30.Et, 75.30.Vn The perovskite manganites have been at the centre of attention[1, 2] recently as systems like Pr1−xCaxMnO3, La1−xCaxMnO3, exhibit colossal magnetoresistance (CMR). These two extensively studied systems have relatively low bandwidths. The observation of CMR even in the wider bandwidth materials like Pr1−xSrxMnO3[3] and Nd1−xSrxMnO3 for x ≥ 0.5[4] have prompted a series of careful experimental work on the magnetic phase diagram of all of these systems particularly in the region x ≥ 0.5 (the so called electron doped regime). This region shows a rich variety of magnetic and orbital ordering and only recently the systematics of the phase diagram with externally controlled bandwidth have begun to emerge[5]. In their study of Pr1−xSrxMnO3, Kajimoto et. al.[6] have summarized the nature of magnetic ordering for a series of manganites having different bandwidths (Fig.1 in ref.[6]). They observe that there is no CE phase in many of these systems and the sequence of phases in the electron doped region for moderate to large bandwidth systems follows the order ferromagnetic (F) → A-type antiferromagnetic (AFM) → C-type AFM and finally to G-type AFM phase. The F phase close to x = 0.5 is very narrow and survives for systems email: [email protected] email: [email protected], [email protected]