Abstract
It is well known that the radius of A-site ion is an important parameter to determine the magnetoresistance (MR) ratio of (R,A)MnO 3 manganites. However, the size effect of A-ion is inevitably mingled with the valence effect of Mn-ion when an R-ion of 3+ valence is replaced by an A-ion of 2+. In order to unravel the mystery of the unusually large MR in these materials, it is essential to distinguish the contribution of size effect from that of the valence effect. In this paper, size effect on various physical properties of two compound systems, Pr 0.7Sr 0.3− x Ca x MnO 3 and Pr 0.6Sr 0.4− y Ca y MnO 3 are reviewed extensively. Based on the transport data, the electronic state of Pr 0.7-system is quite different from that of Pr 0.6-system. However, the size effects in these two systems are very similar from the viewpoint of changing their magnetic state and MR ratio. Adopting the mechanism of composite polaron to explain our observations, CMR arises from the depression of spin–wave excitation and the size effect activates its influence on MR ratio via the changing of the effective mass of composite polaron.
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