Metal-insulator transition in the mixed-valence manganites

Abstract

It is found that the sharp resistivity peak observed near the Curie temperature TC in the manganites R12xAxMnO3 is closely correlated to the residual resistivity r 0 of the sample, suggesting that nonmagnetic randomness plays an important role in determining their anomalous properties. Using the one-parameter scaling theory to study the electronic localization due to both the nonmagnetic randomness and the double exchange spin disorder, we show that the sharp resistivity peak is caused by the Anderson metal-insulator ~M-I! transition and that r 0.r c ~a critical value! is a prerequisite to the occurrence of the M-I transition. TC as a function of r 0 has also been calculated. These results are in good agreement with experimental measurements. @S0163-1829~97!51836-4# The mixed-valence oxides R12xAxMnO3 ~where R5La, Nd, Pr; A5Ca, Sr, Ba, Pb! have recently been subjected to intense experimental 1‐10 and theoretical 11‐18 investigations because of a huge negative magnetoresistance ~colossal magnetoresistance or CMR! exhibited in samples of 0.2,x,0.5. For such a range of doping, the resistivity r vs temperature T curve usually exhibits a sharp peak at a certain temperature T p , indicating a crossover from metallic behavior (dr/dT.0) below T p to activated behavior ( dr/dT,0) above T p . The application of an external magnetic field strongly suppresses r and moves the resistivity peak to higher temperatures, thereby producing a CMR near T p .I t is generally accepted that the anomalous transport phenomena in these R 12x A x MnO 3 systems are closely related to their magnetic properties, in particular the paramagnetic ~PM!ferromagnetic ~FM! phase transition upon cooling. Most experimental measurements 1‐10 indicate that T p is very close to the Curie temperature TC , which is reminiscent of the double exchange ~DE! model 19 based on the exchange of electrons between Mn 31 and Mn 41 ions. For the Mn oxides, metallic ferromagnetism occurs in the composition range 0.2,x,0.5, where it is associated with the simultaneous presence of Mn 31 and Mn 41 ions. Each Mn 31 ion has four 3d electrons, three in the t 2g state and the fourth in the e g state. The Hund’s rule coupling is very strong so that spins of all the d electrons on a given site must be parallel. Three t 2g electrons are localized on the Mn site and give rise to a local spin S of magnitude 3/2, while the eg electron may hop into the vacant eg states of surrounding Mn 41 ions. Owing to the strong Hund’s rule coupling, the hopping of an eg electron between Mn 31 and Mn 41 sites is affected by the relative alignment of the local spins, being maximal when the localized spins are parallel and minimal when they are antiparallel. The sharp drop in r below TC can be attributed to the fact that an increase in the magnetization M upon cooling should reduce spin disorder scattering and thus increase the carrier conductivity. However, whether DE alone can account for the anomalous transport behavior is a question at issue. 13,16,17