Critical behavior and transport properties of lanthanum manganite

The resistivity, thermopower, magnetization, and ac heat capacity of La_0.7Ca_0.3MnO₃, La_0.7(Ca,Pb) _0.3MnO₃, and La_0.7Sr_0.3MnO₃ single crystals have been measured over a broad temperature range in fields of up to 70 kOe. Below T_C, all three samples exhibit a strong T2 temperature dependence, characteristic of electron-magnon scattering, which abruptly disappears around 20 K. Above T_c, the transport properties of LCMO are consistent with thermally activated hopping of polarons. The charge carriers appear to consist of two different types which add according to the Nordheim-Gorter rule. The mixing fraction of the two populations around the transition temperature can be extracted with an effective medium approach. Magnetization data further support this two fluid picture. Although the inverse susceptibility follows a Curie-Weiss law, it has an anomalously small slope. This is indicative of the presence of a Griffiths phase where ferromagnetic clusters emerge in a paramagnetic background well before the entire system orders. The manganites' magnetic heat capacity peak has been observed to shift upward in temperature with applied magnetic field unlike that of ordinary ferromagnets. In the double exchange picture proposed by Zener, magnetism is closely linked to the nature of the charge carriers. We postulate that the exchange energy changes with the mixing fraction of the two carrier populations. By modifying the Oguchi model with this assumption we can reproduce the anomalous behavior of the heat capacity.