Hall effect of La2/3(Ca,Pb)1/3MnO3 single crystals near the critical temperature

The Hall resistivity rho(xy) of a La-2/3(Ca,Pb)(1/3)MnO3 single crystal has been measured as a function of temperature and field. The overall behavior is similar to that observed previously in thin films. At 5 K, rho(xy) is positive and linear in field, indicating that the anomalous contribution Rs is negligible. However, the effective carrier density in a free electron model is n(eff)=2.4 holes/Mn, even larger than the 0.85-1.9 holes/Mn reported for thin films and far larger than the 0.33 holes/Mn expected from the doping level. As temperature increases, a strong, negative contribution to rho(xy) appears, that we ascribe to Rs. Using detailed magnetization data, we separate the ordinary (proportional to B) and anomalous (proportional to M) contributions. Below T-c, \R-S\proportional to rho(xx), indicating that magnetic skew scattering is the dominant mechanism in the metallic ferromagnetic regime. At and above the resistivity-peak temperature, we find that rho(xy)/rho(xx)M is a constant, independent of temperature and field. This implies that the anomalous Hall coefficient is proportional to the magnetoresistance. A different explanation based on the two-fluid model is also presented. [S0163-1829(99)06517-0].