Negative magnetoresistance in mixed-valence La0.6Y0.1Ca0.3MnO3: Evidence for charge localization governed by the Curie-Weiss law

The colossal negative magnetoresistance Delta rho observed in La0.6Y0.1Ca0.3MnO3 at B=1 T shows an unusual, nearly perfect symmetry (around the peak temperature T-0=160 K), suggestive of a Curie-Weiss-dominated transport mechanism in this material both above and below the field-dependent Curie temperature T(C)equivalent to T-0. Attributing this symmetry to strong magnetic fluctuations below T-C (which are triggered by the Y substitution and cause a "bootstrap" destruction of the ferromagnetic phase), the data are interpreted in terms of the nonthermal spin hopping and magnetization-dependent charge carrier localization scenario leading to Delta rho=-rho(s)(1-e(-gamma M2)), with the magnetization M(T,B)=CB/\ T-T-C(B)\(nu). The separate fits through all the data points above and below T-C yield C(+)similar or equal to 2C(-) and nu(+)similar or equal to nu(-)similar or equal to 1, in agreement with the predictions of the Landau mean-field theory. (C) 1999 American Institute of Physics. [S0021-3640(99)01314-6].