Estimation of the charge carrier localization length from Gaussian fluctuations in the magneto-thermopower of La0.6Y0.1Ca0.3MnO3

The magneto-thermoelectric power (TEP) Delta S(T,H) of perovskite-type mang anese oxide La0.6Y0.1Ca0.3MnO3 is found to exhibit a sharp peak at some tem perature T*=170 K. By approximating the true shape of the measured magneto- TEP in the vicinity of T* by a linear triangle of the form Delta S(T,H) sim ilar or equal to S-p(H)+/-B+/-(H)(T* - T), we observe that B-(H)similar or equal to 2B(+)(H). We adopt the electron localization scenario and introduc e a Ginzburg-Landau (GL)-type theory which incorporates the two concurrent phase transitions, viz., the paramagnetic-ferromagnetic transition at the C urie point T-C and the "metal-insulator'' (M-I) transition at T-MI. The lat ter is characterized by the divergence of the field-dependent charge carrie r localization length xi(T,H) at some characteristic field H-o. Calculating the average and fluctuation contributions to the total magnetization and t he transport entropy related magneto-TEP Delta S(T,H) within the GL theory, we obtain a simple relationship between T* and the above two critical temp eratures (T-C and T-MI). The observed slope ratio B-(H)/B+(H) is found to b e governed by the competition between the electron-spin exchange JS and the induced magnetic energy MsHo. The comparison of our data with the model pr edictions produce T-C = 195 K, JS = 40 meV, M-o = 0.4M(s), and xi(o) = 5 An gstrom for the estimates of the Curie temperature, the exchange coupling co nstant, the critical magnetization, and the localization length, respective ly. The magneto-TEP data obtained by other authors are discussed and Found to be consistent with the model predictions as well. [[S0163-1829(99)00734- 1].