Estimation of the charge carrier localization length from Gaussian fluctuations in the magneto-thermopower of La_{0.6}Y_{0.1}Ca_{0.3}MnO_3

Abstract

The magneto-thermoelectric power (TEP) $\Delta S(T,H)$ of perovskite type manganise oxide $L{a}_{0.6}{Y}_{0.1}C{a}_{0.3}Mn{O}_3$ is found to exhibit a sharp peak at some temperature $T^{\ast }=170K$. By approximating the true shape of the measured magneto-TEP in the vicinity of $T^{\ast }$ by a linear triangle of the form $\Delta S(T,H)\simeq S_p(H)\pm B^{\pm }(H)(T^{\ast }-T)$, we observe that $B^{-}(H)\simeq 2B^{+}(H)$. We adopt the electron localization scenario and introduce a Ginzburg-Landau (GL) type theory which incorporates the two concurrent phase transitions, viz., the paramagnetic-ferromagnetic transition at the Curie point $T_C$ and the "metal-insulator" (M-I) transition at $T_{MI}$. The latter is characterized by the divergence of the field-dependent charge carrier localization length $\xi (T,H)$ at some characteristic field $H_0$. Calculating the average and fluctuation contributions to the total magnetization and the transport entropy related magneto-TEP $\Delta S(T,H)$ within the GL theory, we obtain a simple relationship between $T^{\ast }$ and the above two critical temperatures ($T_C$ and $T_{MI}$). The observed slope ratio $B^{-}(H)/B^{+}(H)$ is found to be governed by the competition between the electron-spin exchange $JS$ and the induced magnetic energy $M_s{H}_0$. The comparison of our data with the model predictions produce $T_C=195K$, $JS=40meV$, $M_0=0.4M_s$, ${\xi }_0=10\mathring{A}$, and $n_e/n_i=2/3$ for the estimates of the Curie temperature, the exchange coupling constant, the critical magnetization, the localization length, and the free-to-localized carrier number density ratio, respectively.

6 pages (REVTEX), 2 PS figures (epsf.sty); submitted to Phys.Rev.B