The magneto-thermoelectric power (TEP) $\Delta S(T,H)$ of perovskite type manganise oxide $La_{ 0.6}Y_{ 0.1}Ca_{ 0.3}MnO_3$ is found to exhibit a sharp peak at some temperature $T^{ *}=170K$. By approximating the true shape of the measured magneto-TEP in the vicinity of $T^{ *}$ by a linear triangle of the form $\Delta S(T,H)\simeq S_p(H)\pm B^{ \pm}(H)(T^{ *}-T)$, 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 $T^{ *}$ 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 $M_sH_0$. The comparison of our data with the model predictions produce $T_{ C}=195K$, $JS=40meV$, $M_0=0.4M_s$, $\xi_0=10\AA$, 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.
Categories: cond-mat.str-el