Magnon-polaron and spin-polaron signatures in the specific heat and electrical resistivity of La0.6Y0.1Ca0.3MnO3 in zero magnetic field and the effect of Mn - O - Mn bond environment

in Physical Review B (2002), 66(17),

La0.6Y0.1Ca0.3MnO3, an ABO(3) perovskite manganite oxide, exhibits a nontrivial behavior in the vicinity of the sharp peak found in the resistivity rho as a function of temperature T in zero magnetic ... [more ▼]

La0.6Y0.1Ca0.3MnO3, an ABO(3) perovskite manganite oxide, exhibits a nontrivial behavior in the vicinity of the sharp peak found in the resistivity rho as a function of temperature T in zero magnetic field. The various features seen on drho/dT are discussed in terms of competing phase transitions. They are related to the Mn-O-Mn bond environment depending on the content of the A crystallographic site. A Ginzburg-Landau type theory is presented for incorporating concurrent phase transitions. The specific heat C of such a compound is also examined from 50 to 200 K. A log-log analysis indicates different regimes. In the low temperature conducting ferromagnetic phase, a collective magnon signature (Csimilar or equal toT(3/2)) is found as for what are called magnon-polaron excitations. A Csimilar or equal toT(2/3) law is found at high temperature and discussed in terms of the fractal dimension of the conducting network of the weakly conducting (so-called insulating) phase and an Orbach estimate of the excitation spectral behaviors. The need of considering both independent spin scattering and collective spin scattering is thus emphasized. The report indicates a remarkable agreement for the Fisher-Langer formula, i.e., Csimilar todrho/dT at second order phase transitions. Within the Attfield model, we find an inverse square root relationship between the critical temperature(s) and the total local Mn-O-Mn strain. [less ▲]

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

in Physical Review. B : Condensed Matter (1999), 60(17), 12322-12328

The magneto-thermoelectric power (TEP) Delta S(T,H) of perovskite-type manganese oxide La0.6Y0.1Ca0.3MnO3 is found to exhibit a sharp peak at some temperature T*=170 K. By approximating the true shape of ... [more ▼]

The magneto-thermoelectric power (TEP) Delta S(T,H) of perovskite-type manganese oxide La0.6Y0.1Ca0.3MnO3 is found to exhibit a sharp peak at some temperature 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) similar 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 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-o. 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 MsHo. The comparison of our data with the model predictions produce T-C = 195 K, JS = 40 meV, M-o = 0.4M(s), and xi(o) = 5 Angstrom for the estimates of the Curie temperature, the exchange coupling constant, the critical magnetization, and the localization length, respectively. 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]. [less ▲]