Analysis of low temperature specific heat in the ferromagnetic state of the Ca-doped manganites

The reported specific heat C (T) data of the perovskite manganites La$_{1-x }$CaxMnO3, with x = 0.1, 0.2 and 0.33, is theoretically investigated in the temperature domain $4 \le T \le 10$ K. Calculations of C (T) have been made within the two component scheme: one is the Fermionic and the other is Bosonic (phonon or magnon) contribution. Lattice specific heat is well estimated from the Debye model and Debye temperature for Ca doped lanthanum manganites is obtained following an overlap repulsive potential. Fermionic component as the electronic specific heat coefficient is deduced using the band structure calculations for ferromagnetic metallic phase. Later on, for x = 0.1, following double exchange mechanism the role of magnons is assessed towards specific heat and find that at much low temperatures (T < 10 K), specific heat increases and show almost $T^{3/2}$ dependence on the temperature. We note that, the lattice specific heat is smaller for x = 0.1 when compared to that of magnon specific heat below 10 K. For x = 0.2, i.e., in the ferromagnetic metallic phase the magnon contribution is larger with the electron contribution while the reverse is true for x = 0.33. It is further noticed that in the ferromagnetic metallic phase, electronic specific heat is small in comparison to the lattice specific heat in low temperature domain. The present investigations allow us to believe that electron correlations are essential to enhanced density of state over simple Fermi liquid approximation in the metallic phase of La$_{1-x}$CaxMnO3 (x = 0.2, 0.33). The present numerical analysis of specific heat shows similar results as those revealed from experiments

Electrical resistivity in the ferromagnetic metallic state of La-Ca-MnO 3\mathsf{_{3}} : Role of electron-phonon interaction

The temperature-dependent resistivity of the perovskite manganites La 1-x Ca x MnO 3, with x=0.33, is theoretically analysed within the framework of the classical electron-phonon model of resistivity, i.e., the Bloch-Gruneisen model. Due to inherent acoustic (low-frequency) phonons ( $\omega_{ac})$ as well as high-frequency optical phonons ( $\omega_{op})$ , the contributions to the resistivity have first been estimated. The acoustic phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity compared to the contribution of optical phonons. Furthermore, the nature of phonons changes around T=167 K exhibiting a crossover from an acoustic to optical phonon regime with elevated temperature. The contribution to resistivity estimated by considering both phonons, i.e. $\omega_{ac}$ and $\omega_{op}$ , when subtracted from thin film data, infers a power temperature dependence over most of the temperature range. The quadratic temperature dependence of $\rho_{\it diff.}=[ \rho_{\exp} . - \{\rho_{0} + \rho_{e\text{-}ph} (= \rho_{ac} + \rho_{op}) \} ]$ is understood in terms of electron-electron scattering. Moreover, in the higher temperature limit, the difference can be varies linearly with T 4.5 in accordance with the electron-magnon scattering in the double exchange process. Within the proposed scheme, the present numerical analysis of temperature dependent resistivity shows similar results as those revealed by experiment. Copyright Springer-Verlag Berlin/Heidelberg 2004

Magnetotransport, thermoelectric power, thermal conductivity and specific heat of Pr2/3Sr1/3MnO3 manganite

Magnetotransport and thermal studies of Pr2/3Sr1/3MnO3 polycrystalline sintered bulk sample are reported here. The resistivity �(T) and thermoelectric power S(T) data show an insulator to metal (I-M) phase transition at TP≈294 K and TS≈290 K, respectively. Magnetization measurement confirms that the sample undergoes a transition from paramagnetic to ferromagnetic phase at a defined Curie temperature TC=280 K. A substantial increase in magnetoresistance from 2.5% at 280 K to 5% at 77 K has been noticed in a low magnetic field 0.15 T. Small polaron hopping model is found to be operative above the transition temperature TP, whereas electron-electron and electron-magnon scattering processes govern the low temperature metallic behavior. A detailed analysis of thermoelectric power in the ferromagnetic regime suggests that the complicated temperature dependence of S may be understood on the basis of electron-magnon scattering. A transition from decreasing high temperature thermal conductivity (due to local anharmonic distortions associated with small polarons), to an increasing thermal conductivity (due to decreasing of phonon-phonon scattering) and thereafter a peak at ∼100 K (signifying a crossover from Umklapp to defect-limited scattering) have also been noticed. Specific heat measurements depict a pronounced anomaly near the TC, indicating the magnetic ordering and magnetic inhomogeneity in the sample