Search

Magnetotransport and thermal studies of Pr2/3Sr1/3MnO3 polycrystalline sintered bulk sample are reported here. The resistivity ρ(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. [ABSTRACT FROM AUTHOR]

Abstract: We have studied the electrical resistivity, magnetoresistance (MR), and specific heat of (1− x) Pr0.67Ba0.33MnO3:xAg2O (x =0–30mol% Ag2O) composite manganites. Two insulator–metal-like transitions (T P1 ∼194K and T P2 ∼160K) are observed in the electrical resistivity behaviour of the pristine material Pr0.67Ba0.33MnO3 (PBMO). With Ag2O addition, T P1 becomes sharper whereas T P2 disappears beyond 25mol% Ag2O addition. Electrical resistivity also decreases in the composites in the whole temperature range of measurement. Intrinsic MR (due to grain) gets enhanced from 22% for the pure sample to ∼40% for 30% Ag2O sample with 0.6T field. Extrinsic MR (due to grain boundary), however, is found to decrease with Ag2O addition. Peak MR enhances in composite at the same temperature and at low temperature, beyond the peak, it is almost constant in a large temperature window. Anomaly observed in the specific heat (C P) near the high temperature insulator–metal transition temperature for various samples signifies the onset of magnetic ordering. Also C P vs T trend is similar in all the samples indicating that Ag2O addition does not produce any magnetic inhomogeneity. [ABSTRACT FROM AUTHOR]

Abstract: Electrical and magnetic properties of orthorhombic Pr2/3Ba1/3MnO3 (PBMO) and La2/3Ca1/3MnO3 (LCMO) manganites with considerable difference in variance factors (σ 2) are reported here. PBMO with higher variance exhibits distinct intrinsic (due to grains) and extrinsic (due to grain boundaries) transitions in the resistivity behaviour. Extrinsic effects, however, are not observed in the lower σ 2 LCMO system. Low field magnetoresistivity (LFMR) data also substantiate these results. Increase in the density of states obtained through Mott''s 3-D variable range hopping mechanism in the paramagnetic insulating regime indicates the suppression of magnetic domain scattering with applied magnetic field. Ferromagnetic metallic regime below the extrinsic transition in PBMO seems to emanate from the electron–magnon scattering process. LFMR at 77 K also points towards the higher canting of spins in the vicinity of grain boundary regions in PBMO compared to that in LCMO. [Copyright &y& Elsevier]

We report here the electrical resistivity, ac susceptibility, low field magneto-resistance (LFMR) and structural parameters of the Pr1-xBaxMnO3(x= 0.33–0.80) system. Samples with 33% and 40% Ba-content exhibit two clear transitions in their resistivity-temperature behavior while signatures of higher temperature transition are also seen in the 50% Ba sample. The higher temperature transition (insulator-metal, Tp1), like in other CMR materials, results due to competition between the double exchange and super exchange mechanisms. The lower temperature broad hump (Tp2) in resistivity reflects the strain effect at the grain boundaries due to the ionic radii mismatch between Pr+3 and Ba+2. The low field magneto-resistance (at 0.15T) also corroborates the above results as MR peaks near Tp1 and increases further at lower temperatures, reflecting the contribution from the grain boundaries. Susceptibility measurements show the Curie transition (Tc) to be near Tp1 of the ρ-T data. Results are found to be consistent with the (NdBa) MnO3 and (LaBa) MnO3 systems. Resistivity data on the Pr0.67Sr0.33MnO3 system, where the lower temperature transition at Tp2 is not seen due to the smaller ionic size difference of ( Pr+3–Sr+2) compared to (Pr+3–Ba+2), also substantiate these results. The observed resistivity upturn at low temperatures is considered in terms of the ensuing localization of carriers due to the varying degree of ionic size mismatch in these systems. [ABSTRACT FROM AUTHOR]

Abstract: Electrical resistivity, magnetoresistance (MR), temperature coefficient of resistance (TCR) and magnetocaloric effect of (1−x) Pr2/3Ba1/3MnO3:x PdO (x=0–30mol% PdO) composite manganites are reported here. Pristine sample Pr2/3Ba1/3MnO3 (PBMO) shows two insulator–metal like transitions (TP1 ~194K and TP2 ~160K) in the electrical resistivity behavior. With PdO, TP1 becomes sharper whereas TP2 disappears beyond 10mol% PdO addition. The intrinsic MR gets enhanced from 22% for the pristine sample to ~42% for 27% PdO sample. However, the extrinsic MR is found to decrease in the composites. The TCR also increases from a negligible value for PBMO to 8% for 25mol% PdO sample. These features have been explained on the basis of opening of new conducting channels and decrease in spin dependent scattering and the overall decrease in electrical resistivity. The magnetic entropy change and relative cooling power (RCP) for the PBMO sample are 5.3418J.Kg-K and 304.5428J/Kg respectively. However, these values decrease in the composites. [ABSTRACT FROM AUTHOR]