Influence of Preparation on Structure and Properties of La0.67Ca0.33MnO3 Bulk Material

Abstract

During growth of La 0.67 Ca 0.33 MnO 3 in an usual solid state reaction process, the diffusion of La-site metal ions is a rate-limiting factor. In the initial stage of the growth process, La-deficient manganite with rhombohedral structure was easily formed. With the repetition of the grinding±pressing± sintering process, the metal content on the La-site was gradually increased, and the phase was changed from rhombohedral to cubic structure. For slightly La-deficient (La,Ca) 1Àx MnO 3 manganites, the ferromagnetic±paramagnetic (F±P) transition temperature T M is less sensitive to the Lasite deficiency, but their electric transport properties are greatly affected. The slightly La-site deficient compound was characterized by double bumps on its resistivity versus temperature (R±T) curve, one of which was located at higher temperature very near to its temperature T M . For the La 0.67 Ca 0.33 MnO 3 single phase, only one resistance peak was observed at T R on its R±T curve. For the sample sintered at lower temperatures (< 1100 C) in air T R is much less than T M , and no magnetoresistance (MR) peak was observed within the temperature range of 80 K < T < 300 K. But for the sample sintered at 1200 C in air T R is well in agreement with T M and its MR is strongly peaked very close to T M . All the bulk samples showed substantial MR in a magnetic field of 0.6 T at low temperatures that increased almost linearly with decreasing temperature.

Double MITs and magnetoresistance: an intrinsic feature of Ru substituted La0.67Ca0.33MnO3

“…Another important point to be noted is that MR(T ) curves under 5 T show a sharp maximum close to T MI1 and a broad maximum close to T MI2 (figure 8). This is in contrast to the reports wherein the latter peak is absent in the MR curve for compounds exhibiting double maxima in the R(T, H ) curve due to GS [42,56].…”

“…A significant magnetoresistance with a weak temperature dependence could also be seen well away from the MR peaks, and this is shown to be a characteristic feature of the polycrystalline manganites [41]. Indeed, there are numerous reports on the presence of double maxima in the ρ(T ) curve, and most of these reports infer the origin of the double maxima feature to extrinsic factors such as the synthesis conditions [42], chemical inhomogeneity [43,44], grain size [45,46], grain boundary (GB) [47,48], and oxygen off-stoichiometry (δ) [49]. For the present work, it is pertinent to explore if the aforementioned factors are the cause of the LTM of Ru substituted La 0.67 Ca 0.33 MnO 3 .…”