Two Peak Effect in GMR: A Chemical Effect?
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EXPERIMENTAL Lao.7Ca03 -.NaxMnO 3+8 samples were prepared by three different methods, starting from an aqueous solution of the metallic cations : sol-gel method with urea as gelifiant agent (x = 0.1), combustion of a nitrate-urea mixture (x = 0 and 0.1) and combustion by decomposition of ammonium nitrate (x = 0.1). Two different thermal treatments were applied to the asformed precursors. After a first treatment at 773 K for 15 h, the powders were calcined at 269
Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
powders were next pressed into pellets and sintered at 1523 K for 10 h. In order to make the discussion easier, labels for the different samples are indicated in table I. The homogeneity of the samples was checked by X-ray diffraction and EDX analysis. A small amount of Mn 3 0 4 was found in the combustion samples. The average grain size was estimated by scanning electron microscopy (SEM). The electrical resistance as a function of temperature was measured using the conventional four-probe method. TABLE I: Summary of labels, synthesis conditions and average grain sizes synthesis method thermal Composition label treatment shorta sol-gel Lao. 7Ca0.2Nao. 1MnO 3 LCNSG1 shorta combustion with urea LCNCU1 longe combustion with urea LCNCU2 LCNC1 combustion without urea shorta shorta combustion with urea LCCU1 Lao. 7Cao. 3MnO3 : 773 K (15 h)- 1523 K (3 h)- 1523 K (10 h) b:773 K(15 h)- 1573 K (24 h)-
1average grain size 4 ptm 4 pin 15 Pim 5 ýLm 1.5 ýtm 1523 K (10 h)
RESULTS Transport measurements Figure 1 shows the temperature dependence of the resistance for the LCNCU1 combustion sample. Contrary to the "canonical" behavior (a sharp single-peak curve as in a Lao.7Cao. 3MnO 3 sample prepared by the conventional solid state reaction [6]), two peaks can be seen in the resistance as a function of temperature [R(T)] curve : a broad peak at low temperature (ca. 240 K) and a sharper one at high temperature (ca. 300 K). Further experiments were performed to determine the origin of this unusual behavior. The R(T) curve of the LCCU1 combustion sample is shown in figure 2. The presence of a double peak indicates that the presence of two different doping cations on the La site is not responsible for the double-peak behavior seen in figures 1 and 2. A double peak is also observed in the R(T) curve of the LCNSG1 sol-gel sample (Figure 3). Contrary to the combustion samples, this sol-gel sample contains no Mn 3 0 4. We can conclude that neither a combustion synthesis nor the presence of Mn30 4 are necessary conditions for the appearance of the double-peak behavior. The R(T) curve of the LCNCI combustion sample shows only one peak (Figure 4). This peak is situated at about the same temperature as the high-temperature peak in the LCNCU1 sample. The LCNCU1 and LCNC1 samples have been respectively prepared in presence and in absence of urea : thus, synthesis conditions seem to play a significant role in the appearance of a double peak in the R(T) curve. Given that the LCNC1 sample contains a small amount of Mn
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