Study of the magnetoresistive properties of some divalent alkaline doped manganite material

Abstract

The rewlL<. of the study or magnctorcsistivc properties for n=3 m~rnbcr of the Ruddlesden- poper compound of (LaA)I-rnMn"OJnH , where A= Ca, Ba ;md Sr is reported. Polycrystallinc bulk samples of (LaA)H"Mnn{h"+J was prepared by conventional solid-stale reaction technique. The magnctorcsistivc properties of this bulk polycrystalline samples have been invesligated from room kmperaturc dOl'vn to liquid nitrogen temperature by standard l'lUT point-probe technique. An analysis of samples structural homogeneity was carried out by meanS of X-ray diffraction. The DC electrical resistivity was measured as a fI.mction oftempenllurc both at an applied magnelic field of OT and 0.71 respoctivcly. Most of the samples display a metal-insulator (M:-I) tramilion with a peak in the electrical resi~tivity at a lempenllure Ip. Ca substituted sample La2Ca2Mn30w shows highe~t M-l transition temperattIrC. Substitution of Ba in plaee of Ca in the same composition decreases M-I transition temperature. Partial substitution of Sr and Ba in samples l.;llCaSrMn)Ow and La1CaBaMn.10IG also redu"es the M-l tran~ition temperature. This i~ probably due::\0 the different atomic sizes of divalent cations of Ca, Ba and Sr, which substantially changcs Mn~-Mn~ exchange interaction by bending the Mn-Mn bond angle. Thc application of a Ulagnctic field of 0.7T ~uppresscd the interfacial Mn spin disorder in this manganite system which ultimately improve the conductivity and decrease the resistivity. Magneloresi~tance was measured bolh at room temperature and liquid nitrogen temperature. Room tempendurc MR is found to be very low and is almost linear with field. At 78 K, a sharp increase of maglletore~i~tance was ob~crved at low magneti" lields followcd by a weak and linear dependence at high fields. The ubserved low temperature MR in these manganites can be attributed to b'l"ainand grain boundary effects. The plot of In p(l)/P(RT)-r' suggest that conduction occurred through a thermally activated proces~ abovc the transition temperature.