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.
