Abstract:
A series ofpolycrystaUine bulk samples of Sr.",LaxMnJOIOwere prepared using conventional
solid state reaction technique and sintered at temperature !lOOT for 5 hours. The
magnetoresistive properties of the polycrystalJine bulk samples were investigated from room
temperarure down to liquid nitrogen temperature using standard four-probe technique.
Analysis of X-ray diffraction pattern gives the single-phase structure of all the polycrystalJine
samples without any trace of impurity. The transition between ferromagnetic metallic WId
paramagnetic insulating states in Sr,_,La.Mn10IO perovskites is characterized by a maximum
in the electrical resistivity at transition temperature. The transport behavior especially the
metal-insulator (M-I) transition in mangenites Sr4-xLa"MnjOIOmay be understood in terms of
the double exchange mechanisms which involves the ferromagnetic coupling between
MnJ'(t~, e~) and Mn4+(ti. e;) spins. As a result of strong Hund's exchange coupling, tbe
itinerant e~electrons (holes) interact with the localized Ii, electrons, and thus medIate
ferromagnetic ordering and thc simultaneous M-I transition. Substitution of La in place of Sr
enhances % of MnJ+ which ultimately increases M-I transition temperature in these
manganites. The M-l transition temperature is also found to increases due to the external
magnetic field. The external field might suppress the spin fluctuations which improves the
conductivity. The characteristic magnctoresistance was measured both at room temperature
and liquid nitrogen temperature. At room temperature MR is very low and almost linear with
field. At 78 K a sharp increase of MR was observed at low magnetic field but the MR
gradually decreases with the increase of applied magnetic field. The activation energy curves
show that the conduction occurred just above transition temperature through a thermally
activated process.