Study of magnetoresistive properties of double layer perovskite manganite

Abstract

The polycrystalline samples of (La,.,Nd,)(Ba,.,.Ca,)Mn,07 were synthesized by using the solid state reaction method. Powders of La,O, (99.99%), CaCO, (99.9"10), Nd,O, (999%), MnO, (99.9%) and BaCO, (99.9%)'were mixed thoroughly in an appropriate atnount in dry and acetone media. The mixed powders were calcined at high temperature I 100°C for 72 hours with intermediate grinding after 24 hours. The calcined powder was ground thoroughly to fine powder and pressed uniaxilly into pellets of 12 mm diameter and 1-3 mm thick under a pressure of 12000 P.SJ for about 1 to 3 minutes. The prepared pellets were sintered at 1400°C and 1500°C in air for 5 hours. The structural characterization was carried out using an X-ray diffractometry. All samples were of single phase within the resolution of the X-ray measurements. The de electrical resistivity of the samples was measured by using the four-point probe technique from room temperature down to liquid nitrogen temperature in zero applied field and in presence of O.65T magnetic field. The normalized resistivity p(1)/p(RT) as a function of temperature for various (La,.,Nd,)(Ba,.,.Cay}Mn,07 polycrystalline samples were measured. It was observed that most of the samples show metal-insulator (M-I) transition at certain temperature, Tp' The M-I transition temperature, Tp and the value of p(1)/p(RT) curves were found to vary with compositions. The variation of M-I transition temperature, Tp and p(1)/p(RT) are due to the charge transfer Mn'"-O_Mn4 +, substitution of the trivalent by divalent cation and also different atomic size of the molecules. It was also observed that as the sintering temperature increase, resistivity of the samples decrease throughout the measured temperature range. This is probably due to the decrease of pores and increase of grain size of the samples, which result in the decrease in the grain boundary and hence decrease in resistivity of the samples. In the presence of 0.65T magnetic field, M.I transition temperature enhance by few Kelvin as the magnetic field accelerate magnetic ordering. This is suggested to be due to the fractional ordering of magnetic moment in the paramagnetic regime hence change the transition temperature, Tp due to the applied magnetic field. The magnetoresistance (AiR) as a function of temperature, MR(T) was measured. Also the magnetoresistance (MR) as a function of applied magnetic field, MR(H) for all polycrystalline samples were measured at room temperature (295K) and liquid nitrogen temperature (77K). The MR behavior at room temperature for all samples show almost a linear field dependence of MR. The MR at 17K for all polycrystalline samples exhibit two distinct regimes as a function of magnetic field. Highly sensitive low field MR observe up to an applied magnetic field H'. For H<I-( the MR change rapidly with H followed by a gradual change as H is increase further. Beyond H',MR has a weak dependence in applied magnetic field.