Investigation of magnetoresistance in lanthanum strontium manganites

Abstract

The Lao.7Sr0.3Mnl_xFex03manganite samples for various x have been synthesized using standard ceramic technique .Three sets of specimens have been prepared at 3-different sintering temperatures of 1273 K(1000°C),1373 K(llOO°C) and 1473 K(l200°C). The X-Ray investigation has been carried out on the samples. The X-ray diffraction analysis shows that the samples are homogenous and single phase. Electrical resistance has been measured as function of temperature in the range of 78K to room temperature of 297K with 0 and 0.6 Tesla magnetic fields. The effect of iron substitution and sintering temperature has been investigated. The sintering temperature enhances the metal insulator transition temperature. When hole doped at a concentration of 30% holeslMn ions by Sr substitution for La, the material display a transition from a high temperature paramagnetic insulator to a low temperature ferromagnetic metal associated with a magnetoresistance. The measurement on Fe doped samples shows substitution of Fe up to 4% for Mn enhances the transition by few Kelvin, beyond 4% the transition temperature starts to drop. This suggests a ferromagnetic coupling is favoured between Mn3+ and Fe3+ for x = 0.04 and anti-ferromagnetic coupling beyond x = 0.04. Low temperature MR at 78K shows strong field dependence up to an applied field H' beyond which the MR is a very weak function of H. This is probably due to the reason that at T<<fc the material is in the ferromagnetic regime and in the absence of field the individual spins at the grain boundary region are randomly oriented. In the absence of field, a carrier suffers scattering from the unaligned magnetic domain, as well as disordered spin at the grain boundary region. At a low magnetic field, the magnetization vectors of each grain starts to align towards the direction of the external magnetic field and attains saturation at H*. However a large magnetic field is required to align the spins ofthe grain boundaries resulting in a weak dependence on H.