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Impact of size mismatch induced quenched disorder on phase fluctuation and low field magnetotransport in polycrystalline $Nd_{0.58-x} Gd_x Sr_{0.42} Mn O_3$

We report the magnetic and electrical transport properties of the bulk polycrystalline NGSMO (x ~ 0.0, 0.04, 0.08, 0.12, 0.16, 0.20, 0.25, 0.30, 0.35 and 0.42) synthesized by solid state reaction at 1350 degree Celsius. All the samples are single phase and have grain size ~1-2 micrometer. The variation of magnetic and magnetotransport properties as a function of the variance sigma^2 show some interesting trends. As sigma^2 increases, the paramagnetic-ferromagnetic (PM-FM) transition shows a gradual decrease and broadening, while the decrease in insulator metal transition (IMT) is accompanied by sharpening. However, near equality of TC and TIM at intermediate values of sigma2 could be understood in terms of the competing quenched and the grain boundary disorder. The variation of peak MR (maximum MR around TC/TIM) with sigma2 shows that maximum low field MR =35 % at H=3 kOe (68 % at H=10 kOe) is centered around sigma2 = 0.009857 {A}2 (x=0.25). This shows that huge intrinsic MR can be obtained at relatively higher temperatures and lower magnetic fields in the region of enhanced phase fluctuations.