Here we report the optimized magneto-transport properties of polycrystalline La0.70Ca0.3-xSrxMnO3 and their composites with Ag. The optimization was carried out by varying the Sr and Ag contents simultaneously to achieve large temperature coefficient of resistance (TCR) as well as low field magneto-resistance (MR) at room temperature. Sharpest paramagnetic (PM)-ferromagnetic (FM) and insulator-metal (IM) transition is observed in the vicinity of the room temperature (TC=300 K=TIM) for the composition La0.70Ca0.20Sr00.10MnO3:Ag0.20. Partial substitution of larger Sr2+ ions at the Ca2+ ions sites controls the magnitude of the FM and IM transition temperatures, while the Ag induces the desired sharpness in these transitions. For the optimized composition, maximum TCR and MR are tuned to room temperature (300 K) with the former being as high as 9% and the later being 20 and 30 percent at 5 and 10 kOe magnetic fields respectively. Such sharp single peak (TCR= 9 percent) at room temperature can be used for the bolometric and infrared detector applications. The achievement of large TCR and low field MR at T~300K in polycrystalline samples is encouraging and we believe that further improvements can be achieved in thin films, which, by virtue of their low conduction noise, are more suitable for device applications.
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