Substantial Enhancements of Low-Field Magnetoresistance at Temperatures near 77 K with Doping 4% Cu at Mn Sites of La_2/3Ca_1/3MnO₃ Synthesized at 1000°C

We report on investigations of magnetoresistance (MR) in polycrystalline La_2/3Ca_1/3Mn_1-xCu_xO₃ (x = 0 and 4%) samples prepared by the sol-gel method followed a sintering treatment at 1000°C. For the x = 4% sample, the most remarkable MR effect appears at temperatures near 77 K and for magnetic fields lower than 1 T. For fields ～ 0.3 T and at temperatures ～ 77 K, the x = 4% sample exhibits giant MR effect with M₀ (≡ Δρ/ρ (H = 0)) between ～ 50 and ～ 70%, comparable to the so-called colossal MR response observed in the x = 0 sample for the 3 T field.