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Summary form only given. Materials which display large changes in resistivity in response to an applied magnetic field (magnetoresistance) are currently of great interest due to their potential for applications in magnetic sensors, magnetic random access memories, and spintronics-a new kind of electronics based on spin instead of charge. Although ferromagnetic manganites show colossal magnetoresistance (CMR) effects around their Curie temperature, the low field and nearly temperature independent magnetoresistance properties important for spintronics are found only at low temperatures. Guided by striking features in the electronic structure of several magnetic compounds, we prepared the Heusler compound Co/sub 2/Cr/sub 0.6/Fe/sub 0.4/Al. Based on our band structure calculations, we have chosen this composition in order to obtain a ferromagnetic half metal with a van Hove singularity in the vicinity of the Fermi energy in the majority spin channel and a gap in the minority spin channel. We find evidence for spin polarized transport at room temperature, showing up in a magnetoresistive effect of 35% in a small magnetic field of 0.1 Tesla. This demonstrates the feasibility of developing spintronics at room temperature based on intermetallic compounds.