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The magnetoresistance (MR) of single Co nanowires of various width is investigated at low temperatures in magnetic fields up to 5 T. The in-plane longitudinal MR shows pronounced features at coercive fields indicating the magnetization reversal process. The nanowires are prepared by electron beam lithography. Some of the wires are covered with a 2 nm thick Pt layer, the others are uncovered. The MR behavior of the Pt covered Co nanowires shows significantly different behavior from that of the uncovered nanowires. This is interpreted mainly as arising from an oxidation of the Co surface leading to a formation of an antiferromagnetic CoO layer on top of the uncovered Co nanowires. The CoO layer hinders the magnetization reversal process by domain wall pinning, which is reflected by the MR behavior of these samples. In contrast, the MR behavior of the Pt covered Co nanowires shows no pinning effects. Thus, we conclude that covering the Co nanowires in situ with a 2 nm thick Pt layer prevents oxidation effects. © 2003 American Institute of Physics.