The structural and magnetic properties of cobalt nanowires embedded in epitaxial CeO2 layers are investigated. Combining electron microscopy with x-ray absorption spectroscopy, we show that polycrystalline metallic Co nanowires are formed during the growth. These nanowires are oriented in average along the growth axis with a misalignment distribution, have a predominant hcp structure and a narrow distribution of diameters. Their magnetic properties are overall dominated by shape effects with an easy axis oriented along the mean direction of the wires axis. The reversal mechanism is studied in details for samples with 3 and 5 nm mean diameters. The competition between shape and magnetocrystalline anisotropies is evidenced at 3 nm diameter. Upon reduction in the lateral dimensions, the nanowires tend to behave like aligned Stoner-Wohlfarth elongated particles with effective uniaxial anisotropy resulting from this competition.
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