Synthesis and magnetic properties of cobalt nanocubes

We report on the synthesis, structure, and magnetic properties of cobalt nanoparticles with cubic shape. The nanoparticles with an average cube-edge length of about 50 nm were prepared by wet-chemical processing and characterized by electron microscopy, x-ray diffraction, and magnetometry. The x-ray data indicate a mixture of hcp Co and ϵ-Co. From the magnetization measurements we deduce on the presence of a ferromagnet/antiferromagnet Co/CoO interface, which gives rise to a pronounced exchange-bias effect. Between 350 K and 5 K the coercivity HC increases by almost a factor of 6, whereas the exchange-bias field HE takes on a value of about 380 Oe at 5 K and vanishes above 75 K. The variation of HC with temperature cannot be understood in terms of the various conventional types of magnetic anisotropy.