Morphology dependence of low temperatures exchange bias Co/CoO core-shell nanoparticles/spheres by eco-friendly solvothermal route

The present work reports the magnetic effect of particle shape and magnetic property in a Co/CoO core-shell exchange bias system. Co/CoO core-shell nanoparticles/spheres (NPSs) of different sizes including spherical and octahedral morphologies were prepared by the decomposition of cobalt acetate under eco-friendly solvothermal conditions. Spherical and octahedral Co/CoO NPSs all display superparamagnetism at room temperature due to the superparamagnetic critical size of cobalt. And it can be seen in hysteresis loop patterns that the saturation magnetization (MS) values of the Co/CoO NPSs are 100.6, 76.1, and 75.9 emu/g, whose diameters are about 16, 200 and 300 nm, respectively. In addition, all of spherical Co/CoO NPSs reveal exchange bias field at low temperature, whereas octahedral ones disappear because excessive PVP inhibits the formation of exchange coupling interface between ferromagnetic Co and antiferromagnetic CoO. These results were only observed after field cooling at 5 T.