Enhanced Magnetism in Cobalt Nanoparticles

We report an enhanced magnetic response for dilute Co nanoparticles with diameters d $<$ 2 nm in frozen organic matrices. Slight variations of an air- and water-free inverse micellar synthesis produced isolated Co particles with two sizes, d = 1.8 nm and d $<$ 1 nm. The 1.8 nm particles exhibit a Langevin response from 5 to 200 K with a saturation magnetization 10-20\% above bulk hcp Co; size estimates from Langevin fits and HRTEM images were consistent. Interface effects were studied by changing the surfactant, resulting in increased (amines) and decreased (thiols) magnetism at high fields. The smaller Co particles have a magnetic response at 5 K that fits a Brillouin function with a spin of 8-10 Bohr magnetons and an estimated saturation magnetization well above both bulk Co and Co(II) ions. However, the magnetic moment of these small clusters decreases rapidly with increasing temperature.