Probing the Growth and Aging of Colloidal Cobalt Nanocrystals: A Combined Study by Transmission Electron Microscopy and Magnetic Measurements

We have combined transmission electron microscopy (TEM) and magnetic measurements to probe the growth and aging of colloidal cobalt (Co) nanocrystals and demonstrated that these two techniques together yield structure and property information in a manner that neither can do alone. During the growth, TEM shows the formation of Co nanocrystals (4.8 nm ± 1.7 nm), while magnetic measurements indicate the presence of paramagnetic Co cluster complexes and weakly interacting Co nanocrystals. At the completion of the synthesis, TEM shows that the average size of the Co nanocrystals has increased, but with a narrower size distribution (10.5 nm ± 1.0 nm). Meanwhile, magnetic measurements demonstrate the strong interactions between the Co nanocrystals. Exchange bias and increased coercivity are observed for the aged Co colloid under field-cooled conditions, indicating the existence of antiferromagnetic/ferromagnetic (AFM/FM) coupling. High-resolution TEM confirms that AFM face-centered cubic cobalt(II) oxide grows on the surface of the FM ε-Co nanocrystals, but this oxide layer is thin and inhomogeneous. These combined results suggest that not only the AFM/FM exchange coupling within individual aged nanocrystal matters but also the strong magnetostatic coupling between the neighboring nanocrystals significantly contributes to the observed exchange bias.