A one-pot route to a unique type of thin cobalt film, with potential applications in high-density magnetic storage media, has been designed by Chinese scientists.

Thin films of cobalt have been widely investigated due to their giant magnetoresistance properties, where large differences in electrical resistance occur in response to the application of very small external magnetic fields. It is this property that has led to thin films of cobalt, and other elements, being used in modern high-capacity magnetic media, such as hard disk drives.

Most cobalt films made to date consist of spherical cobalt nanoparticles, but now Shijun Liao and Hao Li from the South China University of Technology1 have developed a one-step route to making a unique type of film containing non-spherical particles. This work is important because films of shape-anisotropic, non-spherical particles may have different properties to those of conventional films.

Liao and Li’s films consist of two-dimensional nanosheets of cobalt metal in a hexagonal close-packed arrangement. “This type of cobalt film is unique and has never been reported before,” says Liao.

The researchers produced their films using a one-pot method they refer to as a magnetic-field-assisted solution-phase route. First, cobalt chloride and ethylenediamine tetraacetic acid (EDTA) were combined in solution to make a Co2+–EDTA complex. Next, the solution was placed under a magnetic field and heated, causing the complex to be chemically reduced. After nearly a day under these conditions, a thin film of pure cobalt was formed around the edges of the reaction vessel. This technique has been used before for preparing one-dimensional magnetic nanowires and nanorods, says Liao, “However, it has never been reported for preparing magnetic thin films.”

Fig. 1: Scanning electron microscope image of a thin cobalt film prepared by a magnetic-field-assisted solution-phase route.

Using scanning electron microscopy, the team saw that the surface of the film looks like an irregular blanket of flowers (Fig. 1). X-ray diffraction analysis revealed the cobalt atoms to be hexagonally close-packed within the thin film.