  Magnetism of 3d transition metals at high density has always received wide
interest due to existence of magnetism at the core of the Earth. For
ferromagnetic cobalt, although there is a theoretical prediction that its
magnetic moment would vanish when the density increases to about 1.4 times the
normal value, so far there is no experimental evidence. We have discovered the
existence of ultrathin superdense nonmagnetic cobalt layers in a
polycrystalline cobalt thin film. The densities of these layers are about
1.3-1.4 times the normal density of Co. This has been revealed by X-ray
reflectometry experiments, which provide a depth profile of the electron
scattering length density, and corroborated by polarized neutron reflectometry
(PNR) experiments. The superdense Co layer has an fcc structure, unlike hcp
structure for bulk Co, as revealed by transmission electron microscopy. The
magnetic depth profile, obtained by PNR, shows that the superdense Co layers at
the film-substrate interface and near the top of the film are nonmagnetic. The
major part of the Co film has the usual density and magnetic moment.
