  Nickelates are a rich class of materials, ranging from insulating magnets to
superconductors. But for stoichiometric materials, insulating behavior is the
norm, as for most late transition metal oxides. Notable exceptions are the 3D
perovskite LaNiO$_3$, an unconventional paramagnetic metal, and the layered
Ruddlesden-Popper phases R$_4$Ni$_3$O$_{10}$, (R=La, Pr, Nd). The latter are
particularly intriguing because they exhibit an unusual metal-to-metal
transition. Here, we demonstrate that this transition results from an
incommensurate density wave with both charge and magnetic character that lies
intermediate in behavior between the metallic density wave seen in chromium
metal and the insulating stripes typically found in layered nickelates. As
such, R$_4$Ni$_3$O$_{10}$, which appears to be the first known example of an
itinerant spin density wave in a 3d transition metal oxide, represents an
important bridge between the paramagnetism of 3D metallic LaNiO$_3$ at higher
nickel valence and the polaronic behavior of quasi-2D R$_{2-x}$Sr$_x$NiO$_4$ at
lower nickel valence.
