  The cuprate high-temperature superconductors (HTSC) have been the subject of
intense study for more than 30 years with no consensus yet on the underlying
mechanism of the superconductivity. Conventional wisdom dictates that the
mysterious and extraordinary properties of the cuprates arise from doping a
strongly correlated antiferromagnetic (AFM) insulator (1,2). The highly
overdoped cuprates$-$those beyond the dome of superconductivity (SC)--are
considered to be conventional Fermi liquid metals (3). Here, we report the
emergence of itinerant ferromagnetic order (FM) below 4K for doping beyond the
SC dome in electron-doped La$_{2-x} $Ce$_x$CuO$_4$ (LCCO). The existence of
this FM order is evidenced by negative, anisotopic and hysteretic
magnetoresistance, hysteretic magnetization, and the polar Kerr effect, all of
which are standard signatures of itinerant FM in metals (4,5). This surprising
new result suggests that the overdoped cuprates are also influenced by electron
correlations and the physics is much richer than that of a conventional Fermi
liquid metal.
