Magnetism in monatomic metal wires

The first investigation of magnetism in one-dimensional (1D) monatomic chains of metal atoms is reported. High-density arrays (5 × 10 6 cm −1 ) of parallel Co atomic wires have been grown using the vicinal Pt(997) surface as a template. Angle-resolved photoemission experiments evidence the presence of a 3d exchange-split band for the Co wires giving rise to enhanced localized spin magnetic moments. X-ray magnetic circular dichroism shows further that the orbital magnetic moment is about five times larger compared to that of bulk hcp Co as a result of the reduced atomic coordination of the 1D wires. Whereas statistical models forbid long-range ferromagnetic order in infinite 1D spin chains at any temperature greater than zero, we show that finite monatomic Co chains display both short- and long-range ferromagnetic order. The chains consist of thermally fluctuating segments of ferromagnetically coupled atoms which evolve into a ferromagnetic metastable long-range-ordered state below 15 K. Ferromagnetism in 1D is stabilized by unusually large magnetic anisotropy energy barriers (2 meV/atom) which arise from the reduced dimensionality of the wires and related large orbital magnetization.