The magnetocrystalline anisotropy energy (MAE) for ferromagnetic and antiferromagnetic freestanding monowires of $4d$ transition metals is investigated on the basis of first-principles calculations. Across the $4d$ series, the easy axis of the magnetization oscillates between two directions: perpendicular and along the wire axis. The largest values of the MAE occur at the end of the series. Giant values of $30–60\mathrm{meV}/\mathrm{\text{atom}}$ can be obtained upon stretching Ru or Rh wires. Ru and Pd chains change the magnetization direction upon wire stretching, opening new perspectives in controlling the spin-dependent ballistic conductance in these structures.

• Received 17 November 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.147201