  Metallic Mn-based alloys with a nearest-neighbor Mn-Mn distance greater than
0.4 nm exhibit large, well-localized magnetic moments. Here we investigate the
magnetism of tetragonal Au4Mn with a Curie temperature of 385 K, where
manganese has a spin moment of 4.1 muB and its orbital moment is quenched.
Since 80% of the atoms are gold, the spin orbit interaction is strong and Au4Mn
exhibits uniaxial magnetocrystalline anisotropy with surface maze domains at
room temperature. The magnetic hardness parameter of 1.0 is sufficient to
maintain the magnetization along the c-axis for a sample of any shape. Au also
reduces the spin moment of Mn through 5d-3d orbital hybridization. An induced
moment of 0.05 muB was found on Au under a pulsed field of 40 T. Density
functional theory calculations indicate that the Mn-Mn exchange is mediated by
spin-polarized gold 5d and 6p electrons. The distance-dependence shows that it
is ferromagnetic or zero for the first ten shells of Mn neighbors out to 1.041
nm (64 atoms), and very weak and oscillatory thereafter.
