Fully unconstrained non-collinear magnetism in triangular Cr and Mn monolayers and overlayers on Cu(111) substrates

A recently developed fully unconstrained approach to non-collinear magnetis m has been applied to the investigation of the magnetic ground state of tri angular free-standing Cr and Mn monolayers and overlayers on Cu(111) substr ates. Such systems represent a physical realization of a frustrated two-dim ensional antiferromagnet. We find that the ground state of the Cr monolayer s is non-collinear; it shows root 3 x root 3 periodicity with +/-120 degree s angles between the directions of the magnetic moments on neighbouring sit es. Mn monolayers on the other hand have a collinear ground state with anti ferromagnetically coupled rows and c(2 x 2) periodicity. The fully unconstr ained description allows for a detailed investigation of the spin densities in the interstitial regions.