Noncollinear magnetism of Cr and Mn monolayers on Cu(111)

Cr and Mn monolayers on a triangular lattice are prototypical examples of f rustrated spin systems in two dimensions. Collinear and noncollinear magnet ic structures of these monolayers on Cu(111) substrate are investigated on the basis of first-principles total-energy calculations using the full-pote ntial linearized augmented plane-wave method extended by the vector spin-de nsity description for the interstitial and vacuum region. The search for th e magnetic minimum-energy configurations included unit cells with one, two, and three atoms. For Cr the minimal energy was found for a 120 degrees spi n configuration in a (root 3x root 3)R30 degrees unit cell, which is in agr eement with the classical nearest-neighbor Heisenberg model with antiferrom agnetic exchange interaction. The same behavior is expected for Mn, but a s urprising result was found: the minimal energy was found for a collinear ro w-wise antiferromagnetic structure. (C) 2000 American Institute of Physics. [S0021-8979(00)51008-1].