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

Cr and Mn monolayers on a triangular lattice are prototypical examples of frustrated spin systems in two dimensions. Collinear and noncollinear magnetic structures of these monolayers on Cu(111) substrate are investigated on the basis of first-principles total-energy calculations using the full-potential linearized augmented plane-wave method extended by the vector spin-density description for the interstitial and vacuum region. The search for the magnetic minimum-energy configurations included unit cells with one, two, and three atoms. For Cr the minimal energy was found for a 120° spin configuration in a (∛×∛)R30° unit cell, which is in agreement with the classical nearest-neighbor Heisenberg model with antiferromagnetic exchange interaction. The same behavior is expected for Mn, but a surprising result was found: the minimal energy was found for a collinear row-wise antiferromagnetic structure.